How Things Work - Additional Demonstrations
96. A Jackscrew Elevator
Description: A person stands on a metal plate that's supported from a jackscrew automobile
jack. Another person furiously turns the crank that rotates the nut that
supports the jackscrew and the metal plate slowly rises upward.
Purpose: To show that, while a jackscrew elevator is possible, it isn't very practical.
Supplies:
1 jackscrew
automobile jack, rigidly attached to plates at its top and bottom and equipped
with a crank to turn the nut that lifts the screw
Procedure: Place the jackscrew elevator on the floor and have someone stand on the upper
plate. Then have another person turn the crank so that the screw slowly rises
upward. Point out that the person turning the crank is doing work on the crank
(pushing it in the direction that it's turning) and that this work is lifting
the person on the plate. A small force exerted over a long distance is
providing a large force exerted over a short distance. Note, however, that it
takes a long time to lift the person on the plate, making this jackscrew
elevator rather impractical.
Explanation: The jackscrew and the nut that lifts it are really inclined planes (ramps) that
are wrapped into cylinders. The person turning the crank is effectively sliding
one inclined plane across the other and causing the inclined plane that is the
jackscrew to rise upward. They are obtaining mechanical advantage—a small force
exerted over a long distance is providing a large force exerted over a small
distance.
97.
A Hydraulic Elevator
Description: You rest an uninflated balloon on the table and hold a small weight on its top.
You then inflate that balloon and the weight rises.
Purpose: To show that a solid object can rise up on a trapped volume of hydraulic fluid.
Supplies:
1 rubber
balloon
1 small weight
Procedure: Place the uninflated balloon on the table and hold a small weigh on top of it.
Now carefully inflate the balloon. The balloon will inflate, carrying the
weight upward with it.
Explanation: As you blew high pressure air into the balloon, you did work on that air and
the air in turn did work on the weight, lifting it upward against the force of
gravity.
98.
A Bigger Hydraulic Elevator
Description: A person sits on a large plate that's on top of a plastic garbage bag. When air
is pumped into the garbage bag by a vacuum cleaner (running backward), the bag
inflates and the person rises into the air.
Purpose: To show that even a small pressure inside a large container can exert enormous
forces on the walls of that container.
Supplies:
1 heavy-duty
plastic garbage bag (or better yet, several of them, one inside the other)
1 stiff plate
that's almost the size of the garbage bag
1 vacuum cleaner
that can be run so that it pumps air into its hose
duct tape
Procedure: Tape the plate to the top surface of the plastic bag and then seal the open end
of the bag around the outlet of the vacuum cleaner hose. It's best to seal most
of the bag's open end to itself with the duct tape (like closing a zipper) and
then duct tape the small open corner of the bag to the hose. Place the assembly
on the floor or on a large, sturdy table and have a person sit on the plate.
Hold that person's hand so that they don't fall over as the bag inflates. Now
turn on the vacuum cleaner so that it inflates the bag. The person will rise
into the air as air enters the bag and inflates it.
Explanation: Although the air pressure inside the bag is only slightly above atmospheric
pressure, it acts on the whole surface area of the plate. The upward force that
the plate experiences, due to the high pressure below it than above it, is more
than the person's downward weight and the plate accelerates upward.
Alternative
Procedure: Inflate a dozen or more rubber balloons and put them on the
floor underneath an inverted table. If the area under the inverted table is
packed densely enough with balloons, 4 or 5 students will be able to
stand on the table without popping any of them. Make sure that someone stands
outside the table and holds onto its legs to keep it from tipping over as
people climb onto the table.
99.
A Hydraulic Jack
Description: Pumping the handle of a hydraulic jack many times causes its main piston to
rise only a small distance. However, the piston exerts an enormous force on the
object above it. (We have a hydraulic jack that crushes the object above it
against a fixed plate—we typically crush a broken electronic device such as an
old calculator.)
Purpose: To show that a small force exerted over a long distance on the small piston of
a two-piston hydraulic system can exert a large force over a small distance on
the large piston of the system.
Supplies:
1 hydraulic
jack system, with a hand-operated pump
1 heavy object to
put on the jack (or an object to crush, if you have a plate bolted in place
above the large piston)
Procedure: Put the jack on the floor or table and place the heavy object on top of its
large piston. Explain that moving the handle of the pump up and down pushes a
small piston in and out of a narrow hydraulic cylinder. Pushing the handle down
pressurizes the hydraulic fluid and squeezes it out of the cylinder, through a
one-way valve, and into the large cylinder beneath the large piston. The
pressure that you create in the small cylinder by pushing the piston into it
also appears in the large cylinder, where it pushes upward on the large piston.
But because the large piston has much more surface area than the small piston
upon which you push, the upward force that the large piston experiences is much
greater than the downward force you exert on the small piston. That's why you
can lift a very heavy object with the large piston while exerting only a modest
force on the pump handle. Show them that this is so by pumping the handle and
lifting the heavy object. Point out that you must move the pump handle a very
long distance to raise the heavy object even a small distance. That's because
the mechanical advantage in the hydraulic system is allowing you do the work of
lifting the heavy object—a large upward force exerted over a small distance—by
exerting a small downward force over a very large distance.
Explanation: Pushing the large piston out of the large cylinder requires much more fluid
than is provided by pushing the small piston into the small cylinder. Thus you
must push the small piston into the small cylinder many times, replenishing its
fluid each time from a reservoir, before the large piston moves a reasonable
distance out of the large cylinder.
100.
A Simple Cable-Lift Elevator
Description: A chair is lifted from above by pulling on a rope that's attached to it.
Purpose: To show that tension in a cord can convey an upward force to an object that's
attached to that rope.
Supplies:
1 chair
1 rope sling that
attaches to all four legs of the chair and provides a single loop above the chair
to which you can attach a rope
1 rope
access to the space
above the front of the lecture hall, or else a ladder
Procedure: Attach the rope sling to the chair and attach the rope to the sling. The rope
should descend to the chair from the space above the front of the lecture hall.
Go up to the top of the rope and lift the chair upward from above at constant
velocity. Announce that to make things simple, you will assume that the rope
weighs nothing. Point out that you are pulling upward on the rope with a force
equal in magnitude to the weight of the chair and that the chair is pulling
downward on the rope with a force equal in magnitude to its weight. The rope is
thus conveying your force to the chair and the chair's force to you. It has a
tension in it equal to the magnitude of the force you and the chair exert on it
(not twice that amount).
Explanation: The rope acts as an intermediary between you and the chair. When you pull
upward on the rope with a certain force, the rope pulls upward on the chair with
that same force (assuming the rope itself doesn't have any weight or mass). The
force "passing through" the rope is called the tension in the rope.
101.
A Simple Cable-Lift Elevator with a Pulley
Description: A chair is lifted from above by pulling down on a rope that's attached to it
and that passes over a pulley near the ceiling.
Purpose: To show that tension in a cord can convey a force to an object, even when that
rope passes over a pulley and the directions of the two forces are no longer
the same.
Supplies:
1 chair
1 rope sling that
attaches to all four legs of the chair and provides a single loop above the
chair to which you can attach a rope
1 rope
1 pulley attached
near the ceiling
Procedure: Attach the rope sling to the chair. Drape the rope over the pulley near the
ceiling and attach one end of it to the top of the rope sling. Again, announce
that to make things simple, you will assume that the rope weighs nothing. Pull
downward on the rope so that the chair rises at constant velocity. Point out
that you are pulling downward on the rope with a force equal in magnitude to
the weight of the chair and that the chair is pulling downward on the rope with
a force equal in magnitude to its weight. The rope is thus conveying your force
to the chair and the chair's force to you, even though the pulley is changing
the directions of those forces. The rope has a tension in it equal to the
magnitude of the force you and the chair exert on it (not twice that amount).
Explanation: The rope acts as an intermediary between you and the chair. When you pull on
the rope with a certain force, the rope pulls on the chair with that same force
(assuming the rope itself doesn't have any weight or mass). The force
"passing through" the rope is called the tension in the rope. But the
pulley is changing the directions of the force so that, while the tension
remains constant throughout the rope, the direction of the force that you exert
on the rope isn't the same as the direction of the force the rope exerts on the
chair.
102.
A Cable-Lift Elevator with a Multiple Pulley
Description: A chair is lifted from above by pulling down on a rope that's part of a
multiple pulley system extending from the ceiling to the chair. Even though a
person is sitting in the chair, it takes only a modest force exerted on the
rope to lift the chair upward.
Purpose: To show that tension in the cord passing through a multiple pulley can be used
several times to exert enormous inward forces on the two ends of a multiple
pulley system.
Supplies:
1 chair
1 rope sling that
attaches to all four legs of the chair and provides a single loop above the
chair to which you can attach a rope
1 multiple pulley
system that hangs from the ceiling
Procedure: Attach the top of the multiple pulley to the ceiling. Attach the rope sling to
the chair and connect the top of the rope sling to the bottom of the multiple
pulley. The end of the multiple pulley's cord should extend downward from the
upper portion of the multiple pulley. Have someone sit in the chair. Now pull
downward on the rope. The chair and person will rise upward. Point out that you
are exerting a relatively small force on the rope and thus creating only a
modest tension in the rope. However, because that same cord extends many times
between the top and bottom of the multiple pulley, the inward forces exerted by
its tension is used several times. If there are 5 rope segments between
the two ends of the pulley, then the tension is used 5 times and the force
pulling upward on the chair is 5 times the tension in the rope. Note that
although you only have to exert a modest force on the cord to lift the chair
and person, you must exert that force over a long distance to lift them a short
distance.
Explanation: You obtain mechanical advantage with the multiple pulley—a modest force exerted
on the rope over a large distance exerts an enormous force on the chair over a
small distance.
103.
Lifting Yourself with a Cable-Lift Elevator
Description: You sit in the chair of the previous demonstration and lift yourself upward.
Purpose: To show that you can even lift yourself with a multiple pulley.
Supplies:
The setup for the
previous demonstration
Procedure: Sit in the chair of the previous demonstration and ask whether you will be able
to lift yourself upward. The answer is yes. Point out that you are holding in
your hand an additional segment of the multiple pulley—should that make it
easier or harder to lift yourself upward? Now pull down on the rope in your
hand and you will begin to rise upward.
Explanation: When you sit on the chair and pull downward on the end of the cord of the
multiple pulley, there is one additional segment of cord pulling upward on you
and the chair. It takes even less tension in the cord to lift you and the chair
than it would if someone else were lifting you by pulling on the cord.
104.
The Value of a Counterweight
Description: You operate a toy elevator system with an elevator car on one side and a
counterweight on the other. As the elevator car rises, the counterweight
descends and vice versa.
Purpose: To show how a counterweight can store energy as the elevator car descends and
then provide energy to lift the car upward the next time.
Supplies:
1 toy
cable-lift elevator—consisting of a "car" and a counterweight, both
suspended from the same string that passes over two horizontally separated
spools that turn easily on a support
2 pretend
passengers (small weights)
Procedure: Start with the elevator car low and the counterweight high. Put the two
passengers into the elevator car and begin to raise the elevator car by turning
one of the spools. Point out that while the cord is doing work on the elevator
car by lifting it upward, the counterweight is doing work on the cord by
pulling its other end downward—energy is flowing from the counterweight to the
car. Now begin to lower the elevator car. Point out that while the elevator car
is doing work on the cord by pulling its end downward, the cord is doing work
on the counterweight by pulling it upward.
Explanation: The counterweight provides some of the energy needed to lift the car upward as
the car rises and it stores some of the energy released by the car as the car
descends.
138. A Fan in a Pipe
Description: A small fan located between two sections of pipe causes the pressure to rise in
one pipe and drop in the other.
Purpose: To show that a fan increases the total energy of the air passing through it.
Supplies:
1 small
"boxer" fan (a computer fan)
2 segments
of pipe that fit tightly against the outer edges of the fan
2 pressure
gauges (manometers) for the two pipe segments
Procedure: Attach the two segments of pipe to the two sides of the fan and note that both
pressure gauges read atmospheric pressure. Now start the fan. The upwind
pressure gauge will drop, showing that the air in that portion of pipe has
converted some of its pressure potential energy into kinetic energy so that it
can flow toward the fan. The downwind pressure gauge will rise, showing that
the air in that portion of pipe has an increased total energy—both its kinetic
energy and its pressure potential energy are greater than they were before you
turned on the fan.
Explanation: The fan does work on the air that passes through its blades and increases the
total energy of that air. This increased total energy is reflected in a rise of
both the air's pressure and speed.
139. Chalk Dust in the Air
Description: Two chalk erasers are pounded together, releasing a cloud of dust that hangs in
the air. A piece of chalk is released and drops quickly to the table.
Purpose: To show that chalk dust isn't supported by buoyant forces—it's supported by
viscous drag forces.
Supplies:
2 chalky
erasers
1 piece of
chalk
Procedure: Smack the two erasers together and observe the cloud of chalk dust that hangs
in the air. Now drop a piece of chalk and observe that it falls quickly. Note
that the chalk is far more dense than the air it displaces, so that neither the
piece of chalk nor the chalk dust is support by buoyant forces. Note instead
that the chalk dust is supported by viscous drag forces—as the chalk begins to
descend through the air, the air molecules exert an upward viscous drag force
on it and support it against the force of gravity.
Explanation: Chalk dust has so much surface area relative to its volume that its motion is
dominated by air resistance. Because of the dust's small size, the air flow
around it is generally laminar and the only drag force it experiences is
viscous drag—the molecular friction that occurs when the dust moves relative to
the air. In effect, the dust pulls the surrounding air with it because of
viscous interactions in the air. This pulling of the air slows the dust's
decent and limits its downward speed to only a few millimeters per second (its
terminal velocity).
174. Poor Conductors of Heat -
Glass
Description: You hold a glass rod in your hand and heat its other
end until that end melts.
Purpose: To show that glass is a very poor conductor of heat.
Supplies:
1 glass
rod (about 20 cm is fine)
1 gas
burner
1 piece
of paper (to burn)
matches
water
(to put out the burning paper, if necessary)
Procedure: Light the burner. Hold one end of the glass rod in your
hand and place the other end of the rod in the burner flame. After a few
seconds, the rod will begin to melt. Note that the end you are holding is still
cool. Note also that you can barely see the red glow emitted by the hot
glass—it's emissivity is very low in the visible (it's transparent and doesn't
couple well to visible light). Now remove the glass rod from the flame and
touch it to the paper (be sure that nothing else flammable is nearby). The paper
will burst into flames. Carefully extinguish the paper.
Explanation: Glass is a poor conductor of heat primarily because it
has no mobile electrons—it's an electric insulator. Heat flows so slowly
through glass that you can heat one end of the rod red hot while the other end
of the rod remains cool.
175. Which Feels Hotter, Glass or Metal?
Description: Your students touch two chilled plates with equal
temperatures. One is glass and the other is metal. The metal plate feels much
colder than the glass plate.
Purpose: To show that thermal conductivity plays a role in our
perception of temperature.
Supplies:
1 glass
plate (or a plastic plate)
1 metal
plate (copper would be best, but aluminum will do, too)
ice
Procedure: Chill both plates in the ice so that they have equal
temperatures. (Or you can even use room temperature plates, assuming that the
room isn't too warm.) Remove the plates from the ice, dry them completely, and
have the students touch them. The metal plate will feel much colder than the
glass plate.
Explanation: You perceive an object as cold because it extracts
heat from your skin. The surface of the glass plate quickly warms up as heat
flow into it from your skin, so the rate at which heat flows out of you soon
decreases. As a result, the glass doesn't feel very cold. The metal plate
conducts heat well, so that you will be unable to heat its surface
significantly. Heat will continue to flow rapidly out of your skin to the
plate, so the plate will feel very cold.
Follow-up: Why is it more hazardous to touch your tongue to a metal
surface in freezing cold weather than it is to touch it to a glass surface?
176. Glass Wool - Insulating with Air
Description: You place a coin on a thick layer of glass wool that
you're holding in your hand and heat that coin red hot with a blowtorch. Your
hand remains cool.
Purpose: To show that air trapped in a fibrous material is an
excellent thermal insulator.
Supplies:
1 thick
pad of glass wool (I use the glass fiber wrapping material that's used with
large hot or cold water pipes)
1 coin
(a solid copper penny works well; but don't use a recent penny—after about
1982—because it will be made of zinc and will melt)
1 hand-held
propane torch
matches
water
(in case you have to cool anything quickly)
Procedure: Light the propane torch. Hold the pad of glass wool in
one hand and place the coin on top of it. Make sure that the pad completely
covers your hand and be prepared to get your hand out of the way instantly if
you begin to feel heat. Now carefully heat the coin with the torch. If you feel
heat on your hand, something is wrong and you should stop the experiment immediately.
The coin should soon begin glowing red hot but you should feel essentially no
heat from the torch. (The glass fibers themselves will begin to melt somewhat.
That's normal and won't cause trouble unless your pad isn't thick enough. Once
the pad has thinned noticeably, it's time to stop the experiment and discard
the pad. However, let the pad and the coin cool completely before touching them
or discarding them.)
Explanation: Because the glass fibers trap the air and prevent it
from undergoing convection, the only way that heat can flow from the coin to
your hand is via conduction through the air and glass. Since neither of these
materials is a good conductor of heat, that heat flow is very slow. Even though
the coin is red hot, your hand remains cool.
177. Countercurrent Exchange in Your Arm
Description: You immerse your hand in a bucket of ice. Even though
your hand becomes quite cold, your body remains warm.
Purpose: To show how heat exchange processes in your arm allow your
hand to become much colder than your body without wasting very much thermal
energy.
Supplies:
1 bucket
of ice
2 skin
thermometers (thin plastic strips with liquid crystals inside and numbers on
their surfaces that indicate the current temperature of your skin)
Procedure: Place one of the skin thermometers on your hand and the
other on your upper arm. Note the temperatures at these two locations. Then
immerse your hand in the ice. Despite the continuing flow of blood to and from
your hand, only your hand becomes cold. The temperature of your hand drops
substantially while the temperature of your upper arm remains unchanged.
Explanation: As blood flows toward your hand, it gives up heat to
the blood returning from your hand in a process called countercurrent exchange.
The temperature of the blood decreases on its way to your hand and increases on
its way back.
178. Blocking Thermal Radiation
Description: A thermopile measures the heat emitted by a warm
surface. You put various materials over that warm surface and the amount of
thermal radiation detected by the thermopile decreases.
Purpose: To show how various barrier layers reduce radiative heat
transfer.
Supplies:
1 warm,
black surface
1 thermopile
1 plate
of glass
1 sheet
of aluminum foil
1 sheet
of cloth
Procedure: Point the thermopile at the warm surface and note how
much thermal radiation that surface is emitting. Now insert the glass, aluminum
foil, and cloth in between the surface and thermopile, one at a time. In each
case, the amount of thermal radiation will decrease.
Explanation: Glass is a good absorber and emitter of
room-temperature thermal radiation (infrared light). Since the glass is cooler
than the warm surface, the glass absorbs more of the warm surface's thermal
radiation than the glass emits itself. The thermopile sees the glass's weaker
thermal emission rather than the warm surface's stronger thermal emission.
Aluminum foil is a good reflector of thermal radiation. It blocks the thermal
radiation from the warm surface and allows the thermopile to see a reflection
of its own weak thermal radiation. The cloth absorbs hot surface's thermal
radiation and emits thermal radiation of its own. Since the cloth exchanges
heat readily with the air around it and is at room temperature, it emits less
thermal radiation than the warm surface.
179. A Thermos Bottle or Dewar
Description: A Thermos bottle or dewar holds a very hot or very
cold liquid and keeps it hot or cold for a long time.
Purpose: To show that it's possible to stop virtually all heat
transfer in some cases.
Supplies:
1 Thermos
bottle or dewar flask
liquid
nitrogen or another hot or cold liquid
Procedure: Pour the liquid nitrogen into the Thermos bottle or
dewar flask. After a few seconds of violent boiling, the liquid will settle and
boil relatively gently. The liquid will take a very long time to boil away completely.
Explanation: The Thermos bottle or dewar has a double wall
structure, with a vacuum in between. The double wall makes it difficult for
conduction to transport heat to or from the contents of the bottle. The walls
are made of a material with a poor thermal conductivity (either glass or
stainless steel). The space between the two walls contains a vacuum, so that
convection can't carry heat between the walls. And in many Thermos bottles or
dewar flasks, the inner surfaces of the double walls are mirrored to prevent
radiation from carrying heat between the walls. With almost no way for heat to
flow to or from the liquid in the container, it remains hot or cold for a very
long time.
180. An Illustration of Thermal
Expansion - Cans and Rubber Bands
Description: Several beverage cans are connected with rubber bands
to form a lattice. When they're "heated" to higher temperatures and
vibrate relative to one another, their average spacings increase—the lattice expands.
Purpose: To show why heating most substances causes them to occupy
more volume.
Supplies:
2 or
more beverage cans
rubber
bands
Procedure: Place the cans upright on a table and arrange them to
form a lattice. Now attach adjacent cans to one another with rubber bands—loop
each rubber band around two adjacent cans. When you hold up this lattice and
don't push on it, the lattice is analogous to a solid at absolute zero. But if
you begin to stretch and release the rubber bands, so that the cans begin to
vibrate against one another, the lattice is analogous to a solid at a finite
temperature. The more vigorous the vibrations, the hotter the solid. If you now
observe the average spacings between the cans (the atoms), you'll see that they
become larger as the system's temperature increases.
Explanation: At absolute zero, the atoms in a classical solid are
all in equilibrium and don't move. This equilibrium is stable, so that if you
displace one of the atoms, it will experience a restoring force. However, this
restoring force isn't symmetric—the repulsive forces between atoms that are too
close are stiffer than the attractive forces between atoms that are too
distant. As a result, atoms that are vibrating when the temperature is above
absolute zero spend more time too far apart than they spend too close together.
As a result, the lattice expands. The same situation holds for the can/rubber
band analogy: the repulsive forces between squeezed cans are stiffer than the
attractive forces of stretched rubber bands. Thus as the cans vibrate back and
forth more vigorously, their lattice expands, too.
181. Expansion of Metals - a
Ball and a Ring
Description: A metal ball is too large to fit through a metal ring
when they are both at the same temperature. But heating the ring and/or cooling
the ball makes it possible for the ball to pass easily through the ring.
Purpose: To show that metals expand when heated.
Supplies:
1 ball
and ring set (from a scientific supply company)
1 gas
burner
1 container
of liquid nitrogen
matches
Procedure: First show that the ball can't fit through the ring—it's
too large for the ring. Now chill the ball by dipping it in liquid nitrogen.
The ball will contract and it will then be able to fit through the ring. Allow
the ball to warm to room temperature and repeat the experiment. However, this
time light the burner and heat the ring until it's almost red hot. While it
might seem as though heating the ring should make it expand and shrink the hole
inside it, the entire ring will expand outward and the hole will become larger.
The ball will now fit through the ring.
Explanation: Raising the temperature of most metals moves their
atoms farther apart on average and causes them to increase in size in all
directions. Hollow spots, such as the hole in the ring, expand in size as the
overall metal expands. While having the innermost layer of atoms in the ring
expand inward would move those atoms farther from the atoms in the next to
innermost layer, it would also move the atoms of the innermost layer closer to
one another. In thermal expansion, every atom moves farther from its neighbors,
and that can only occur when all the atoms expand outward from the center of
the object. This rule applies even to the innermost layer of atoms in the
ring—they move outward from the center of the ring.
Follow-up: Drill a hole in an aluminum plate. The diameter of that
hole should be just too small for an aluminum rod to enter it. Now chill the
aluminum rod in liquid nitrogen and insert the contracted rod in the hole.
Allow the rod to warm up. It will be impossible to remove the rod from the
hole.
182. Expansion of an Metal Tube
with Temperature
Description: When steam flows through an aluminum tube, the tube's
temperature increases and so does its length.
Purpose: To show that metals expand when heated.
Supplies:
1 aluminum
tube (roughly 5 mm in diameter and 30 cm long)
1 hose
1 steam
boiler
1 stand
for the steam boiler
1 gas
burner
matches
1 flat
weight
1 paper
pointer with a pin through it
tape
Procedure: Use the hose to attach the aluminum tube to the boiler.
Lay the aluminum tube along the edge of the table or a heat-resistant surface
and tape the hose end of the tube firmly to the table. Insert the pin of the
paper pointer under the open end of the aluminum tube and lay the weight over
the tube to press it against the pin. As the open end of the tube moves toward
or away from the hose end, the pin will rotate and the pointer will change its
direction of pointing. Place the steam boiler on the stand, light the burner,
and allow the boiler to make steam. As the steam flows through the aluminum
tube, the tube's length will increase and the pointer will turn.
Explanation: The steam will heat the aluminum tube and cause it to
expand a small fraction of its length. The pointer makes that small expansion
more visible.
Alternative Procedure: Make the simple plastic ruler thermometer of
described in the opening of Chapter 6.
183. Glass/Liquid Thermometer
Description: When a common glass/liquid thermometer is heated or
cooled, the level of liquid inside it rises or falls, even though both the
glass and the liquid are experiencing the same changes in temperature.
Purpose: To show that liquids normally expand or contract more with
temperature changes than solids do.
Supplies:
1 glass/liquid
thermometer
1 container
of hot water
1 container
of ice water
Procedure: Observe the reading of the thermometer at room
temperature. Immerse the thermometer in hot water and watch as the level of
liquid inside it rises. Point out that both the glass and the liquid are expanding
with temperature, but that the liquid is expanding more rapidly than the glass.
Now immerse the thermometer in ice water and water as the level of liquid
inside it falls. Again, the liquid is contracting more rapidly than the glass.
Explanation: When the temperature of a solid changes, its atoms
vibrate more vigorously and their average spacings increase. However, they
don't normally rearrange much. When the temperature of a liquid changes, its
atoms not only vibrate more vigorously, so that their average spacings
increases, but they also tend to rearrange and adopt less tightly packed
formations. That's why liquids expand more rapidly with temperature than solids
do. In the case of the glass/liquid thermometer, the rapidly expanding liquid
is force to flow up the thin tube inside the glass because the liquid expands
more rapidly than the hollow volume inside the glass does.
184. A Rubber Band Thermometer
Description: A weight hangs from the end of a rubber band. When you
heat the rubber band, its length decreases and the weight rises.
Purpose: To show that not all materials simply expand with
temperature.
Supplies:
1 rubber
band
1 weight
1 support
for rubber band
1 heat
gun or hairdryer
Procedure: Attach the rubber band to the support and hang the
weight from the rubber band. The rubber band should be stretched almost to its
elastic limit. Note how far the rubber band has stretched. Now heat the rubber
band without touching it. The rubber band will shrink and pull the weight
upward.
Explanation: The rubber band contracts upon heating because the
long organic molecules inside it develop more and more kinks as their
temperatures rise. In a cold, unstretched rubber band, these molecules are
wound up into random coils. Stretching the rubber band unwinds those coils.
However, the random coils are the more thermodynamically favorable arrangement
so the molecules recoil and shorten when you either relieve the tension on the
rubber band or heat the rubber band up.
185. A Bimetallic Strip
Thermometer
Description: A thin metal strip, made of a sandwich of two metals,
bends whenever its temperature changes.
Purpose: To show that different metals expand differently with
temperature and that this can be used to make objects that bend with
temperature.
Supplies:
1 bimetallic
strip (typically iron and brass)
1 gas
burner
matches
1 container
of ice water
Procedure: Examine the bimetallic strip, pointing out that it's
made of two different metals that have been bonded together. Note that at room
temperature the strip is flat. Now light the burner and heat the strip gently.
It will curl in one direction as the outer metal (brass) surface expands more
rapidly than the inner metal surface (iron). Now immerse the strip in ice water
and watch as it curls the other way. The surface of the strip that expanded
more rapidly when heated (brass) also shrinks more rapidly when cooled and
becomes the inner surface of the curling strip.
Explanation: The characteristics of the forces between atoms varies
from metal to metal, so that different metals have different coefficients of
volume expansion. The strip is made of two different metals with different
coefficients of volume expansion. As a result, it only remains flat at one
temperature.
186. An Electric Light Blinker
Description: A bimetallic strip is used as an electric switch,
opening a circuit when it becomes hot. When the circuit is used to power a
heater near the bimetallic strip, the strip repeated opens and closes the
circuit. A light bulb attached to the circuit also blinks on and off.
Purpose: To illustrate how both a light blinker and a thermostat
work.
Supplies:
1 bimetallic
strip
1 switch
mount for the bimetallic strip (see below)
1 powerful
battery
1 nichrome
wire heater
1 small
light bulb
wires
Procedure: Mount the bimetallic strip so that it barely touches a
metal contact while the strip is at room temperature and bends away from that
contact when it's somewhat hotter than room temperature. Connect one terminal
of the battery to the bimetallic strip. Connect the metal contact to one
terminal of the nichrome wire heater and also to one terminal of the light
bulb. Place the nichrome wire heater very close to the bimetallic strip (or
touching it, if the nichrome is insulated). Now connect the other terminals of
the nichrome wire heater and the light bulb to the other terminal of the
battery. Current will begin to flow through the bimetallic strip and metal
contact. It will continue through both the heater and the light bulb before
returning to the battery. The light bulb will light and the heater will heat.
When the bimetallic strip's temperature exceeds some value, it will bend away
from the metal contact and current will stop flowing. The light will go out and
the heater will stop heating. After a few seconds, the strip will have cooled
enough to straighten out and will again touch the metal contact. The light will
turn back on and so will the heater. The system will switch on and off
indefinitely.
Explanation: This blinker arrangement oscillates indefinitely because
whenever the heater is on it soon turns itself off and whenever the heater is
off it soon turns itself on.
187. Liquid Crystal Thermometers
Description: Numbers on a flat plastic strip change colors as the
strip's temperature changes. A larger sheet of plastic changes colors as you
rub your hand across it.
Purpose: To show how temperature can affect the ordering of liquid
crystals.
Supplies:
1 liquid
crystal room or aquarium thermometer (a flat plastic strip thermometer that
measures temperatures near room temperature)
1 sheet
of liquid crystal film that changes colors in the temperature range only
slightly above room temperature
Procedure: Show that the liquid crystal thermometer is reading room
temperature—that only one or two of its numbers are brightly colored. Then warm
the thermometer with your hand and show that the different numbers appear and
disappear as the strip's temperature rises. Each number appears when the liquid
crystal it contains achieves the proper ordering characteristics. Now examine
the liquid crystal film. At room temperature, it should be mostly colorless.
But when you begin to heat it with your hands, it will become brightly colored.
As its temperature increases, the liquid's order becomes such that it reflects
visible light.
Explanation: These temperature sensitive films contain chiral
nematic liquid crystals that naturally form spiral structures within the film.
The pitch of these spirals is temperature dependent. When the temperature of a
particular liquid crystal mixture is such that its spiral pitch is equal to the
wavelength of visible light, that liquid crystal will reflect some of the
visible light. The liquid crystals in the various numbers of the thermometer
are slightly different and achieve the right pitch at different temperatures.
In the large film, only the portions of the film that are with the right
temperature range reflect visible light.
188. Color-Changing Toys
Description: Toys ranging (from shirts to pens) made from special
plastics change colors when heated by body heat, friction, or contact with hot
water.
Purpose: To show another type of crude thermometer.
Supplies:
1 color
changing toy (BIC makes a set of color changing pens call
"wavelengths")
Procedure: First examine the toy or pen while it's at room
temperature. Then warm the toy or pen by holding it or rubbing its surface
(sliding friction). The toy or pen's color will become much lighter.
Explanation: The plastic contains tiny bubbles. Each of these bubbles
contains a mixture of chemicals, one of which melts at a temperature slightly
above room temperature. At room temperature, that chemical is solid and the
remaining liquid chemicals are brightly colored. But when the plastic is heated
and the solid chemical melts, it interferes chemically with the coloring
molecules of the liquid. The liquid loses its color. Upon cooling, the melted
chemical solidifies again and the liquid's color returns. The plastic often
contains a second, temperature-insensitive dye that becomes visible when the
other color vanishes at elevated temperatures.
189. Thermocouples as
Temperature Sensors
Description: You measure the voltage developed between the two
wires of a thermocouple when that thermocouple is immersed in liquid nitrogen
or heated over a burner.
Purpose: To show that mobile electrons not only make metals good
conductors of heat, they also gives metals interesting electric properties when
the metals are exposed to temperature gradients.
Supplies:
1 thermocouple
(two different wires of, for example, iron and constantin, that have been
twisted or welded together at one end)
1 sensitive
voltmeter (or thermocouple readout)
1 gas
burner
matches
1 container
of liquid nitrogen
Procedure: Form the thermocouple by removing about 1 cm of
insulation from each end of the two different wires and twisting the pair of
wires together at one of their ends. Attach the voltmeter to the free ends of
the two wires. With everything at room temperature, the voltmeter will read
zero volts. But when you heat or cool the twisted end of the thermocouple, the
voltmeter will read a small non-zero voltage (in the tens of millivolts range).
Explanation: When you heat or cool the twisted end of the
thermocouple, there is a temperature gradient along each wire. Because the
mobile electrons in each wire are moving fastest at the hotter end, they tend
to migrate to the colder end and make the colder end of each wire negatively
charged (the Seebeck Effect). However, the extent of this negative charging
depends on the wire. By comparing the charging of two different wires, you can
determine the temperature difference across the wires.
190. Thermistors as Temperature
Sensors
Description: The electric resistance of a thermistor decreases as
its temperature increases.
Purpose: To show that semiconductors become better conductors as
their temperatures rise.
Supplies:
1 thermistor
1 ohm
meter
hot
water
ice
water
Procedure: Connect the two wires of the thermistor to the ohm meter
and determine its resistance at room temperature. Now immerse the thermistor in
hot water (or simply pinch it in your fingers) and observe its decrease in
resistance. Finally, immerse the thermistor in cold water and observe its
increase in resistance.
Explanation: The thermistor is a semiconductor that would normally
not conduct current if it weren't for thermal energy. At absolute zero, the
semiconductor would have all of its valence levels filled and all of its
conduction levels empty and it would be unable to transport electric charge.
Thermal energy transfers electrons from filled valence levels to unfilled
conduction levels and makes it possible for the semiconductor to transport
charge—to carry current. The more thermal energy (i.e. the higher the temperature),
the more easily the semiconductor carries charge.
191. Copper wire as a
Temperature Sensor
Description: The electric resistance of a coil of copper wire
decreases as its temperature decreases.
Purpose: To show that metals become better conductors as their
temperatures fall.
Supplies:
1 coil
of thin copper wire
1 light
bulb (one that requires a fair amount of current)
1 battery
1 container
of liquid nitrogen
wires
Procedure: Form a complete circuit by connecting one terminal of
the battery to one end of the coil of copper wire, the other end of the coil of
copper wire to one end of the bulb, and the other end of the bulb to the other
terminal of the battery. The bulb should glow dimly because the current should
be losing most of its energy in the copper wire. Now immerse the coil of copper
wire into liquid nitrogen. As its temperature drops, the copper will become a
better electric conductor and the light bulb will become much brighter.
Explanation: As current flows through room temperature copper wire,
the individual charges collide with the vibrating copper atoms and transfer
some of their energies to the copper atoms. When the copper is chilled to low
temperature, the copper atoms vibrate less and their increased order makes them
less likely to be hit by moving charge. Copper's electric resistance drops as
its temperature drops and it becomes a better conductor.
227. Removing Dust from the Air
Description: You clap chalky erasers together and note how slowly
gravity removes the chalk particles from the air.
Purpose: To show that gravity is slow and ineffective at removing
tiny particles from the air.
Supplies:
2 chalky
erasers or another source of visible, nontoxic dust
1 piece
of chalk (or a bulk form of whatever dust you choose)
Procedure: Clap the erasers together and show that it hangs in the
air for a long time. Drop the piece of chalk to show that it falls
rapidly—buoyancy isn't supporting the chalk dust, air resistance (viscous drag)
is. Discuss the concept of terminal velocity; of the falling particles
experiencing upward forces that balance their weights when they reach very
small downward velocities relative to the air. Point out that to be drawn
through the air at larger terminal velocities, the particles need to be exposed
to stronger forces than gravity—for example, to Coulomb forces!
Explanation: Small particles have such large surface-to-volume
ratios that their interactions with air dominate their dynamics. To pull them
through the air at more than a snail's pace, they need to be exposed to forces
stronger than those of gravity.
228. Electric Charge and Coulomb Forces
Description: Two pith balls hang from threads. One of them is given
negative charge by a negatively charged Teflon rod and the two objects repel
one another. The other pith ball is given positive charge by a positively
charged acrylic rod and the two objects also repel one another. Finally, the
two pith balls are carefully brought toward one another. They suddenly draw
together and touch, showing that they attract one another.
Purpose: To demonstrate the strong repulsive and attractive forces
between electric charges and to show that there are two types of electric
charges: positive and negative.
Supplies:
2 silvered
pith balls hanging from threads and supports (we sometimes use carbon-coated
latex rubber balloons, which work very nicely but age badly and must be made
fresh for each use. The carbon-coating is done with Aerodag colloidal carbon
spray and makes the balloons electrically conducting.)
1 Teflon
rod
1 Acrylic
rod
1 piece
of silk
Procedure: Set the two pith balls so that they hang about
40 cm apart. Rub the Teflon rod with the silk, a process that will
transfer negative charge to the Teflon and leave the silk positively charged.
Touch the Teflon rod to one of the pith balls. The pith ball will immediately
repel the Teflon rod. Demonstrate this repulsion.
Now rub the acrylic rod with the
silk, a process that will transfer negative charge to the silk and leave the
acrylic positively charged. Touch the acrylic rod to the other pith ball. The
pith ball will immediately repel the acrylic rod, although you may have to
recharge the acrylic rod with the silk and repeat the charge transfer once or
twice (acrylic doesn't work as well as Teflon). Demonstrate this repulsion,
too.
Finally, shift the supports for
the pith balls slowly toward one another so that the balls move closer and
closer. When they are near enough, they will pull together and
"kiss." Once they have touched, they will drop limply because they
have little net charge left. Point out that this attraction between the pith
balls is evidence that the two pith balls were oppositely charged and that
there are two different charges present in our universe. Identify them as
positive and negative and discuss how sliding friction tends to move them
between objects (which is how you charged the rods with the silk.) Note that
like charges repel but opposite charges attract.
Explanation: Sliding friction rubbed negatively charged electrons
off the silk and onto the Teflon. It also rubbed negatively charged electrons
off the acrylic rod and onto the silk, leaving the acrylic rod with a net
positive charge.
229. Detecting Charge with an Electroscope
Description: You transfer charge from a Teflon rod to the foils of
an electroscope and they repel outward to indicate the presence of charge.
Purpose: To show how a simple apparatus can detect the presence of
electric charge.
Supplies:
1 electroscope
1 Teflon
rod
1 silk
cloth
Procedure: Rub the Teflon rod with the silk to give the rod a net
negative charge. Touch the Teflon rod to the top of the electroscope so that
negative charge flows onto the foils. They will repel one another and swing
outward. Point out that the electroscope uses this repulsion between like
charges to indicate the presence of charge on the foils.
Explanation: When you touch the Teflon rod to the electroscope,
negative charges flow onto the foils. Since like charges repel one another, the
two foils are swung outward by the repulsions between their charges.
230. Electric Conductors and Electric Insulators
Description: A metal rod connected to the foils of an electroscope
conduct charge to the foils when you touch the rod with a charged Teflon rod. A
plastic rod connected to the foils doesn't conduct charge to the foils when you
touch it with the charged Teflon rod.
Purpose: To show that some materials can transport electric charge
and are electric conductors, while other materials can't transport electric
charge and are electric insulators.
Supplies:
1 electroscope
1 metal
rod that can attach to the electroscope
1 plastic
rod that can attach to the electroscope
1 Teflon
rod
1 piece
of silk
Procedure: Start with the electroscope uncharged and with the metal
rod attached to its foils. Charge the Teflon rod by rubbing it with the silk.
Now touch the Teflon rod to the metal rod so that the foils swing outward.
Point out that the metal rod has transported the charge to the foils and is
thus an electric conductor.
Now remove the metal rod and
replace it with the plastic rod. Again start with the electroscope uncharged.
Touch the charged Teflon rod to the plastic rod and show that the foils don't
swing outward. Point out that the plastic rod hasn't transported the charge to
the foils and is thus an electric insulator.
Explanation: The metal rod has mobile electrons (conduction level
electrons or perhaps empty levels in its valence bands) that allow it to
transport electric charges from one end to the other. The plastic rod has no
such mobile electrons (its valence levels are completely filled and it has no
conduction level electrons) and can't transport electric charges from one end
to the other.
231. Faraday's Ice Bucket
Description: You transfer electric charge to an isolated metal cup
and then use an electrometer to look for that charge. You find that it's on the
outside of the cup, not on the inside.
Purpose: To show that charge distributes itself relatively uniform
around the outsides of conducting objects.
Supplies:
1 metal
cup on an insulating stand (a cylindrical metal can with a bottom but no top)
1 metal
ball on an insulating stick (for charge transfers)
1 electroscope
1 Teflon
rod
1 piece
of silk
Procedure: Rub the Teflon rod with the silk to give the rod a
negative charge. Transfer this charge to the metal cup (Faraday's ice bucket)
by rubbing the rod lightly against the cup. Now locate the charge on the cup.
First look for the charge inside the cup by carefully inserting the transfer ball
into the cup (don't touch the lip of the cup) and by touching the inside
surface of the cup. Remove the ball from the cup and touch it to the
electroscope. There will be no deflection of the foils, indicating no charge on
the ball and no charge on the inside surface of the cup.
Now touch the ball to the outside
surface of the cup. Again touch the ball to the electroscope. The foils will
bend outward, indicating charge on the ball and charge on the outside surface
of the cup.
Explanation: Like charge becomes more widely separated by spreading
itself on the outside surfaces of a conducting object. No charge is found on
the inside surfaces of a conducting object.
232. A Van Der Graaf Generator
Description: A van der Graaf generator operates like an automated
version of Faraday's ice bucket. A belt delivers charge into a conducting ball
and this charge runs quickly to the outside surfaces of the ball.
Purpose: To show how a large quantity of like charge is accumulated
on the surface of a van der Graaf generator.
Supplies:
1 van
der Graaf static generator
Procedure: First examine the components of the van der Graaf
generator. It has a conducting metal sphere on top that will store like charge
on its surface. It has an insulating rubber belt that will deliver charge to
the inside of the conducting metal sphere. It has a charging system at the base
of the belt that deposits charge on the belt. And finally it has a motor that
turns the belt and pushes the charged belt toward the like-charged metal
sphere.
Now turn on the van der Graaf
generator and allow it to begin producing sparks. Point out that the motor is
doing work on the charges in order to push them onto the sphere (the charges
already on the sphere are repelling the newly arriving charges).
Explanation: Whenever the belt carries a charge into the sphere and
allows that charge to transfer to the sphere, the charge quickly moves onto the
outer surface of the sphere. Once on the outer surface of the sphere, the
charge can only leave through a spark or on a passing air molecule. As more and
more charges accumulate on the sphere, their potential energies increase and
thus the voltage of the charges increase (voltage is energy per charge).
(However, our van der Graaf generator accumulates negative charge, so it
reaches a very large negative voltage.)
233. Launching a Styrofoam Cup
Description: A Styrofoam Cup placed upside down on a van der Graaf
generator lifts itself into the air.
Purpose: To show the tendency for electric charges to transfer from
the surface of the van der Graaf generator onto nearby objects and to show that
like charges repel.
Supplies:
1 van
der Graaf static generator
1 Styrofoam
cup
1 grounding
ball, stick, and wire
Procedure: Turn on the van der Graaf generator and ground the
sphere of the van der Graaf generator (we use a metal ball on a long insulating
stick, with a wire that connects the ball to earth ground) to make it safe (or
less painful) to touch. Put an inverted Styrofoam cup on top of the ball and
remove the grounding ball. As charge accumulates on the van der Graaf
generator's sphere, some of it will transfer to the nearby cup. Soon the sphere
and cup will repel one another strongly enough for the cup to lift up into the
air.
Explanation: An electric charge on the surface of the van der Graaf
generator can lower its total energy by moving to the Styrofoam cup. It does so
with the help of passing air molecules, which serve as ferries for the charges.
Once the cup and the sphere are each sufficiently charged, the upward Coulomb
force on the cup exceeds its weight and the cup accelerates upward.
234. Making the Strands of a Pom-Pom Stand Up
Description: A plastic Pom-Pom is attached to the sphere of a van
der Graaf Generator. As charge accumulates on its strands, they spread outward
until the Pom-Pom resembles a dandelion tuft.
Purpose: To demonstrate the repulsion between like charges.
Supplies:
1 van
der Graaf static generator
1 Pom-Pom
(a ball of thin plastic stripes attached to a stick)
1 suction
cup
1 grounding
ball, stick, and wire
Procedure: Turn on the van der Graaf generator and ground its
sphere to make it safe to touch. Attach the stick of the Pom-Pom to the top of
the van der Graaf generator with the suction cup. Remove the grounding ball and
allow charge to accumulate on the sphere and on the Pom-Pom. The plastic
strands of the Pom-Pom will soon spread outward into a large uniform ball of
straight plastic strips.
Explanation: Air molecules ferry electric charges from the van der
Graaf generator to the plastic surfaces of the Pom-Pom. Once there are enough
charges on those strands, they repel one another strongly and stand up to form
a round ball.
235. Making Peoples' Hair Stand Up
Description: A person stands on a plastic stool and touches the
sphere of a van der Graaf generator. As charge accumulates on the sphere and
their body, their hair begins to stand up.
Purpose: To demonstrate the repulsion between like charges (and to
have fun).
Supplies:
1 van
der Graaf static generator
1 plastic
stool (a one-step stool, about 30 cm tall)
1 grounding
ball, stick, and wire
Procedure: Place the van der Graaf generator at the edge of a table
and put the plastic stool a short distance away on the floor. The volunteer who
will stand on the stool (for electric insulation from the ground) should be
able to reach out and touch the sphere of the van der Graaf generator
comfortably, but without coming too close to anything else, particularly the
base of the van der Graaf generator. Before the volunteer arrives, turn on the
van der Graaf generator and touch the grounding ball to the van der Graaf
generator's sphere to eliminate any charge from its surface. Have the volunteer
stand on the stool (it's not a matter of how tall they are—they need the
electric insulation that the stool provides) and touch the sphere of the van
der Graaf generator. They should feel absolutely no shock while they’re doing
this because you are still grounding the sphere.
When the volunteer is ready and
not near anything besides the sphere and the stool, move the grounding ball
away from the van der Graaf generator's sphere. Never move the grounding ball
back to the van der Graaf generator's sphere while the person is still touching
the sphere because the volunteer will feel a shock. As charge accumulates on
the sphere and the volunteer, that person's hair will begin to stand up. Some
people's hair works better than others and there is simply no predicting whose
hair will work best. It's completely trial and error! The only exception to
that rule is with children—young children with fine, straight, white-blond or
jet black hair always work well.
Explanation: The charge that migrates onto the volunteer's body
through their conducting skin also works its way onto their hairs. When each
hair is sufficiently charged, the Coulomb repulsions between the hairs lift
them upward against their own weights.
236. Sharp Points and Charge - Lightning Rods
Description: When you approach the sphere of a van der Graaf
generator with a smooth grounded object, sparks occur. But when you approach
the sphere with a sharp grounded object, the sphere loses its charge quietly
without any sparks.
Purpose: To show that sharp points are particularly good at
emitting electric charges into the air.
Supplies:
1 van
der Graaf static generator
1 grounding
ball, stick, and wire
1 pin,
needle, or sharpened metal rod
Procedure: Turn on the van der Graaf generator and allow charge to
accumulate on the surface of its sphere. Approach that sphere with the grounded
ball and show that sparks leap from the sphere to the ball. Now attached the
pin to the surface of the grounding ball and repeat the same experiment. No
sparks will occur. Moreover, you can hear the motor of the van der Graaf
turning more easily—the pin is helping charge to move between the sphere and
the ball so that very little charge accumulates on the sphere of the van der
Graaf generator! (I do this experiment with my bare hands. I approach the
charged sphere with my knuckles and it sends sparks at them—unpleasant, but not
particularly painful. I then approach the charged sphere with a sharp pin in my
hand and it doesn't send any sparks at all.)
Explanation: As you approach the sphere with the sharp pin, charges
that are opposite to those on the sphere begin to leap off the pin's point and
onto passing air molecules—a corona discharge. These charges quickly move
toward the sphere and land on it, neutralizing the sphere's charge. Although
the motor and belt try to recharge the sphere, the charge transfer from the pin
is so effective that the sphere loses most of its net charge and can't produce
any sparks.
237. An Electrostatic Bell
Description: A metal ball, hanging from a string between two
oppositely charged plates, begins to move back and forth between those plates.
It's ferrying charge and creating lots of noise.
Purpose: To show that opposite charges attract one another and that
like charges repel.
Supplies:
1 Wimshurst
static generator
2 vertical
metal plates, about 10 cm square, supported on insulators
1 ball
string
1 support
for ball
2 wires
Procedure: Use the string to hang the ball from the support and
place it between the two plates. The two plates should be just far enough apart
to give the ball a little room to move. The ball should just barely touch one
of the two plates. Touch the two contacts of the Wimshurst static generator
together to eliminate any charges they may have and connect the two contacts to
the two plates. Now separate the two contacts and begin cranking the Wimshurst
generator. When enough charge has accumulated on the two plates, the ball will
be repelled by the plate that it's touching and will accelerate toward the
other plate. As soon as it touches the other plate, it will reverse its charge
and accelerate in the opposite direction. It will shuttle back and forth
between the plates as long as you continue to turn the crank of the Wimshurst
generator.
Explanation: The metal ball is repelled by the like charge of the
plate that is has just touched and attracted to the opposite charge of the
other plate. It accelerates back and forth between the two.
238. Putting Out a Candle with Static Electricity
Description: A candle that's placed between two oppositely charged
plates is ripped apart by the Coulomb forces it experiences and extinguishes
itself.
Purpose: To show that a candle flame contains some electrically
charged particles and Coulomb forces acting on those charged particles can make
it impossible for the flame to operate.
Supplies:
1 Wimshurst
static generator
2 vertical
metal plates, about 10 cm square, supported on insulators
1 candle
matches
Procedure: Space the two metal plates about 4 cm apart and put
the candle between the two plates. Touch the two contacts of the Wimshurst
static generator together to eliminate any charges they may have and connect
the two contacts to the two plates. Light the candle. Now separate the two
contacts and begin cranking the Wimshurst generator. When enough charge has
accumulated on the two plates, the candle flame will become severely distorted
and will probably extinguish itself.
Explanation: The charged particles in the flame are pulled toward
opposite charges and the flame becomes a very horizontal, rather than vertical,
structure. In its new shape, the flame has trouble sustaining itself and tends
to put itself out.
239. A Simple Electrostatic Precipitator
Description: Smoke drifts upward through a metal can containing a
thin metal wire. When opposite electric charges are placed on the can and the
wire, the smoke suddenly disappears.
Purpose: To demonstrate the principles of electrostatic
precipitation.
Supplies:
1 large
coffee can, open at both ends
1 extremely
thin metal wire
1 insulated
support for the metal wire
1 insulated
support for the coffee can
1 weight
for the metal wire
1 Wimshurst
static generator (or another high voltage power supply)
2 wires
1 smoke
source (for example, unscented incense sticks)
matches
Procedure: Support the coffee can about 50 cm above the table
and lower the metal wire through its center. Support the top of the wire and
hang the weight from the bottom of the wire to pull the wire straight. Touch
the two contacts of the Wimshurst static generator together to make sure that
they have no charges on them and connect one contact to the coffee can and the
other contact to the wire. Be careful not to break the wire. (It does matter
somewhat which charge you put on the wire and which charge you put on the can,
but you'll have to experiment to see which works best.)
Now light the smoke source and
allow its smoke to drift upward through the coffee can. To demonstrate the
electrostatic precipitator, separate the two contacts of the Wimshurst machine
and turn its crank. As charge begins to accumulate on the can and wire, the
smoke will abruptly disappear as it travels through the can.
Explanation: A corona discharge occurs around the electrically
charged wire and this discharge transfers charge onto passing air molecules and
smoke particles. These ionized particles are then repelled by the wire and are
attracted to the inside surfaces of the coffee can. The missing smoke is
actually coating the inside of the coffee can as a thin film of particles.
240. Deflecting a Stream of Water with a Charged Comb
Description: A thin stream of water is deflected by a nearby comb.
Purpose: To show that a charged object can electrically polarize
another object and the two will attract.
Supplies:
1 hose
1 support
for the hose
1 rubber
or plastic comb (or a Teflon rod)
flowing
water
Procedure: Connect the hose to a water faucet and support its end
over the drain. Adjust the water flow so that a thin but continuous stream of
water flows from the hose. Now charge a comb either by drawing it through your
hair several times or by rubbing it with a piece of silk. Hold the comb near the
upper part of the water stream and watch as the water stream bends toward the
comb.
Explanation: The comb's electric charge attracts opposite charges
onto the water stream and repels like charges out of the water stream. Since
the stream is now polarized, with charges that are opposite to those on the
comb closer then charges that are like those on the comb, the stream is
attracted to the comb and bends toward the comb.
241. Sticking a Balloon to the Wall with Charge
Description: You rub a balloon through your hair and then stick it
to the wall. Its electric charge holds it in place.
Purpose: To show that a charged particle is naturally attracted to
any uncharged surface because it will polarize that surface and obtain an
attractive force.
Supplies:
1 balloon
(a long, thin one oriented vertically works well because it can't roll down the
wall)
1 wall
Procedure: Charge the balloon by rubbing it through your hair (or
rubbing it with a silk cloth). Hold it against the wall and observe that it
sticks.
Explanation: The electrically charged balloon pulls opposite
charges in wall toward it and repels like charges in the wall away from it.
This polarization of the wall makes it possible for the balloon to stick to the
wall through Coulomb forces.
244. The Forces Between Magnets
Description: A bar magnet on a horizontal pivot always turns so
that its north pole faces the south pole of a magnet you're holding in your
hand, or vice versa.
Purpose: To show that magnets have two different poles and that
like poles repel while opposite poles attract.
Supplies:
2 bar
magnets
1 horizontal
swivel mount for one of the bar magnets
Procedure: Suspend one of the bar magnets on the horizontal mount.
Hold the second magnet in your hand and show that its poles repel like poles of
the horizontally supported magnet and that its poles attract opposite poles of
that magnet.
Explanation: As with electric charges, magnetic poles come in two
types: north and south. But unlike electric charges, its impossible to find an
isolated north pole or an isolated south pole. Each bar magnet has a north and
a south pole. Like poles on two bar magnets experience repulsive forces and
opposite poles on two bar magnets experience attractive forces.
245. Visualizing a Magnetic
Field
Description: A small bar magnet is inserted into a magnetic field
visualizer and the magnetic flux lines become visible.
Purpose: To show how the magnetic field extends from a magnet's
north pole outward and around to the magnet's south pole.
Supplies:
1 magnetic
field visualizer (a clear plastic rectangle, filled with iron powder and oil,
with a hollow region into which you can put a small bar magnet)
1 bar
magnet
Procedure: Shake the visualizer to disperse the iron powder evenly.
Insert the bar magnet into the visualizer and watch as the iron powder
accumulates along the magnetic flux lines. Point out that these lines indicate
the direction of the force that an isolated north pole would experience if it
were at one of those locations. (The fact that isolated north poles aren't
available doesn't alter the meaning of the magnetic field lines.)
Explanation: The iron powder particles are magnetized by the
magnetic field and line up along the flux lines because they respond to the
magnetic forces associated with those flux lines.
246. Magnetic Levitation - First
Attempt
Description: You place one magnet over another so that the upper
magnet is supported by repulsive forces from the lower magnet. However, you
must put a stick through the two magnets to keep the upper magnet from falling
off the lower magnet's magnetic cushion.
Purpose: To show that, while you can suspend one disk or ring
magnet over another by magnetic repulsion, the equilibrium created by that
levitation technique is unstable.
Supplies:
2 ring-shaped
magnets
1 wooden
dowel
Procedure: Show that when the two ring-shaped magnets are stacked
so that they have like poles facing one another, they repel strongly enough to
support the upper magnet. Show also that you can't balance the upper magnet
above the lower magnet. Show that only when you put the dowel through the holes
in the two rings can you can get a stable arrangement.
Explanation: They same repulsive force that supports the weight of
the upper magnet also tends to push it to the side so that it falls off the
magnetic cushion provided by the lower magnet. It’s in an unstable equilibrium.
247. Magnetic Levitation -
Second Attempt
Description: You place one bar magnet above another so that their
like poles are on top of one another. While the magnetic repulsion supports the
upper magnet, it tends to fall of the magnetic cushion. Only when you box in
the upper magnet so that it can't move horizontally will it float over the
lower magnet.
Purpose: To show that, while you can suspend one bar magnet over
another by magnetic repulsion, that the equilibrium created by this levitation
scheme is unstable.
Supplies:
2 strong
bar magnets
1 frame
that prevents horizontal motion of the bar magnets
Procedure: Show that when the two bar magnets are aligned with
their like poles on top of one another, that the upper magnet can be suspended
by the repulsive forces. Now show that you can't balance the upper bar magnet
over the lower bar magnet—the equilibrium there is unstable. Add the frame and
show that only with its help to prevent horizontal motion can you suspend one
bar magnet over another.
Explanation: The same repulsive forces that support the upper bar
magnet also tend to push it to the side so that it falls off its magnetic
cushion.
248. Magnetic Levitation - An
Almost Free Bearing
Description: A magnetic toy spins above a magnetic base. While it
appears that the magnetic toy is levitating, it’s actually touching the base a
one point. Without that contact, it would be unstable.
Purpose: To show that magnetic suspension with permanent magnets is
inherently unstable.
Supplies:
1 magnetic
bearing toy (available from scientific supply companies)
Procedure: Suspend the magnet bearing toy in its base and give it a
spin. Show that while the bearing remains suspended by repulsive forces above
its magnetic base, it's equilibrium is unstable in one horizontal direction. It
touches the base at one point in order to avoid falling off the base in the
unstable direction.
Explanation: The repulsion between the floating bearing toy and its
base leaves the bearing’s equilibrium stable in the up-down and back-front
directions. However, that equilibrium is unstable in the left-right direction
and the toy needs the contact point to avoid falling off its magnetic cushion.
249. Electronic Feedback -
Newton's Folly
Description: A magnetized metal marble hangs in midair beneath an
electromagnet. When you block the electric eye that senses the marble's height,
it either falls or sticks to the electromagnet.
Purpose: To demonstrate that feedback can be used to make an
unstable system stable.
Supplies:
1 Newton's
Folly (available from Edmunds Scientific)
Procedure: Plug in Newton's Folly and carefully raise the
magnetized marble toward the electromagnet from below (as per the
instructions). Be careful not to block the electric eye. When the ball is in
the correct position, it should become stably suspended—you can let go and
lower your hands. Show that the marble is truly suspended by putting a business
card between it and the electromagnet above it. But show also that it the
device needs to monitor the marble's height continuously in order to avoid
dropping it or attracting it all the way to the electromagnet. You can show
this by blocking the electric eye (the small holes on either side of the
frame). Depending on how you block the electric eye system, the marble will
either fall downward or leap upward toward the electromagnet.
Explanation: The basic system uses attraction between two opposite
poles to suspend the marble. This arrangement is stable in the horizontal
directions but unstable in the vertical direction. Only through the use of feedback
can this system be made stable.
250. AC Magnetic Levitation -
Jumping Rings
Description: A small aluminum ring is placed around a group of iron
rods that pass through a coil of wire connected to the AC power line. When AC
current flows through the wires, the ring is repelled by the coil of wire and
leaps upward.
Purpose: To show that an electromagnet that's powered by
alternating current repels nearby metal.
Supplies:
1 AC
electromagnet with an iron-rod pole piece that extends vertically above the
wire coil
1 solid
aluminum ring that fits around the iron pole pieces
1 cut
aluminum ring (cut so that it isn't a complete ring and can't conduct
electricity in a full circle)
Procedure: Place the aluminum ring around the pole piece and lower
it onto the coil of wire. Now allow AC current to pass through the coil of
wire. An AC current will begin flowing through the ring and the ring will
become magnetic. The ring will experience a strong repulsion from the coil of
wire and will leap up into the air.
Repeat this process with the
cut aluminum ring. Because that ring can't conduct electricity, it won't become
magnetic and won't be repelled by the wire coil.
Explanation: When AC current flows through the coil of wire, the
electromagnet's poles reverse rapidly. The changing magnetic field induces an
AC electric current in the aluminum ring and, in accordance with Lenz's law,
the upward pointing pole of the coil is always the same as the downward
pointing pole of the aluminum ring. The two objects repel.
251. Eddy Current Pendulum
Description: A metal pendulum swings freely through the pole pieces
of an inactive electromagnet. But when the electromagnet is on, the pendulum
slows to a stop as it tries to swing through the pole pieces of the
electromagnet.
Purpose: To show that a conducting object that enters a magnetic
field experiences a repulsive force that slows it down.
Supplies:
1 strong
DC electromagnet
1 copper
or aluminum pendulum with support (don’t use iron, steel, or any other
ferromagnetic metal in the pendulum)
Procedure: With the DC electromagnet off, arrange the pendulum so
that it swings smoothly between the electromagnet's pole pieces. Show that the
inactive electromagnet has no effect on the pendulum. Now turn on the
electromagnet and repeat the demonstration. The pendulum will slow dramatically
as it enters the pole pieces and will probably come to a stop between them.
Explanation: As the pendulum approaches the pole pieces, the
changing magnetic field it experiences induces currents in its surface. It
becomes magnetic and, in accordance with Lenz's law, it repels the poles of the
electromagnet. This repulsion slows its motion. The currents that gave rise to
the magnetization in the pendulum quickly lose energy in the metal and the
pendulum comes to rest between the pole pieces.
Follow-up: Repeat the experiment with another pendulum that can't
conduct electricity (either a plastic pendulum or a metal pendulum with cuts
through it that prevent currents from flowing). This modified pendulum will
swing through the electromagnet even when that electromagnet is on.
252. A Magnet Falling Through A
Copper Pipe
Description: A small magnet falls incredibly slowly through a
copper pipe.
Purpose: To demonstrate the repulsive magnetic fields that appear
when a magnet moves across a conductive surface.
Supplies:
1 small
neodymium-iron-boron magnet
1 metal
cylinder the same size as the magnet
1 narrow
copper pipe
1 support
for the copper pipe
Procedure: Support the copper pipe so that it's vertical. Drop the
metal cylinder through the copper pipe and note how quickly it falls. Now drop
the magnet through the pipe and watch how slowly it descends.
Explanation: As it falls, the magnet induces currents in the copper
pipe and these currents exert repulsive magnetic forces on the magnet. These
repulsive forces slow the magnet’s descent.
253. A Magnet Sliding Through a
Half-Copper, Half-Plexiglas Track
Description: A small disk magnet rolls through a narrow track
that's made of Plexiglas at one end and copper at the other. The magnet rolls
quickly through the Plexiglas portion of the track but slows dramatically when
it enters the copper portion of the track.
Purpose: To demonstrate the repulsive forces that occur when a
magnet moves past a conducting surface.
Supplies:
1 small
disk neodymium-iron-boron magnet
1 track
for the magnet, cut from a square copper bar at one end and from a square Plexiglas
bar at the other end. The two bars are joined and framed in Plexiglas to keep
them together and to keep the magnet in the track.
Procedure: Tilt the track so that the magnetic disk rolls along the
track. Show that the disk rolls quickly through the Plexiglas portion of the
track but slows when it rolls through the copper portion of the track.
Explanation: The moving magnet induces currents in the conducting
copper and experiences repulsive magnetic forces from the currents it induces.
The magnet rolls freely through the Plexiglas because currents can't flow in
the Plexiglas.
254. Electrodynamic Magnetic
Levitation of Magnet on a Spinning Metal Disk
Description: A large disk magnet floats above a spinning aluminum
disk.
Purpose: To demonstrate electrodynamic levitation.
Supplies:
1 large
neodymium-iron-boron disk magnet (the larger and thinner, the better)
1 sturdy
aluminum disk about 40 cm in diameter, with a spindle attached
1 variable-speed
motor for spinning the aluminum disk
1 sturdy
mount for the motor
Procedure: Mount the aluminum disk on the motor and attach the
motor to a sturdy table so that the aluminum disk spins in a horizontal plane.
Be sure that everything is well balanced and strong enough to tolerate high
rotational speeds. The disk's surface should be able to reach speeds of
200 km/h without any damage! If you are concerned about the disk coming
apart at these high speeds, build a safety fence around the spinning disk.
Support the magnet on a flexible strap that will keep it horizontal but will
allow it to rise or fall vertically.
Turn on the motor and bring
the aluminum rotor to a relatively high surface speed of at least
100 km/h. Use the strap to lower the magnet carefully toward the outer
surface of this disk. The strap should be oriented tangent to the disk's edge,
with the disk turning in the direction that leads from your hand toward the
magnet. The magnet will be pulled in the direction of the disk's rotation by
magnetic drag forces and you should hold the strap tightly so that it isn't
pulled out of your hand. Before the magnet touches the aluminum disk, it will
experience a strong magnetic repulsion and it will begin to hover a few
centimeters above the spinning aluminum disk. The faster the aluminum disk turns,
the higher the magnet will hover and the less magnetic drag force it will experience.
Be carefully not to spin the disk so fast that it flies apart. Safety first!
Explanation: The magnet induces currents in the aluminum disk and
the disk becomes magnetic. It repels the magnet, suspending the magnet in the
air and giving rise to the magnetic drag force that tends to pull the magnet
along with the disk. The magnetic drag force diminishes with higher speeds
because the currents in the aluminum have less time to waste energy.
255. Superconductors and
Magnetic Levitation
Description: A small permanent magnet hovers above the surface of a
high temperature superconductor.
Purpose: To demonstrate the perpetual current flow and
magnetization of a superconductor when approached by a magnet.
Supplies:
1 high-Tc superconductor disk
1 small
neodymium-iron-boron magnet
1 Styrofoam
cup
1 thin
foam rubber or sponge pad
liquid
nitrogen
Procedure: Cut the Styrofoam cup to form a shallow tub and place
the superconductor disk on the foam rubber pad in the middle of this tub. Fill
this tub with liquid nitrogen and allow the disk to cool until the liquid
nitrogen is barely boiling. Now lower the permanent magnet onto the disk and
watch as it floats above the disk.
Explanation: The approaching magnet induces currents in the
superconductor disk and the two repel one another. This repulsion suspends the
magnet in midair. Because the currents in the superconductor don't decay away
or lose energy, the suspension continues indefinitely.
Follow-up: Even if you leave the magnet on the superconducting disk
while it's cooling down, the magnet will lift up off the surface of the
superconductor as soon as the superconductor becomes cool enough to superconduct.
This behavior, in which magnetic fields are excluded from a superconductor, is
called the Meissner effect and is something not seen in normal electrodynamic
levitation. It's unique to certain types of superconductors.
265. Magnetic Domains
Description: An array of magnetic arrows (tiny compasses) forms aligned domains.
Purpose: To show that magnetic domains tend to form in any extended ferromagnetic
system.
Supplies:
1 array
of magnetic arrows (available from a scientific supply company)
1 bar magnet
Procedure: Set the array of magnetic arrows on the table and inspect its arrows. You'll
find groups of nearby arrows that are aligned with one another, but overall
they will have little or no average alignment. These local regions of alignment
are analogous to the domains in a ferromagnetic solid.
Now bring one pole of the bar magnet near
the edge of the array. The array will change so that virtually all of the
arrows will be aligned. They will all point either toward or away from the pole
of the bar magnet. You have magnetized the array—its domains have changed so
that they have a net magnetic alignment.
Take the bar magnet away from the array and
show that much of its magnetic alignment remains. The array is permanently magnetized.
Finally, wave the bar magnet across the
array carefully and gradually move it farther and farther away until it has no
more effect. The array will once more consist of small aligned domains that
have no average overall alignment. You have demagnetized the array.
Explanation: If all the magnetic arrows were to point in the same direction, the array would
be a large magnet and would have considerable magnetic potential energy. The
array normally lowers its energy by breaking up into domains and allowing the
magnetizations of these domains to cancel one another. But when you bring the
strong external magnetic field near the array, you force it into alignment.
Even when you take away the external magnet, the array remains aligned—it needs
a disturbance to break up into domains once again. When you jiggle the magnet
nearby, you create this disturbance and the array breaks up into domains.
266.
A Magnet and Steel Nails
Description: Steel nails normally don't stick to one another. But when you touch the pole of
a permanent magnet to one of the nails, the nail becomes a magnet. When this
nail touches another nail, that nail becomes magnetic, and so on. When you
remove the permanent magnet, the nails slowly lose most of their
magnetizations.
Purpose: To show how the presence of a strong magnetic pole magnetizes steel or iron.
Supplies:
1 bar
magnet
3 or more
steel nails
Procedure: First show that the nails don't normally stick to one another. Then touch the
north pole of the bar magnet to a nail. The nail will stick to the bar magnet
because it will become magnetized. The presence of the nearby north pole
rearranges the magnetic domains inside the steel so that their south poles all
point toward the north pole of the permanent magnet. As a result, the other end
of the nail becomes a north pole. Show that this nail can magnetize another
nail it touches in a similar manner. Form a chain of nails dangling from the
bar magnet.
Finally, remove the bar magnet from the first
nail. The chain of nails will slowly fall apart as the domains in the nails
gradually return to their original random orientations. A few domains won't
return to normal, so the nails will remain slightly magnetized as a result of
their exposure to the bar magnet.
Explanation: Iron and most steels contain magnetic domains. Until these materials are
exposed to magnetic fields, the domains are randomly aligned and their
magnetization cancel one another. However, when these materials are exposed to
magnetic fields, the domains grow or shrink until the materials exhibit
substantial overall magnetizations. These magnetizations only remain while the
external magnetic fields persist. The domains in very pure iron rearrange
easily when the external fields vanish, so that very pure iron completely loses
its magnetization. But in steels, the impurities in the crystals prevent the
domains from rearranging so easily. Steel is a little harder to magnetize when
an external magnetic pole approaches it and it doesn't demagnetize completely
when the external magnet is taken away.
267.
Aluminum and Copper are Non-Magnetic
Description: While steel sticks to a bar magnet, aluminum and copper do not.
Purpose: To show that most metals are non-magnetic (they are not ferromagnetic).
Supplies:
1 strip
of steel (not stainless steel!)
1 strip of
copper
1 strip of
aluminum
1 bar magnet
Procedure: Show that steel sticks to the bar magnet while copper and aluminum do not.
Explanation: The steel contains magnetic domains that can be aligned by the proximity of a
strong magnetic pole. The copper and aluminum have no magnetic domain structure
at all, so a nearby magnetic pole has no effect on their internal magnetic structures.
268.
Domain Flipping in a Piece of Soft Iron
Description: An iron rod sits in a coil of wire that's attached to a sensitive audio
amplifier. As a bar magnet is brought up to the iron, the domains inside the
iron flip into alignment with the magnet. These flipping domains induce currents
in the coil of wire and create a "shoop" sound from the amplifier's
speakers.
Purpose: To show that the domains in iron flip when the iron is magnetized.
Supplies:
1 iron
rod
1 coil of
wire
1 preamplifier,
amplifier, and speaker
1 bar magnet
Procedure: Connect the coil of wire to the preamplifier, amplifier, and speaker. Insert
the iron rod inside the coil. Turn on the amplifiers and slowly bring one pole
of the magnet up to the iron rod. You will hear a "shoop" sound
emerge from the speaker. Each component of the "shoop" corresponds to
a domain flipping in the iron rod. Since there are so many domains and they
flip at random moments between the start to the finish, their overall sound is
the "shoop" sound. If you reverse the bar magnet, you can repeat the
experiment and hear the "shoop" again.
Explanation: Each time you magnetize the iron, the domains in the iron rod align with the
bar magnet. Their rearrangement creates a changing magnetic field through the
coil and induces a current in its wire.
269.
Reversing the Magnetization of a "Permanent" Magnet
Description: A bar magnet is inserted in a magnetizer and its poles are permanently
reversed. A second trip through the magnetizer flips its poles back to normal.
Purpose: To show that the poles of a "permanent" magnet can be reversed during
the magnetization process.
Supplies:
2 bar
magnets, with their ends clearly labeled north and south (or red and white)
1 horizontal
swivel mount for one of the bar magnets
1 bar magnet
magnetizer (available from a scientific supply company)
Procedure: Support one of the bar magnets on the swivel mount. Hold the other magnet in
your hand and show that the opposite poles of the two magnets attract and the
like poles repel. Now insert the magnet that you have in your hand into the
magnetizer and magnetize it backwards! When you again hold it in your hand, its
"north" pole will attract the north pole of the magnet in the swivel.
Show that the poles of the hand-held magnet are completely reversed. Finally,
reinsert the magnet into the magnetizer and magnetize it properly. Show that
its poles are back to normal.
Explanation: A permanent magnet is a material that, once magnetized in a certain direction,
remains magnetized in that direction. While the factory may have magnetized the
bar magnet in a particular direction, you can reverse that direction if you
have the right equipment (typically a coil of wire and a highly charged
capacitor).
270.
Sprinkling Iron Fillings on a Credit Card
Description: You sprinkle iron filings on the magnetic strip of a credit card. The filings
align in patterns, indicating that there is a pattern to the magnetization of
the permanent magnet particles in the magnetic strip.
Purpose: To show how the magnetization of a credit card strip contains information.
Supplies:
1 credit
card (this test is non-destructive; you can clean off the credit card and it
will still work)
1 shaker of
iron filings (finely ground)
Procedure: Sprinkle iron filings on the magnetic strip of a credit card and gently tap the
card to allow the loose filings to slip away. You'll see a pattern to the
filings that shows that there is a pattern to the magnetization of the magnetic
strip.
Explanation: The magnetic strip of a credit card is like a very coarse magnetic tape. The
magnetic patterns on the credit card strip are so huge that you can see them
with your eye, or at least with a magnifying glass.
271.
A Simple Tape Player
Description: You construct of simple tape player by inserting an iron rod in a coil of wire
that's attached to an amplifier and speaker. You then pull a long refrigerator
magnet strip across the iron "playback head" and hear a humming sound
from the speaker. The faster you pull the strip across the iron rod, the higher
the pitch of the hum.
Purpose: To demonstrate how a tape recorder plays back a tape.
Supplies:
1 long
magnetic strip (a long refrigerator magnet or a magnetic strip for a office
organizational bulletin board)
1 iron rod
1 coil of
wire
1 preamplifier,
amplifier, and speaker
Procedure: Connect the coil of wire to the preamplifier, amplifier, and speaker. Insert
the iron rod into the coil. Turn on the amplifiers and draw the long magnetic
strip across the iron rod. A humming sound will emerge from the speaker. The
faster you move the magnetic strip, the higher the pitch of the hum. Point out
that the magnetic strip has many poles on it and that they reverse every few
millimeters (you can show this with iron fillings if you like). As you pull the
strip across the iron rod, the iron's magnetization reverses periodically and
it induces fluctuating currents in the coil of wire. The amplifiers and speaker
use this fluctuating current to produce the humming sound.
Explanation: Just as in a magnetic tape that has recorded sound on it, the magnetic strip
has a fluctuating magnetization on its surface. As you draw it across the
"playback head," the amplifiers and speaker produce a fluctuating air
pressure that is the humming sound.
272.
A Reconstructed Tape Recorder
Description: A piece of magnetic tape slides across the playback head of a tape recorder.
The amplifier and speaker of the tape recorder reproduce the sound.
Purpose: To show how the parts of a tape recorder work.
Supplies:
1 cassette
tape recorder (to be disassembled)
1 cassette
tape
parts, time, and
perseverance
Procedure: Extract the playback head of the tape recorder (or a tape player) and mount it
and the preamplifier on a board that allows them to be inserted into the center
of a cassette cartridge so that the head touches the tape. Position a variable
speed motor so that it will pull the tape through the cassette tape cartridge
at a steady, slow speed. Connect the playback head and preamplifier to an
amplifier and speaker.
Now start the tape moving through the
cartridge and bring the playback head into contact with the tape as the tape
moves through the middle of the tape cartridge. The speaker will reproduce the
sound recorded on the tape. Getting all of this working correctly takes a
little time and energy, but it's pretty satisfying when it works. It really
helps demystify tape recorders.
Explanation: The moving tape induces currents in the playback head and these currents are
amplified and delivered to the speaker to reproduce the sound.
Follow-up: Tape player kits exist and can be modified to make it easy to see how the tape
recorder works.
273. Generating Electricity - A
Coil and a Magnet
Description: When a magnet moves past a coil of wire, a current
flows through the wire.
Purpose: To show that changing or moving magnetic fields can induce
currents in electric conductors.
Supplies:
1 coil
of wire
1 bar
magnet
1 current
meter (one that reads both positive and negative currents)
Procedure: Connect the two ends of the coil of wire to the two
terminals of the current meter. Now move one pole of the bar magnet past the
coil. You'll observe that the meter needle moves first one way and then the
other. Show that as the pole approaches the coil, the needle moves one way and
as the pole moves away from the coil, the needle moves the other way. Try
reversing the magnet (use its other pole)—the effect will reverse.
Explanation: The changing magnetic field through the coil produces
an electric field around it and this electric field pushes charges through the
coil's windings. The meter registers this flowing current. The direction of
current flow is determined by the direction in which the electric field points
and that direction depends on how the magnetic field is changing.
274. Generating Electricity - A
Coil and a Two-Color LED
Description: When a magnet moves past a large coil of wire, a
current flows through it and illuminates an LED. The LED's color depends on
which way the magnet moves and on which of its poles is being used.
Purpose: To show that moving a magnet past a conductor can cause a
current to flow through that conductor.
Supplies:
1 large
coil of wire (several hundred or even a thousand turns)
1 two-color
LED (actually two different LEDs connected in parallel in the same package. One
LED glows when the current flows one direction and the other LED glows when the
current flows in the opposite direction. Alternatively, use two LEDs connected
in parallel but in the opposite directions)
1 strong
bar magnet
Procedure: Connect the LED to the two ends of the wire coil. Now
hold the bar magnet in your hand and bring one of its poles toward the wire
coil. The faster you move the magnet, the more effective it will be. The LED
should light with one of its colors. Now pull the magnet out of the coil
quickly. The LED should light with its other color. Repeat this process rapidly
several times and point out that you are generating alternating current. If you
were to attach the magnet to a spinning rotor, the LED would blink back and
forth rapidly as the magnet swept by. Show that reversing the pole of the
magnet reverses its effects.
Explanation: The changing magnetic field in the coil of wire
induces currents in the coil. The coil is large enough (has enough turns) that
these induced currents reach the high voltages (about 3 to 5 V)
needed to power an LED.
275. An AC Generator
Description: You turn the crank of an AC generator and illuminate a
light bulb. You show that it's much harder to turn the crank of the generator
when current is flowing through the light bulb than when the circuit to the
light bulb is open.
Purpose: To show how an AC generator works.
Supplies:
1 AC
generator
1 suitable
light bulb for the generator
1 light
bulb holder
2 wires
Procedure: Connect the two terminals of the generator to the two
terminals of the light bulb holder and light bulb. Turn the generator and show
that the light bulb lights up. Allow a student to turn the generator and open
and close the circuit to show that it's much harder to turn the generator while
current is flowing and the generator is producing electric power.
Explanation: The generator moves a magnet past a coil (or a coil
past a magnet) and generates an alternating electric current in the coil and
the circuit to which that coil is attached. In this case, the current flows
back and forth through the light bulb and its filament becomes hot.
276. Two DC motors Connected in
Parallel
Description: Two DC motors (with permanent magnets) are connected
to one another by wires. When you turn one of the motors, the other motor also
turns. Reversing the direction in which you turn the first motor reverses the
direction in which the other motor turns.
Purpose: To show how a DC generator works.
Supplies:
2 DC
motors (good bearings and permanent magnets are essential—we use two 12 V
motors that are large and powerful; probably about 1/10 hp or so)
2 wires
Procedure: Connect the two DC motors together with the two wires so
that you form one large circuit. Now spin the rotor of one of the motors and
observe that the other motor spins. That's because you're generating
electricity with the first motor (effectively a generator) and that electricity
is powering the second motor. Do the same with the second motor and show that
the two motors are interchangeable. Now show that reversing the direction in
which you spin the first motor causes the second motor to reverse its direction
of rotation. That's because the motors are acting as DC generators—they contain
switching systems that ensure that the current flows in one direction that's
determined only by the direction in which you spin the rotor. Similarly, the
direction of current flow through a DC motor determines its direction of
rotation.
Explanation: When you spin the rotor of the DC motor, you are
moving a permanent magnet past a coil (or vice versa) and generating a current
in that coil. A switching system inside the motor/generator changes the
connections regularly so that current always flows in the same direction
through the external portions of the circuit (as long as you don't reverse the
direction in which the motor/generator's rotor is spinning). The DC electricity
that you generate with the first motor/generator powers the second
motor/generator, which turns in a direction determined by the direction of
current flow through the circuit.
277. Hero's Engine
Description: Steam produced by water boiling in a spherical vessel
emerges from that vessel through two arms that are arranged in a Z shape. As
the arms push the steam in one direction, the steam pushes back and the vessel
experiences a torque. It begins to spin rapidly.
Purpose: To show how steam can be used to create rotational motion
(a primitive turbine-like heat engine).
Supplies:
1 Hero's
engine (available from a scientific supply company)
1 suspension
for the Hero's engine (preferably with a swivel clip)
1 gas
burner
matches
water
Procedure: Partly fill the Hero's engine with water and install the
cap. Suspend the Hero's engine from its support and place the burner beneath
it. Ignite the burner and allow the water to boil. When steam begins to emerge
from the arms of the Hero's engine, the reaction forces on the arms will
produce a torque on the engine and it will begin to spin rapidly. Turn off the
burner so that it doesn't get out of control.
Explanation: The ejected steam exerts a torque on the engine, which
undergoes angular acceleration. The steam is doing work on the engine,
converting a small amount of its thermal energy into work as heat flows from
the hot steam to the colder room air. The Hero's engine is a simple heat
engine.
278. An Air Turbine or Windmill
Description: You blow air from a compressed air line or tank at a
turbine or fan and it begins to spin. With the turbine or fan attached to a
generator, it produces electric power.
Purpose: To show that a high-pressure (or high-speed) fluid can be
used to generate electricity.
Supplies:
1 turbine
or fan assembly, attached to a generator
1 light
bulb
1 light
bulb holder
2 wires
1 hose
compressed
air or a tank of high pressure gas
Procedure: Insert the bulb in the holder and use the two wires to
connect it to the generator. Allow the air or gas to flow through hose and
direct the stream of air or gas toward the turbine blades. The blades will begin
to spin, turning the generator and generating electricity. The light bulb will
illuminate.
Explanation: As the air or gas flows through the turbine blades,
they experience lift forces. These lift forces produce torques on the blades
about their central pivot and the blades begin to turn. They turn the generator,
which produces electricity.
279. Diodes - One Way Devices
for Current
Description: A battery and light bulb are connected in a circuit so
that the bulb lights up. When a diode is inserted into the circuit in one
direction, it has essentially no effect and the bulb remains bright. But when
the diode is reversed, no current flows through the circuit and the bulb is
dark.
Purpose: To show that a diode only carries current in one
direction.
Supplies:
1 12 V
battery
1 12 V
light bulb
1 light
bulb holder
1 power
diode
3 wires
Procedure: Insert the light bulb in the holder and use two of the
wires to connect the battery to the bulb. The bulb will glow brightly. Now
insert the diode into the circuit so that the battery's positive terminal connects
to the anode of the diode and the diode's cathode connects to the light bulb.
The light bulb will continue to glow. Finally, reverse the diode's connection,
so that its anode is connected to the light bulb and its cathode is connected
to the positive terminal of the battery. The light bulb will be dark because no
current will flow. Discuss the fact that the diode only permits current
(positive charges) to flow from its anode to its cathode.
Explanation: When the diode is forward biased (its anode is
positively charged and its cathode is negatively charged), conduction level
electrons in the cathode's n-type semiconductor can approach the diode's p-n
junction and leap across the junction into empty conduction levels in the
anode's p-type semiconductor. The anode's positive charges can then meet the
oncoming electrons so that there is a net flow of charge and current through
the diode. But when the diode is reverse biased (its anode is negatively
charged and its cathode is positively charged), the depletion region near the
p-n junction widens and no charges cross the junction.
280. A Solar Cell
Description: A solar cell is connected to a small motor. When the
cell is exposed to light, the motor turns.
Purpose: To show that a solar cell can produce electricity directly
from light.
Supplies:
1 solar
cell
1 ultra-low
friction motor (specially designed for solar cell operation—available from a
scientific supply company)
2 wires
1 100 W
(or more) incandescent spot light
Procedure: Use the two wires to connect the solar cell to the
motor. Now expose the solar cell to the bright light from the spot light.
Current will begin flowing through the solar cell and receiving power. This
power will be delivered to the motor and will cause it to turn.
Explanation: The solar cell is a specially designed diode. Light
energy transfers electrons from the n-type semiconductor of the cathode to the
p-type semiconductor of the anode. The anode becomes negatively charged and the
cathode becomes positively charged. Since the electrons can't return through
the diode's p-n junction, they flow through the circuit (including the motor).
The light energy is causing this current flow and is powering the motor.
Section 9.5 Electric Motors
281. Hanging from an
Electromagnet
Description: A strong electromagnet hangs for the ceiling. A steel surface is touched to it
and it's turned on. The forces between the electromagnet and the steel are so
strong that you can hang from the steel without pulling it away from the
electromagnet.
Purpose: To demonstrate the tremendous forces that are possible with electromagnets.
Supplies:
1 strong,
battery-powered electromagnet (available from scientific supply companies)
1 thick
steel plate, the same diameter as the electromagnet
2 strong
steel eyelets with threaded shafts
2 ropes
Procedure: Use a drill and tap to attach one of the eyelets to the back of the
electromagnet and the other to the back of the steel plate. Attach the ropes to
the eyelets and hang the electromagnet from the ceiling. Form a loop in the
rope attached to the steel plate so that you can hold onto the rope tightly.
Now touch the steel plate to the electromagnet and turn the electromagnet on.
The plate will bind very strongly to the electromagnet. Pull downward on the
steel plate to show that it can't be pulled away easily. Try hanging on the
plate (though be prepared for it to pull away from the electromagnet). If the
electromagnet is sufficiently strong, the plate will remain attached.
Explanation: Steel is a ferromagnetic metal, meaning that it contains magnetically ordered
domains. When you bring the steel near the electromagnet, the steel's domains
change size and reorient to give the steel its own magnetic poles. The steel's
poles are opposite to those of the electromagnet and the two bind together
strongly.
282. A Galvanometer
Description: When you send current through the coil of a galvanometer, the coil moves. It
experiences a torque in the presence of a magnetic field.
Purpose: To show that the torque between a current-carrying coil and a fixed permanent
magnet can cause that coil to turn.
Supplies:
1 galvanometer
(or a coil of wire that's supported in a low-friction bearing and surrounded by
permanent magnets)
1 battery
2 wires
1 resistor
(to limit the current through the galvanometer, if necessary)
Procedure: Use the two wires to connect the battery to the galvanometer. If the
galvanometer involves thin wires, you should include a current-limiting
resistor in the circuit. As soon as you complete the circuit and current begins
to flow through the galvanometer, its coil will become magnetic and will
experience a torque due to its interactions with the surrounding magnets. However,
it will turn only once and then settle down. Unlike a motor, the galvanometer
coil has an equilibrium orientation into which it's able to settle.
Explanation: The galvanometer coil will turn to bring its magnetic poles as close as
possible to the opposite poles of the surrounding permanent magnets.
283. A DC Motor
Description: A DC motor with a visible commutator turns rapidly as current passes through it
from a battery.
Purpose: To show how a DC motor works.
Supplies:
1 DC
motor demonstration, with a visible commutator
1 battery
2 wires
Procedure: Use the two wires to connect the DC motor to the battery. The motor will begin
spinning. Reverse the battery and show that the motor turns the other way.
Point out that the motor reverses because all the poles of the coil reverse but
the permanent magnets that surround the coil remain unchanged. As a result, the
torques on the rotor reverse and the motor spins backward. Stop the motor and
discuss how the commutator reverses the flow of current through the coil just
before the coil reaches its equilibrium orientation. This current reversal
ensures that the coil keeps turning because the coil can never actually reach
its equilibrium orientation.
Explanation: The battery provides power to the current that then flows through the coil of
the motor. This current magnetizes the coil and causes the coil to experience a
torque in the presence of the surrounding permanent magnets. The coil rotates
so as to approach its equilibrium orientation within the permanent magnets, but
before it arrives, the commutator causes the current through the coil to
reverse and it must turn further. The coil never reaches an equilibrium
orientation and continues to turn indefinitely.
284. A Very Simple DC Motor
Description: A tiny motor built right on top of a battery turns for hours without stopping.
Purpose: To illustrate just how easy it is to build an electric motor.
Supplies:
1 "D"
battery
1 strong
rubber band
2 large
paper clips
1 square
magnet, about 2 cm on a side and about 0.3 cm thick, with a north
pole on one side and a south pole on the other.
enamel-coated
copper wire, about #24 gauge
fine sandpaper
pliers
tape
1 small
base
Procedure: One end of a paper clip has two metal loops. Locate this end of each clip and
bend the outer loop over the inner loop so that you form an oval opening at
that end of the paper clip. Place one of the modified paper clips at each end
of the battery so that the two oval openings project outward from the same side
of the battery. Hold the two paper clips in place with the rubber band. Lie the
battery on its side so that the paper clips point directly upward and tape the
battery to the base so that the battery won't roll. Use tape to attach the
square magnet to the top of the battery, between the two paper clips.
Now wind a circular coil from the
enamel-coated copper wire. You should form a coil about 2 cm in diameter
that contains about 10 turns of wire. One end of the wire in the coil should
extend about 3 cm to the left from the coil and the other end should
extend about 3 cm to the right. Wrap the wire ends once or twice around
the other 10 turns of wire before extending them outward, to help hold the coil
together. You should end up with a wire ring that has an end wire extending
leftward at 9 O'clock and another end wire extending rightward at
3 O'clock.
Sand away the insulation from one end wire
but be careful with the other end wire. Hold the coil of wire so that the coil
is in a vertical plane with the untouched end wire oriented horizontally. Lower
that end wire onto a firm horizontal surface and sand away only the enamel
that's on the upper half of the end wire. Leave the lower half enamel-coated.
Carefully insert the coil's end wires into
the two oval loops of the two paper clips—one end wire into each oval—and let
the end wires touch the paper clips. If the paper clips are touching the
battery terminals and if the end wires of the wire coil are making contact with
the paper clips, the coil should begin to move. You may have to spin the coil to
get it started. Note that it will only spin properly in one direction,
determined by the direction of current flow through the coil and the
orientation of the magnet. The coil will spin as long as the electric
connections are good and will operate for hours before depleting the battery's
energy.
Explanation: Because of the partial insulation on the enamel wire, the coil is an
electromagnet only for half its orientations. It is attracted or repelled by
the magnet beneath it during half its rotation, but just as it gets to its
equilibrium orientation, the current flow vanishes and it continues on for half
a turn because of its rotational inertia. It continues to turn indefinitely.
285. Sophisticated DC Motors
Description: A DC motor that's attached to a variable-current power supply turns more
rapidly as the current passing through it is increased. When the current
passing through it is reversed, its direction of rotation reverses.
Purpose: To show that a DC motor's rotational speed increases as the current passing
through it increases (assuming that its only load is friction) and that its
direction of rotation reverses as the current through it reverses.
Supplies:
1 good
quality DC motor
1 variable-current
DC power supply
2 wires
Procedure: Use the two wires to connect the power supply to the motor. Show that as the
current through the motor increases, so does its rotational speed. Show also
that when you reverse the current passing through the motor, that its direction
of rotation reverses.
Explanation: The rotational speed of the unloaded motor is limited by its ability to do work
against sliding friction. The faster it turns, the more work it does each
second and the more electric power it requires. Thus increasing the current
passing through the motor and voltage drop of that current increases the power
the motor receives and allows it to turn faster. Since reversing the current
through the motor interchanges all the north and south poles of the motor's
electromagnets, the torques in the motor reverse and it turns backwards.
286. A Simple Induction Motor
Description: An aluminum pizza platter or pie dish floats on water. When you move a strong
magnet around in a circle above the platter, the platter begins to rotate with
the magnet, even though the two aren't touching.
Purpose: To show how magnetic drag forces allow a magnet that's circling a conducting
wheel to pull that wheel around with it.
Supplies:
1 aluminum
pizza platter or pie dish
1 large, shallow
container of water (large enough to float the aluminum dish in)
1 strong
magnet
Procedure: Float the platter or dish in the water and stop it from turning. Now hold one
pole of the magnet a few centimeters above the platter and begin to circle the outer
edge of the platter with the magnet. The platter will experience angular
acceleration and will begin to turn with the circling magnet.
Explanation: The moving magnet induces currents in the platter and makes that platter
magnetic. The repulsive forces between the magnet and platter tend to push the
platter out in front of the magnet. If you could continue this motion steadily
enough, the platter would end up turning just a little more slowly than the
magnet.
287. A Large Single-Phase Induction Motor
Description: A capacitor-start motor leaps into action when you turn it on and rotates
steadily there after.
Purpose: To demonstrate the operation of a powerful induction motor.
Supplies:
1 large
induction motor with a starting capacitor (1/2 hp or whatever you can
find)
Procedure: Hold the induction motor in place (I use my foot) and plug it in. It will jump
as its rotor begins to spin. Point out the raised ridge on its side. This ridge
contains a capacitor that helps to create a magnetic pole in the stator that
circles the rotor in a particular direction as the motor starts up. During its
operation, the rotor turns almost as fast as the circling pole of the stator.
Since the rate at which the stator's pole circles the rotor depends on the
cycling of the power line, the motor's rotational speed is determined by the
power line frequency. Many induction motors complete one full turn for every
two cycles of the power line. These motors turn at almost 1800 rpm (almost 30
turns per second) in the United States or almost 1500 rpm in many other countries.
Explanation: The stator of the induction motor is built from electromagnets. The starting
capacitor provides a delayed phase to some of the electromagnets during the
starting process so that the magnetic poles of the stator circle the rotor in a
particular direction. (Note for the experts: Once the rotor is turning
properly, the delayed phase isn't needed any more. The poles of the stator are
then driven directly from the single phase power and these poles oscillate back
and forth rather than circling the rotor. However, these oscillating poles can
be decomposed into pair of poles that circle with and against the direction in
which the rotor is spinning. It turns out that the torque exerted on the rotor
by the poles that are circling with the rotor are strongest and they keep the
rotor turning steadily and powerfully forward.)
288. An Electric Fan
Description: The induction motor of an electric fan turns at 2 or 3 different speeds,
as determined by the rotation rates of the poles on its stator.
Purpose: To show how varying the rotation speeds of the stator poles can change the
rotation speed of an induction motor's rotor.
Supplies:
1 2-
or 3-speed fan
Procedure: Show that the fan has two or three different speeds of rotation. These speeds
are determined by how rapidly the poles of the stator circle the rotor.
Explanation: The faster the poles of the stator circle the rotor, the faster the rotor must
turn to keep up with the circling poles.
289. A Shaded Pole Motor
Description: A copper disk that can turn on a bearing is held horizontally above the pole
piece of an AC electromagnet. When the electromagnet is operating and another
piece of copper shades half the pole piece from the copper disk, the disk
begins to turn.
Purpose: To demonstrate another type of induction motor—a shaded pole motor.
Supplies:
1 copper
disk, about 10 cm in diameter that turns about a central bearing
1 support
for the copper disk and its bearing
1 AC
electromagnet with a vertical pole piece that extends upward above the
electromagnet
1 thick
piece of highly conductive copper sheet (about 3 or 4 mm thick)
Procedure: Mount the copper disk horizontally above the pole piece of the AC electromagnet.
The pole piece should end about 1 cm below one edge of the disk. Turn on
the AC electromagnet and gradually slide the copper sheet on top of the pole
piece until it covers half the pole piece. The edge of the strip should be
aligned with the radius of the disk. The disk will begin turning so that its
surface moves from above the uncovered portion of the pole piece to above the
covered portion. If you shift the copper sheet to the other side of the pole
piece, the disk will begin to turn the other way.
Explanation: The presence of the copper sheet above the pole piece delays the formation of a
magnetic pole on the copper-shaded side of the pole piece. This delay occurs
because the induced currents in the copper sheet temporarily shield the area
above the sheet from the magnetic field of the pole piece. In effect, the pole
moves from the unshaded portion of the pole piece to the shaded portion. The
copper disk moves with this moving pole and it turns.
Follow-up: You can replace the disk and bearing with a copper ball that floats in
water. The ball will begin to rotate when you cover half the pole piece with
copper.
You may wish to repeat the Ohm's law
demonstration from Section 12.2 to show how a resistor impedes the flow of electric
current and the capacitor demonstration from Section 1.4 to show how a
capacitor stores separated electric charge.
290. The Current from a Microphone
Description: The current from a microphone is displayed on an oscilloscope while you make
various sounds.
Purpose: To show how the air pressure fluctuations at the microphone are represented by
current fluctuations in the circuit to which the microphone is attached.
Supplies:
1 microphone
(with power supply, if necessary)
1 oscilloscope
wires
Procedure: Connect the microphone to the input of the oscilloscope and turn both on. Set
the oscilloscope trigger so that a clear trace appears on the screen when you
make a single-pitch sound (a whistle, for example). Point out that the
oscilloscope displays the current in the circuit on the vertical axis (with
zero appearing at the center of the screen, so that excursions below the center
of the screen represent reversals of the current) and that time is the
horizontal axis. Note that broad fluctuations in the trace represent low
frequency sounds and low frequency alternating currents. Note also that narrow
(rapid) fluctuations in the trace represent high frequency sounds and high
frequency alternating currents. Show that larger volumes produce larger
amplitude alternating currents.
Explanation: The microphone produces currents that are proportional to changes in air
pressure. As sound reaches the microphone, the rising and falling air pressures
are represented by the microphone as forward and backward currents through the
circuit connected to the microphone.
291. A Speaker
Description: A variable-amplitude 60 Hz current flows into a large speaker that rest
horizontally on the table. Several marbles in the cone of that speaker begin to
leap up and down.
Purpose: To show how a speaker uses an alternating current to produce sound.
Supplies:
1 large
(woofer) speaker, without a cabinet
1 low-voltage
transformer (12 VAC, 5 A or so)
1 variable-voltage
autotransformer (a Variac)
3 or more
marbles
wires
Procedure: Connect the primary of the low-voltage transformer to the output of the
variable-voltage autotransformer. Connect the secondary of the low-voltage
transformer to the speaker. Plug in the autotransformer and slowly turn up its
voltage. The speaker should begin to hum more and more loudly. Put the marbles
in the speaker and allow them to bounce up and down. Discuss the motion of the
speaker cone as the alternating current in its coil flows back and forth.
Discuss how this motion produces compressions and rarefactions of the air; thus
producing sound.
Explanation: The AC current flowing through the secondary coil of the low-voltage
transformer and the coil of the speaker magnetizes the coil of the speaker and
causes it to be alternately attracted and repelled by the speaker's permanent
magnet. The speaker's paper cone is connected to its coil and both move toward
and away from the speaker's permanent magnet. This motion causes the marbles to
jump about.
292. A MOSFET
Description: You show that a tiny amount of electric charge (delivered with your finger) on
the gate of a MOSFET can control the flow of a large amount of electric current
between its source and drain. The MOSFET controls a light bulb.
Purpose: To show how charge affects the conductivity of a MOSFET and allows it to
control the current flowing in a circuit.
Supplies:
1 n-channel
enhancement-mode MOSFET with a suitable current and voltage rating (I have
usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the
"1") and 500 V (hence the "50"). However, if you want
to use a high current bulb, an MTP10N40E would be appropriate—10 A at
400 V. In any case, be prepared to replace the MOSFET once in a while when
you damage it with static electricity. It just happens.
1 12 V
light bulb (less than 1 A if you use a 1 A MOSFET, but can be higher
current if you use a more powerful MOSFET)
1 light
bulb holder
1 12 V
battery
wires
Procedure: Before handling the MOSFET, always touch an earth ground to remove any charge
you may have accumulated! Insert the light bulb in the holder. Connect the
positive terminal of the battery to one terminal of the light bulb holder.
Connect the other terminal of the light bulb holder to the drain of the MOSFET.
Connect the source of the MOSFET to the negative terminal of the battery. Now
you're ready to begin switching the light on and off.
To turn the light on, touch one hand to
the positive terminal of the battery and then touch your other hand to the gate
of the MOSFET (in that order! If you touch the MOSFET first, you may have
excess charge on you and may destroy the MOSFET!). Positive charge will flow
onto the gate of the MOSFET and it will conduct current. The light bulb will
turn on.
To turn the light off, touch one hand to
the negative terminal of the battery and then touch your other hand to the gate
of the MOSFET (again battery first!). Positive charge will flow off the gate of
the MOSFET and it will stop conducting current. The light bulb will turn off.
Since the charge (or lack of charge) will
remain on the gate while you are not touching it, the light will remain on or
off indefinitely while you leave the gate alone.
Explanation: When positive charge is present on the gate of the MOSFET, electrons are attracted
into the normally p-type semiconductor of the channel and the channel becomes
effectively n-type semiconductor. Because both the source and drain are already
n-type semiconductor, the p-n junction between the source and channel and
between the channel and drain vanish and the entire MOSFET acts like a piece of
n-type semiconductor. Current can flow through it from the source to the drain.
However, when the positive charge is removed from the gate, the channel becomes
p-type again, the p-n junctions reappear and current can't flow through the
MOSFET anymore.
293. An Audio Amplifier
Description: You build a simple audio amplifier and use it to amplifier sound from a small
tape or CD player so that it can be reproduced by a reasonably large speaker.
The amplifier is so sensitive that you can act as part of the wiring connecting
the tape or CD player to the input portion of the amplifier.
Purpose: To show how an audio amplifier works.
Supplies:
1 n-channel
enhancement-mode MOSFET with a suitable current and voltage rating (I have
usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the
"1") and 500 V (hence the "50"). However, an MOSFET
that's capable of handling more current would also be fine. Be prepared to
replace the MOSFET if you burn it out.
1 1 mF capacitor (20 V or higher)
1 100 mF capacitor (20 V or higher)
1 100 KW resistor
1 50 W resistor (2 Watt)
1 9 V
battery or an equivalent power supply
1 speaker
(8 W or
4 W)
1 small
tape or CD player
wires
Procedure: Construct the amplifier shown in the figure below (also Fig. 13.1.9 in the
book). I do it on a giant, homemade bread board with the components already
mounted on cards with pins that plug into the breadboard. Each component is
labeled with its symbol so that when the amplifier is complete, it looks like
the figure below.
Be careful as you assemble the amplifier
not to burn out the MOSFET. It should be inserted last and you should touch
earth ground (and ground the rest of the amplifier, at least briefly) before
you touch the MOSFET.
When the amplifier is complete, connect
the speaker to its output wires (on the right) and the tape player or CD player
to the input wires (on the left). If you now turn on the tape player or CD
player, sound will come out of the speaker. Discuss how alternating currents in
the input circuit cause charge to flow on and off the gate of the MOSFET.
Discuss how charge on the gate of the MOSFET controls the current flowing
between its source and drain. Discuss how the MOSFET diverts current that flows
down from the battery's positive terminal through the 50 W resistor and keeps that current
from flowing to the speaker. By alternately diverting and not-diverting this
current from the 50 W resistor, the MOSFET produces an fluctuating current in its output circuit and
through the speaker. The speaker produces sound.
For a display of how sensitive the MOSFET
is to charge, disconnect one of the input wires from the tape or CD player and
use your hands to remake the connection. Enough current will flow through you
to allow the amplifier to play the music.
Explanation: Current in the input circuit controls the charge on the MOSFET's gate and the
MOSFET controls the current flowing through the speaker.
Section
10.2 Computers
294. Series and Parallel
Circuits
Description: You create a circuit with a battery and bulb, in which two switches are in
series. Both switches must be closed simultaneously before current will flow
and the lamp will light. You then arrange the switches in parallel and either
switch can close the circuit.
Purpose: To show the differences between series and parallel arrangements for switches.
Supplies:
2 switches
(knife switches, if possible)
1 12 V
battery
1 12 V bulb
1 bulb holder
wires
Procedure: Connect the battery and bulb in a complete circuit and show that the bulb
lights up. Now insert one switch into the circuit and show that it must be
closed in order for the bulb to light. Add a second switch in series with the
first switch and show that both switches must be closed for the bulb to light.
Now disconnect the second switch and reinsert it
in parallel with the first switch. Show that closing either switch causes the
bulb to light. Discuss how in a series arrangement, the same current must flow
through both devices to reach its destination. Discuss how in a parallel
arrangement, current can flow through either device to reach its destination.
Explanation: In general, two devices in series experience the same current but their overall
voltage drop is the sum of their individual voltage drops. Two devices in
parallel experience the same voltage drop, but their overall current is the sum
of their individual currents.
295.
A Simple CMOS Inverter
Description: You build a simple CMOS inverter. You then show that when you deliver positive
charge to its input, it delivers negative charge to its output and vice versa.
Purpose: To show how an inverter works.
Supplies:
1 n-channel
enhancement-mode MOSFET with a suitable current and voltage rating (I have
usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the
"1") and 500 V (hence the "50"). Be prepared to
replace the MOSFET if you burn it out.
1 p-channel
enhancement-mode MOSFET with a suitable current and voltage rating (I have
usually used Motorola MTP2P50 MOSFETs, which are rated at 2 A (hence the
"2") and 500 V (hence the "50"). Be prepared to
replace the MOSFET if you burn it out.
1 9 V
battery
1 voltmeter or
equivalent
wire
Procedure: Connect the two MOSFETs according to the figure below (also Fig. 13.2.6 of
the book), but use the 9 V battery as the supply, rather than the 3 V
shown (the power MOSFETs need 9 V rather than 3 V). The upper MOSFET
is the p-channel MOSFET and its source is connected to the positive terminal of
the 9 V battery. Be careful to ground yourself and the components before
working with them. Attach the voltmeter to the output to monitor its voltage
(and charge).
To deliver positive charge to the input of this
inverter, touch one hand to the positive terminal of the battery and then touch
your other hand to the input wire. The output will go to 0 V (negative
charge).
To deliver negative charge to the input of this
inverter, touch one hand to the negative terminal of the battery and then touch
your other hand to the input wire. The output will go to 9 V (positive
charge).
Explanation: This CMOS inverter is using the charge delivered to its input to control two
MOSFETs. The MOSFETs are arranged so that positive charges on their gates turns
on the n-channel MOSFET and it delivers negative charge to the output. Negative
charges on their gates turns on the p-channel MOSFET and it delivers positive
charge to the output.
296.
A Simple CMOS NAND Gate
Description: You build a simple CMOS NAND gate. You then show that when you deliver positive
charge to both of its inputs, it delivers negative charge to its output. If
either input has negative charge on it, it delivers positive charge to its
output.
Purpose: To show how a computer gate works.
Supplies:
2 n-channel
enhancement-mode MOSFETs with a suitable current and voltage rating (I have
usually used Motorola MTP1N50 MOSFETs, which are rated at 1 A (hence the
"1") and 500 V (hence the "50"). Be prepared to
replace the MOSFET if you burn it out.
2 p-channel
enhancement-mode MOSFETs with a suitable current and voltage rating (I have
usually used Motorola MTP2P50 MOSFETs, which are rated at 2 A (hence the
"2") and 500 V (hence the "50"). Be prepared to replace
the MOSFET if you burn it out.
1 9 V
battery
1 voltmeter or
equivalent
wire
Procedure: Connect the four MOSFETs according to the figure below (also Fig.
13.2.8 of the book), but use the 9 V battery as the supply, rather
than the 3 V shown (the power MOSFETs need 9 V rather than 3 V).
The upper MOSFETs are the p-channel MOSFETs and their sources are connected to
the positive terminal of the 9 V battery. Be careful to ground yourself
and the components before working with them. Attach the voltmeter to the output
to monitor its voltage (and charge).
To deliver positive charge to an input of this
gate, touch one hand to the positive terminal of the battery and then touch
your other hand to the input wire. To deliver negative charge to an input of
this gate, touch one hand to the negative terminal of the battery and then
touch your other hand to the input wire. Don't reverse the touch order or you
will zap the MOSFETs!
When both inputs are positively charged, the
output will be negative (0 V). When either input is negatively charged,
the output will be positive (9 V).
Explanation: This CMOS NAND gate is using the charge delivered to its inputs to control four
MOSFETs. The two p-channel MOSFETs are arranged in parallel and deliver
positive charge to the output when either input is negatively charged. The two
n-channel MOSFETs are arranged in series and deliver negative charge to the
output when both inputs are positively charged. This arrangement gives the
output a NAND relationship to the inputs.
302. Fluorescence
Description: Various materials are exposed to ultraviolet light and glow different colors.
Purpose: To demonstrate fluorescence.
Supplies:
1 ultraviolet
lamp
fluorescent dyes and
materials of various colors
Procedure: Turn on the ultraviolet lamp and show that you can't see its light. Point out
that normal materials remain dark when exposed to only ultraviolet light. Now
put the various fluorescent materials in the ultraviolet light and observe that
they begin to emit visible light of various colors. Discuss the fact that this
light is new light, radiated by the dyes and materials using energy they
obtained from the ultraviolet light.
Explanation: A fluorescent material absorbs a photon of ultraviolet light and emits a new
photon of light. While the new photon can have all of the energy of the
original photon, so that it's just a new version of the original photon, the
fluorescence that we observe most often involves the emission of a lower-energy
photon—usually a visible photon. The missing energy usually becomes thermal
energy.
303.
Fluorescence Caused by Electron Impact
Description: A beam of electrons in a simple cathode ray tube causes the phosphor coating on
the inside of the tube to glow (probably green).
Purpose: To show that energy from a beam of electrons can cause fluorescence.
Supplies:
1 simple
cathode ray tube and its power supply
Procedure: Turn on the cathode ray tube and show that the impact of electrons on its
phosphor screen causes that screen to emit light. The electrons are providing
energy to the phosphors and they turn that energy into visible light.
Explanation: Phosphors can produce light whenever they are shifted to electronically excited
states. Whether that excitation is the result of exposure to high energy light
photons or the result of collisions with particles, the phosphors produce
light.
304.
Deflecting a Beam of Electrons with Electric Fields
Description: An electrostatic field created by a static generator deflects a beam of
electrons in a cathode ray tube.
Purpose: To show that a beam of electrons accelerates in response to electric fields.
Supplies:
1 simple
cathode ray tube and its power supply
2 metal plates
with insulating supports
2 wires
1 Wimshurst
static generator
Procedure: Touch the two contacts of the Wimshurst generator together to be sure that it
doesn't have any stored charge. Use the wires to connect its two contacts to
the two plates, being sure that the wires aren't near anything conductive or
near one another. Position the plates at the sides of the cathode ray tube.
Turn on the cathode ray tube. Separate the two contacts of the Wimshurst
generator and turn its crank to generate static electricity. As charge builds
up on the plates, the beam of electrons in the cathode ray tube will steer
toward the positively charged plate.
Explanation: The beam of negatively charged electrons is attracted toward the positively
charged plate and repelled by the negatively charged plate. The electrons
accelerate toward the positive plate and the beam is deflected.
305.
Deflecting a Beam of Electrons with Magnetic Fields
Description: A magnetic field created by a hand-held magnet deflects a beam of electrons in
a cathode ray tube.
Purpose: To show that a beam of moving electrons accelerates in the presence of magnetic
fields.
Supplies:
1 simple
cathode ray tube and its power supply
1 strong bar
magnet
Procedure: Turn on the cathode ray tube and then hold the bar magnet near its face. The
spot formed when the electrons hit the phosphors will move, indicating that the
magnetic field has deflected the electron beam.
Explanation: Moving electrons experience a transverse force when they move through a
magnetic field. While this force is at right angles to their velocities and
does no work on the electrons, it does alter their trajectories.
306.
Deflecting a Beam of Electrons with a Magnetic Field - in a Black and White
Television Set
Description: You hold a strong magnet up to a black and white television set and the picture
distorts.
Purpose: To show that the television set is using a beam of electrons to form its image
and to show that this beam of electrons can be steered by a magnetic field.
Supplies:
1 black and
white television set (or an old color television set, if you don't mind
spoiling it or are willing and able to demagnetize its shadow mask after the
demonstration)
1 strong bar
magnet
Procedure: Turn on the television and obtain a clear picture. Now bring the bar magnet up
to the surface of the screen and show that the image distorts. If you're using
a color television, the colors will also shift because the electrons no longer
travel in their usual paths through the shadow mask. The effect will vanish
when you remove the bar magnet from a black and white set, but the image may
remain distorted or color-shifted on a color set. To "repair" a color
television set, you need to demagnetize its shadow mask with a large AC
demagnetizing coil.
Explanation: Moving electrons inside the television's picture tube are deflected by their
passage through the extra magnetic field and they hit the screen at unintended
positions.
307.
Mixing the Primary Colors of Light
Description: By mixing various amounts of red, green, and blue light, you can make people
perceive any possible color.
Purpose: To show how the primary colors of light can be mixed (as they are in a
television) to make us see any possible color.
Supplies:
3 light
sources of variable brightness
1 red filter
1 blue filter
1 green filter
Procedure: Place the three filters over the three light sources and partially overlap
their beams on a white screen. Show that by adjusting their relative
intensities, you can form various colors in their overlapping regions. When red
and green are mixed evenly, you see yellow. When green and blue are mixed
evenly, you see cyan. When red and blue are mixed evenly, you see magenta. And
when all three are mixed evenly, you see white.
Explanation: Our eyes are really only sensitive to three types of light: red, green, and
blue. While wavelengths of naturally occurring light that fall in between the
wavelengths of pure red, pure green, and pure blue light cause us to see
intermediate colors, we can be tricked into seeing those colors by the proper
mixture of these primary colors of light.
Before the demonstration about
virtual images, I repeat the demonstration about real images from Section 16.1.
Then I can combine the two demonstrations to form the Keplerian telescope that
follows.
336. Forming a Virtual Image
Description: You hold a magnifying glass in front of a picture and
see an enlarged virtual image of that picture. This image appears behind the
lens, so you can't put your fingers in it the way you can with a real image.
Purpose: To show how a converging lens forms a virtual image of a
very nearby object.
Supplies:
1 picture
1 magnifying
glass (or another converging lens)
Procedure: Hold the magnifying glass a short distance in front of
the picture and show that a virtual image of the picture appears. Point out
that the image is larger than the picture and that it's located on the same
side of the lens as the picture. You can't touch the virtual image. The virtual
image is also upright, in contrast to a real image, which is inverted.
Explanation: The converging lens takes the diverging light rays
that emerge from a particular point on the picture and bends them so that they
don't diverge quite as fast. You see them as coming from a more distant but
much larger virtual image.
337. A Keplerian Telescope
Description: You use a magnifying glass to allow close inspection
of the real image formed by a converging lens, thus producing a Keplerian
telescope.
Purpose: To show how two converging lenses can form a simple
telescope.
Supplies:
1 light
bulb (or another bright, identifiable object)
1 converging
lens (about 50 mm in diameter, with a focal length of about 250 mm or
so)
1 magnifying
glass (or another converging lens)
1 optics
bench (optional—otherwise just use lens and component holders)
Procedure: Place the light bulb several meters from the first
converging lens and locate the real image of this light bulb that the first
lens forms. You can use your hand to find the pattern of light in space because
you can touch a real image.
Now take the magnifying glass
and use it to produce an enlarged virtual image of that real image. You may
want to observe this real image with a television camera and monitor so that
everyone can see it.
Explanation: The first lens forms a real image of the light bulb and
the second lens allows you to make a close (magnified) inspection of that
image. The final virtual image is inverted because it's an upright virtual
image of an inverted real image.
338. Forming a Virtual Image
with a Mirror
Description: You look at an object in a mirror and notice that what
you see is a virtual image.
Purpose: To show that mirrors can form virtual images.
Supplies:
1 mirror
1 light
bulb (or another object)
Procedure: Place the light bulb a short distance in front of the
mirror and observe the image that the mirror forms. This image is located on
the other side of the mirror from the object, where you can't touch it. It's
thus a virtual image.
Explanation: The mirror bends the light rays so that they appear to
come from an object that's located behind the mirror, the same distance behind
the mirror as the object is in front of the mirror.
339. Forming a Virtual Image
with a Curved Mirror
Description: You look at an object in a curved mirror and notice
that what you see is an enlarged virtual image.
Purpose: To show that curved mirrors can form enlarged virtual
images.
Supplies:
1 concave
mirror
1 light
bulb (or another object)
Procedure: Place the light bulb a short distance in front of the
mirror and observe the enlarged virtual image that the mirror forms.
Explanation: The mirror bends the light rays so that they appear to
come from an object that's located behind the mirror. This virtual image is
located at a greater distance behind the mirror than the object is in front of
the mirror. This virtual image is also greatly enlarged relative to the object.
340. Forming a Real Image with a
Curved Mirror
Description: Light from a distant light bulb reflects from a curved
mirror and forms an inverted real image.
Purpose: To show that curved mirrors can form real images.
Supplies:
1 concave
mirror
1 light
bulb (or another object)
1 ground-glass
screen
Procedure: Place the light bulb a long distance in front of the
mirror (perhaps in the back of the darkened room) and observe the real image
that the mirror forms on a nearby ground glass screen.
Explanation: The mirror bends the mildly diverging light rays from
the distant bulb so that they converge together on the screen. The real image
that forms on the screen is inverted in this process.
Follow-up: Use a magnifying glass to inspect the real image,
thereby creating a reflecting telescope. Also, insert apertures in front of the
mirror so that you use less of the mirror. Show that the real image darkens but
remains complete. Point out that one of the values of a large aperture is
light-gathering ability. Large mirrors collect more light and do their jobs
faster.
341. The Importance of Large
Apertures - Diffraction
Description: A laser beam is sent through a series of progressively
smaller pinholes. With each smaller size, the resulting beam spreads outward
more strongly.
Purpose: To show that sending light through an aperture causes it
to spread outward (and that this spreading limits the resolution of a
telescope).
Supplies:
1 laser
beam with a good quality beam (a helium-neon laser is probably better than a
solid state laser pointer. In principle, you want a clean TEM00 mode from the
laser.)
1 set
of pinholes
1 screen
Procedure: Direct the beam from the laser at the screen and notice
how small the beam spot is. Now insert the pinholes into the beam, one at a
time. As these holes get smaller, not only does the light spot get dimmer—it
also gets wider.
Explanation: The light propagates as a wave. When the wave is force
to go through a narrow aperture, it naturally spreads out in its subsequent
travels. The narrower the aperture, the more rapidly the wave spreads. In a
telescope, making the light go through the aperture defined by the mirror's
diameter causes the light spread and limits the telescope's ability to resolve
nearby stars.
345. Radiation and Shielding
Description: You hold a Geiger counter near various radioactive sources and listen to their
decays. When you insert certain materials between the sources and the Geiger
counter, the counts diminish, indicating that the particles released by the
decays are being blocked by the materials.
Purpose: To show that certain materials block the fragments of radioactive decays and
can thus be used to control radiation and induced nuclear reactions.
Supplies:
1 or
more radioactive sources (appropriate licensing, training, and safety
precautions must be followed)
1 Geiger
counter
shielding
materials, ranging from cardboard for beta decays to lead sheets for more
energetic particles
Procedure: Use the Geiger counter to monitor the decays of the various radioactive
sources. Then insert the shielding materials between the sources and the Geiger
counter to show that the decay fragments can be block (absorbed or reflected)
by these materials.
Explanation: The electrons from beta decays are easy to block because the low-mass electrons
are easily deflected. But more massive alpha particles must encounter the
nuclei of massive atoms such as lead to deflect them from their paths. Gamma
rays are also stopped only by large atoms because they interact most strongly
with the tightly bound inner electrons of those giant atoms.
346. Melting Ice
Description: A thermometer inserted in a container filled with a mixture of water and ice
reads 0° C, even when the container is heated by a flame or cooled by dry
ice.
Purpose: To show that the phase transition between water and ice occurs at 0° C,
and that adding or removing heat from a mixture of the two causes one phase to
transform into the other and doesn't change the temperature of the mixture.
Supplies:
1 Pyrex
or Kimax beaker
water and
ice mixture
1 thermometer
1 support
for the beaker
1 support
for the thermometer
1 gas
burner
matches
1 cube
of dry ice
Procedure: Place the beaker on the support and fill it with a mixture of ice and water.
Insert the thermometer in the beaker and support the thermometer so that it
doesn't touch the sides of the beaker. In a few seconds, the thermometer will
read 0° C. To show that adding or removing heat from the mixture of water
and ice won't change its temperature, first add heat to the mixture by heating
it gently with the gas burner (don't heat too aggressively, or you'll break the
beaker). The thermometer will still read 0° C. Finally, put away the
burner and put the beaker on the cube of dry ice. Make sure that the thermometer
doesn't touch the sides of the beaker. The thermometer will still read
0° C.
Explanation: While water and ice are in equilibrium with one another, the temperature must
be 0° C. If you add heat to this mixture, some of the ice will transform
into water but the mixture's temperature will remain at 0° C. If you
remove heat from this mixture, some of the water will transform into ice but
the mixture's temperature will remain at 0° C.
347. Boiling Water with Heat
Description: A beaker of water is heated with a burner. Although water will be seen to
evaporate once the water is hot, it will only begin to boil when the water's
temperature approaches 100° C. Once the water is boiling, additional heat
will not cause its temperature to rise.
Purpose: To show that while evaporation can proceed at any temperature, boiling appears
when evaporation becomes rapid enough to occur within the body of the liquid.
Also to show that during boiling, adding heat to the water causes it to
transform into steam rather than to become hotter.
Supplies:
1 Pyrex
or Kimax beaker
water
1 support
for the beaker
1 gas
burner
matches
1 thermometer
1 support
for the thermometer
Procedure: Place the beaker on the support and fill it half way full of water. Insert the
thermometer into it and support the thermometer so that it doesn't touch the
sides of the beaker. Light the burner and put it under the beaker. Heat the
beaker gently so that it doesn't break. As the water becomes warmer, mist will
appear above the water. A short while later, gas bubbles will appear on the
walls of the beaker. And finally, bubbles of steam will appear within the water
and the water will begin to boil. At that point, the temperature of the water
will be approximately 100° C and this temperature will remain constant,
despite the continued input of heat by the burner.
Explanation: As the water warms up, evaporation from its surface will become faster and
faster. A mist will appear above the water when the evaporation becomes fast
enough to send hot, water-saturated air upward into the cooler air above the
beaker—as this hot, water-saturated air cools, water droplets form in it and
create the mist that you see. The gas bubbles that appear on the walls of the
beaker are dissolved gases that comes out of solution as the water nears its
boiling temperature—most gases are less soluble in hot water than in cold
water. Finally, boiling occurs when evaporation is so rapid that it begins to
occur within the body of the liquid. For these evaporation bubbles to form and
grow, they must be able to withstand the crushing effects of atmospheric
pressure. By the time the water reaches 100° C, the bubbles of steam
inside the water are so dense with water molecules that they have a pressure
equal to atmospheric pressure and can't be crushed by atmospheric pressure.
348. Boiling Water in a Vacuum
Description: A glass of room temperature water is put in a glass bell jar and the air is
removed from that bell jar by a vacuum pump. The water begins to boil. Moments
later, air is admitted to the bell jar and it's removed. The water is still
cool.
Purpose: To show that water's boiling temperature depends on the ambient pressure.
Supplies:
1 glass
water
1 bell
jar and vacuum pump system
Procedure: Fill the glass half way full of water and insert your finger in it to show that
it's cool. Put the glass in the bell jar and turn on the vacuum pump. When
enough air has left the bell jar, the water will begin to boil. Stop the vacuum
pump and allow air to reenter the bell jar. Open the jar and insert your finger
into the water to show that its still cool.
Explanation: While evaporation is always occurring at the surface of cold water, it can't
normally occur in the body of cold water because any evaporation bubble that
appears inside the water will have too low a density and pressure to withstand
the crushing effects of atmospheric pressure. But when a vacuum system has
removed most of the air and air pressure from around a glass of water,
evaporation bubbles that appear inside the water will be able to grow and
expand. The water will boil even at low temperatures.
Follow-up: Try to soft boil an egg in a glass of water that's boiling in a bell jar. The
egg won't cook at all. That's because boiling a three minute egg really means
exposing that egg to water at 100° C for three minutes. In the vacuum
chamber, you're exposing an egg to water at room temperature for three minutes
and that has no effect on the egg at all. Why does it take longer to boil an
egg at high altitude than it does at sea-level?
Another
Follow-up: Try putting ice water in the vacuum. It will also boil if you're
patient enough.
349. Condensing Steam - Crushing a Beverage Can
Description: You heat a small amount of water in an open beverage can until the can fills
with steam. You then quickly invert the can and plunge it into a pan of cold
water. The can is immediately crushed by atmospheric pressure.
Purpose: To show that removing heat from steam causes it to condense into water and that
water occupies a much smaller volume than steam.
Supplies:
1 empty
aluminum beverage can
1 ring
stand
1 gas
burner
1 cooking
pan
tongs
matches
water
Procedure: Fill the cooking pan with about 3 cm of cold water. Pour about 2 ml
of water into the beverage can and place it on the ring stand. Light the burner
and heat the bottom of the can until the water boils. After the can has
completely filled with steam and the steam has completely displaced any air the
can contained (about 20 seconds of boiling), use the tongs to pick the can up,
invert it, and plunge it into the pan of cold water. The can will collapse with
a crunching sound.
Explanation: Boiling water in the can fills it with steam rather than air. When the steam is
immersed in cold water, it gives up heat to the cold water and undergoes a
phase change back into water. Water occupies much less volume than steam and
the can is left virtually empty. With nothing inside it to support its walls,
the can is crushed by the surrounding air pressure.
350. Dissolving Salt, Sugar, and Carbon Dioxide in
Water
Description: You mix sugar, then salt, then carbon dioxide into water. All three dissolve
easily.
Purpose: To discuss the mechanisms whereby added materials dissolve in water.
Supplies:
3 glasses
water
salt
sugar
1 soda
siphon
1 carbon
dioxide cylinder
1 spoon
Procedure: First add a spoonful of salt to a glass of water and stir. In a few seconds,
the salt will have disappeared. Point out that the salt is still there, it has
just decomposed into individual sodium positive ions and chlorine negative
ions, each of which is now wrapped in an entourage of water molecules.
Now add a spoonful of sugar to the
glass of water and stir. Again, it will dissolve. Point out that the sugar
molecules are separated from one another and surrounded by shells of water
molecules.
Finally, fill the soda siphon with
water, put the top on, and charge the siphon with carbon dioxide according to
its instructions. Shake the siphon to disperse the carbon dioxide and wait a
few seconds. Then serve the carbonated water into a glass. It will bubble
merrily. Note that the carbon dioxide molecules have attached themselves to
water molecules to form a weak acid known as carbonic acid.
Explanation: Salt dissolves well in water because water molecules are strongly attracted to
sodium and chlorine ions. They wrap those ions in solvation shells of water
molecules. The negative ends of the water molecules (their oxygen atoms) turn
toward a positive sodium ion and the positive ends of the water molecules
(their hydrogen atoms) turn toward a negative chlorine ion. Sugar dissolves
well in water because water molecules bond relatively well to sugar molecules.
Water molecules form hydrogen bonds with the oxygen-hydrogen groups on a sugar
molecule and construct a solvation shell around the sugar molecule. Finally,
carbon dioxide dissolves well in water because water molecules combine with
carbon dioxide molecules to form a new molecule—carbonic acid. The binding
between these two molecules is modest but it's enough to make it easy for
carbon dioxide to dissolve in water.
351. Depressing the Melting Point of Ice with Salt or
Sugar
Description: A beaker of melting ice initially has a temperature of 0° C. When salt or
sugar is added to the ice, the temperature drops well below 0° C.
Purpose: To show that adding a water-soluble solid to ice depresses its melting
temperature.
Supplies:
1 beaker
ice
salt or
sugar
1 thermometer
1 support
for the thermometer
1 spoon
Procedure: Fill the beaker with ice and carefully insert the thermometer in it. Support
the thermometer so that it doesn't touch the walls of the beaker. After a few
seconds, the thermometer will read 0° C. Now remove the thermometer and
add a large spoonful of salt or sugar to the ice. Stir the mixture and reinsert
the thermometer. After a few seconds the thermometer will read below 0° C.
Explanation: Adding a water soluble solid to ice destabilizes the solid phase at 0° C.
The ice begins to melt to form salty or sugary water at 0° C, but this
melting requires heat. The ice that does melt extracts heat from the ice that
doesn't melt and the remaining ice becomes colder and colder. Soon the entire
mixture, including the salty or sugary water, is at a temperature well below
0° C. The addition of the salt or sugar has caused more of the ice to
become water and, because melting the ice has used some of the mixture's
thermal energy, the mixture is now colder than it was before.
352. Raising the Boiling Point of Water with Salt or
Sugar
Description: A beaker of boiling water initially has a temperature of 100° C. When salt
or sugar is added to the water, the temperature rises well above 100° C.
Purpose: To show that adding a water-soluble solid to water raises its boiling
temperature.
Supplies:
1 beaker
water
salt or
sugar
1 thermometer
1 support
for the thermometer
1 spoon
1 support
for the beaker
1 gas
burner
matches
Procedure: Place the beaker on the support and fill it with water. Carefully insert the
thermometer in it and support the thermometer so that it doesn't touch the
walls of the beaker. Light the burner and gently heat the beaker. Be careful
not to heat the beaker too quickly or it may break. Soon the water will boil
and the thermometer will read about 100° C. Now add a large spoonful of
salt or sugar to the boiling water. Stir the mixture. When the mixture again
begins to boil, the thermometer will read well above 100° C.
Explanation: Adding a water soluble solid to water interferes with its ability to evaporate.
With many of the water molecules involved in stabilizing the dissolved solid,
there are fewer water molecules evaporating at any given temperature. The water
temperature must exceed 100° C before evaporation is fast enough for
evaporation bubbles to become stable within the body of the water so that
boiling can occur.
353. Regelation of Ice
Description: A heavily weighted wire is draped over a melting ice cube. The wire slowly
descends into the ice cube, leaving a healed scare of solid ice above it.
Purpose: To show that pressure depresses ice's melting temperature.
Supplies:
1 large
ice cube (frozen in a rectangular muffin tin)
1 board
to support the ice cube
1 clamp
1 piece
of piano wire
1 heavy
weight
Procedure: Clamp the support board to a sturdy table so that it extends out over the
floor. Place the ice cube on the support. Tie loops at the two ends of the
piano wire, drape the wire over the ice cube, and hang the heavy weight from
the two loops so that the wire is pulled tightly against the ice cube. When the
ice cube warms to 0° C and begins to melt, the wire will begin to cut into the
ice cube and will soon disappear below its surface. The ice will reform above
it, so that the wire will soon be trapped in solid ice.
Explanation: This whole process takes place while the ice cube is at almost exactly
0° C. The elevated pressure below the piano wire depresses the ice's
melting temperature so that water's liquid phase is more stable below the wire
than is water's solid phase. The ice there melts and the wire descends into the
liquid water. Relieved of the pressure, the water returns to its solid phase.
Ice thus melts below the wire and reforms above the wire. In fact, there is a
continual heat transfer from the freezing water above the wire to the melting
ice below the wire. In this manner, the wire drifts right through the solid ice
cube.
You might want to repeat the breaking a
penny demonstration from Section 6.1 to show how reduced temperature prevents
dislocations from moving and makes some metals hard and brittle.
354. Hardening and Annealing a Steel Nail
Description: You try to bend a hardened steel nail and it breaks. You take an identical
nail, heat it red hot, and let it cool slowly. It then bends rather than
breaking. You straighten this nail and reheat it. However, this time you plunge
the red hot nail into water to harden it. Now it breaks rather than bending.
Purpose: To show how heat treatment hardens carbon steels.
Supplies:
2 high
carbon nails (masonry nails—we have found a supply of flat-sided masonry nails
that work very well. They are very hard and very brittle. That's what you want.)
1 propane
torch
matches
1 container
of water
1 vise
1 pliers
1 tongs
safety glasses
Procedure: Clamp one of the nails in the vise and try to bend it with the pliers. Instead
of bending, it will break. Now take the second nail in the tongs and heat it
red hot with the torch. Allow it to cool gradually until it's at room
temperature (a minute or two). Now clamp it in the vise and try to bend it with
the pliers. It will bend without breaking. Straighten it back out and hold it
in the tongs. Reheat it red hot and this time plunge it into the water. This
rapid cooling will harden the steel. When you return it to the vise and try to
bend it, it will break.
Explanation: The steel is hard because it contains tiny particles of hard cementite (iron
carbide) scattered throughout its ferrite crystals (iron). When you try to bend
this hard steel, the cementite particles keep the ferrite crystals from
undergoing plastic deformation (slip) and the nail breaks. But when you heat
and slowly cool the steel, the carbon that dissolves in the hot steel has time
to migrate out of the ferrite crystals. The steel is then a soft mixture of
large ferrite crystals and a few large cementite crystals. It bends easily
because the ferrite crystals have no tiny cementite particles in them to
prevent them from undergoing plastic deformation.
355. Different Steels
Description: You compare the properties of several different steel alloys.
Purpose: To show how small compositional changes and changes in processing can have
substantial effects on the characteristics of steels.
Supplies:
1 piece
of low-carbon steel (common steel)
1 piece of
high-carbon steel (tool steel)
1 piece of
18-8 stainless steel
1 magnet
1 plastic
container of hydrochloric acid
Procedure: Discuss the compositional differences between the steels. Show that the
high-carbon steel can cut the low-carbon steel because the former is much
harder than the latter. Show that both the carbon steels are magnetic while the
stainless steel is not. Show that the carbon steels react with hydrochloric
acid while the stainless steel does not.
Explanation: The high-carbon steel is harder than the low-carbon steel because it contains a
large proportion of iron carbide particles (and perhaps other slip-inhibiting
inclusions). The stainless steel is chemically inert because of its high
content of chromium and nickel atoms.
356. Melting Glass - Quartz vs.
Soda-Lime Glass
Description: You try to melt quartz glass tubing unsuccessfully while soda-lime glass tubing
melts easily.
Purpose: To show that the addition of soda and lime to quartz glass dramatically reduces
its melting and softening temperatures.
Supplies:
1 piece
of quartz glass (or Vycor glass) tubing or rod
1 piece
of soda-lime glass tubing or rod
1 gas
burner or propane torch
matches
Procedure: Try to melt the quartz glass tube with the burner or torch. You will be unable
to do so. Now try to melt the soda-lime glass tube. It will melt and flow
easily.
Explanation: Adding the soda and lime to the quartz makes it much easier to work with. The
sodium ions terminate the covalent networks that are the basis for quartz
glasses and weaken those networks. As a result, soda-lime glasses are softer
and melt more easily than pure quartz glass. In fact, soda-lime glasses are
eutectics—they melt at temperatures below the melting points of the chemicals
from which they are made.
357. Thermal Shock and Glass
Description: You show that heating soda-lime glass rapidly causes it to crack from the
stresses of uneven thermal expansion. Borosilicate glass doesn't suffer such
problems. Quartz glass can handle rapid heating well, too. Upon rapid cooling
in cold water, even the borosilicate glass may break. But quartz glass is still
unaffected.
Purpose: To show that thermal expansion and contraction can cause glasses to tear apart
during uneven heating and cooling.
Supplies:
1 glass
slide (soda-lime glass)
1 pyrex
tube or dish
1 quartz
glass tube (or Vycor glass)
1 propane
torch
matches
1 container
of water
safety glasses
Procedure: Heat the glass slide rapidly with the torch. It will crack or shatter. Now heat
the pyrex tube or dish. Unless you heat it particularly quickly in one spot, it
should survive. Now heat the quartz tube. You can't damage it with heat.
Next reheat the pyrex tube or dish and
plunge it into cold water. It will almost certainly crack or shatter. Try the
same with the quartz tube. It will survive without injury.
Explanation: Soda-lime glass is soft and has a large coefficient of volume expansion. When
you heat part of it rapidly, the heated part expands. The heated and unheated
parts of the glass exert tremendous stresses on one another and they tear the
weak glass apart. Borosilicate glasses are still structurally weak, but they
have much smaller coefficients of volume expansion. The heated and unheated
parts are less able to tear one another apart. However, very rapid temperature
changes (as occur when hot glass is plunged into water) still cause the glass
to tear itself apart. Quartz glass is so strong and has such a small
coefficient of volume expansion that it's very hard to injure with thermal
shock.
358. The Disappearing Glass Container
Description: You pour salad oil into a clear container that has a Pyrex or Kimax item inside
it. The item appears to vanish.
Purpose: To show that there are no reflections when light moves between two materials
with the same index of refraction.
Supplies:
1 bottle
of salad oil (Wesson works well)
1 Pyrex
or Kimax flask or beaker
1 clear
container
Procedure: Put the flask or beaker in the container and observe that it's plainly visible.
Now pour the salad oil into the container and into the flask or beaker. The
flask or beaker will become essentially invisible.
Explanation: The indices of refraction of the salad oil and borosilicate glasses are almost
identical. With no change of speed upon entry or exit from the flask or beaker,
light doesn't refract or reflect, and you can't tell that the flask or beaker
is there.
359. Tempered Glass - A Bologna Bottle
Description: You use a peculiar glass bottle to pound in a nail. You then drop a tiny chip
of sharp crystal into the bottle and it falls apart.
Purpose: To show that the surface stresses experienced by glass determine its resistance
to tearing and breakage.
Supplies:
1 bologna
bottle (available from a scientific supply company, at non-negligible expense. Sargent-Welch
charged $41 for them recently. Still, they are remarkable.)
1 piece
of wood
1 nail
with a large head (just to be safe)
safety glasses
Procedure: Hold the neck of the bologna bottle and tap the nail into the wood with the
side of the round bottle. Having demonstrated that the outside of the bottle is
extremely tough, hold the bottle upright over a garbage can and drop the
crystal chip that came with the bottle into the neck of the bottle. When this
chip hits the inside bottom of the bottle, the bottle will tear itself apart
and its pieces will drop into the garbage can.
Explanation: The bottle is tempered in such a way that the outside surface is experiencing
compression and the inside surface is experiencing tensile stress. Since it's
very hard to start a tear in a layer that is being compressed, it's hard to
tear the outside of the bologna bottle. But the inside is under tension and the
slightest injury to it will cause the surface to tear itself to shreds.
360. Tempered Glass - Rupert Drops
Description: When you break the tail of a small glass drop, the drop crumbles into dust.
Purpose: To show that tempered glass exhibits dicing fracture when its compressed outer
skin is broken.
Supplies:
2 or
3 Rupert drops (available from a scientific supply company)
1 needle-nosed
pliers
cloth gloves
safety glasses
Procedure: Hold a Rupert drop in your gloved hand and break off its tail with the pliers.
If the drop has been properly tempered (I've had mixed luck), it will tear
itself to powder. You may have to try more than one to observe this
self-destruction.
Explanation: The Rupert drops are tempered glass—their outer surfaces are under compression
while their insides are under tension. When you break through the compressed
surface layer and expose the tense inner portion of the drop, it tears itself
apart.
361. Glass Fibers
Description: You heat the middle of a glass rod until it softens and then pull its ends away
from one another. A glass fiber forms in between the ends. This fiber is
relatively flexible and extremely strong for its size.
Purpose: To show how glass fibers are formed.
Supplies:
1 glass
rod
1 gas
burner
matches
safety glasses
Procedure: Light the burner and hold the middle of the glass rod over the flame. When the
glass has softened significantly, pull the two ends of the rod away from one
another in a smooth and steady motion. Stop when you have stretched the rod to
about 1 m long. Allow the pieces to cool briefly. Show that the glass
fiber is flexible (don't bend it too far or it will break!). Be careful with
the hot ends of the glass until they've had enough time to cool completely. Be
careful with eyes.
Explanation: The glass fiber's strength comes in part because of its relative lack of
defects on its surface. With so little surface on any given length of fiber,
there are only a couple of sites for a tear to begin as you bend the fiber.
Many of the demonstrations listed in this section
are standard experiments done by students of organic chemistry. You may be able
to obtain the materials for these experiments from your local chemistry
department already prepared and ready to go.
362.
Natural Polymers
Description: You display several natural polymers.
Purpose: To show that polymers (plastics) are common in nature.
Supplies:
1 sheet of
paper (cellulose)
1 rubber band
(rubber)
1 piece of wool
1 piece of silk
1 box of
cornstarch
Procedure: Simply point out that each of these materials consists of extremely long molecules
that are used to give structure and function to biological systems.
Explanation: Cellulose and starch are both sugar polymers. Rubber is a polymer of isoprene
monomers. Wool and silk are both protein polymers.
363.
Cellulose Derivatives
Description: You show that a piece of nitrocellulose (celluloid) is quite clear and tough,
but that it burns nicely. A piece of cellulose acetate (acetate plastic) is
much more practical.
Purpose: To show some of the early synthetic plastics.
Supplies:
1 piece of
clear nitrocellulose sheet (can be made by allowing collodion to dry on a sheet
of shiny aluminum foil. Because the ether solvent in collodion is dangerously
flammable, you should only do this drying in a fume hood or outdoors. Be
careful!)
1 piece of
cellulose acetate plastic
tongs
matches
water (in case of fire)
Procedure: Show that the nitrocellulose (celluloid) sheet is clear and flexible. But then
hold it in the tongs and light it with a match. The nitrocellulose will burn
rapidly and leave no ash. Note that relatively non-flammable cellulose acetate
replaced nitrocellulose.
Explanation: Both nitrocellulose and cellulose acetate can be reshaped in ways that
cellulose itself cannot. However, nitrocellulose is extremely flammable (in its
highly nitrated form it's a high explosive and the principle component of
smokeless powder), so cellulose acetate is a safer choice. It also ages less
and is less susceptible to light damage.
364.
Reptation in Wet Cornstarch
Description: A mixture of cornstarch and water appears liquid-like when you stir it slowly
but feels hard when you poke it suddenly or try to throw it abruptly out of its
container.
Purpose: To show that the long molecules of cornstarch moves slowly past one another
(reptation) in a solution. If you try to deform the solution quickly, the
cornstarch molecules won't permit it to flow. Only if you're patient will it
behave as a liquid.
Supplies:
2 plastic
cups
1 stirring stick
cornstarch
water
Procedure: Half fill one of the cups with cornstarch and gradually add water to it,
stirring carefully with each addition. After you have added a modest amount of
water, the entire powder will be wet and it will begin to flow as you stir
slowly. (Don't add too much water—be patient and stir carefully.) When the
whole mixture behaves like a very thick liquid when you stir slowly, it's
ready.
First show that you can pour the
"liquid" from one cup to the other. You'll see that it doesn't quite
pour normally…it tends to crack as it pours. Next show that if you poke it
quickly with your finger, it feels hard and doesn't get your finger wet.
Finally, hold the cup by its bottom and try to throw its contents at someone.
The mixture will remain in the cup as long as your motion is very rapid.
Explanation: The cornstarch mixture flows slowly because it must wait for the long molecules
to disentangle themselves in order to change its shape. This disentanglement is
done through reptation of the molecules and depends on their thermal energies
and thermal motions. When presented with large, sudden stresses, the mixture
resists deformation. But with time, it flows to relieve those stress.
365.
Glue Putty
Description: You mix white glue, water, and borax to create a soft putty that flows slowly
like a liquid but that tears when exposed to sudden large stresses.
Purpose: To show that the long molecules in glue take time to disconnect from one
another and to disentangle themselves. With patience, the material will flow
but when stressed suddenly, it tears.
Supplies:
1 large
mixing container
1 smaller
container
1 measuring cup
1 stirring stick
white glue
borax
water
Procedure: In the large container, mix about 125 ml of glue and 125 ml of water.
In the small container, dissolve 5 ml of borax powder in about 125 ml
of water. Slowly add the borax solution to the glue, stirring as you do. The
glue will congeal into a blob of glue putty. If you knead this material
carefully and add the right amount of the borax solution, it will be soft and
relatively non-sticky. Show that with time the putty will drip from your hands
or flatten itself into a puddle, but that if you pull on it suddenly, it will
tear into pieces. Is this material solid or liquid? How does time enter into
the answer to that question?
Explanation: The borax molecules form hydrogen bonded bridges between the long molecules of
the glue and effectively tie the whole mass of molecules together into one big
molecule—like weak vulcanization. The water molecules that originally plasticized
the glue are caught up in this network of molecules. Because the hydrogen bonds
are relatively easy to break, the mass can rearrange and flow if you wait.
366.
Slime
Description: You mix solutions of poly(vinyl alcohol) and sodium borate together to get a
gooey glob of slimy plastic. Like glue putty, this material flows slowly like a
liquid but tears like a solid when exposed to sudden large stresses.
Purpose: To show that the long molecules in poly(vinyl alcohol) take time to disconnect
from one another and to disentangle themselves. With patience, the material
will flow but when stressed suddenly, it tears.
Supplies:
poly(vinyl
alcohol) (a white powdery substance available from a chemical supply company)
sodium borate
water
1 heated magnetic
stirrer
several containers
1 stirring stick
Procedure: Dissolve 4 grams of poly(vinyl alcohol) in 100 ml of water (this
recipe can be scaled up). You will have to heat the water to about 70° C
and stir it with a magnetic stirrer for an hour or two. You may want to filter
the resulting solution through a strainer because some of the material just
won't dissolve, no matter how long you wait. In a second container, dissolve
4 grams of sodium borate in 100 ml of water.
To form the slime, slowly stir some of the sodium
borate solution into the poly(vinyl alcohol) solution—about 5 to 10 ml of
the sodium borate solution will be enough. The mixture will form a gooey
elastic material, commonly called "slime."
Explanation: The borate ions in the sodium borate solution form hydrogen bonded bridges
between the long poly(vinyl alcohol) molecules—like weak vulcanization. A vast
network of molecules forms and the water is caught up on that network. Because
the hydrogen bonds are relatively easy to break, the mass can rearrange and
flow if you wait.
367.
Making Plexiglas
Description: You add a tiny amount of catalyst to a test tube of methyl methacrylate and
heat it to about 90° C. About 20 minutes later, the test tube is full of solid
poly(methyl methacrylate) or Plexiglas.
Purpose: To demonstrate a common polymerization process.
Supplies:
methyl
methacrylate (an irritating chemical that makes your eyes tear. Use only in
good ventilation.)
benzoyl peroxide (a
contact explosive—never keep more than a tiny bit around and never let it come
in contact with metals. Use only ceramic or glass containers or scoops. This
stuff is potentially bad news. It's used frequently in chemistry departments
for this very reaction. It's also used in acne medications.)
1 large test tube
1 glass stirring
rod
1 hot water bath
(at about 90° C—don't let it boil because the methyl methacrylate will also
boil and pop out of the test tube…I spoiled a good jacket with this stuff
several years back)
safety glasses
Procedure: Half-fill the test tube with methyl methacrylate and add a pea-sized amount of
benzoyl peroxide (which acts as a catalyst for the polymerization). Stir. Place
the test tube in the hot water bath and cook the mixture for about 20 minutes
at about 90° C. The test tube will then contain a nearly solid, clear mass
that will harden completely when you allow it to cool. You have made a glassy
plastic called poly(methyl methacrylate), Plexiglas, or Lucite.
Explanation: The benzoyl peroxide forms free radicals that initiate the polymerization of
the methyl methacrylate molecules. With the help of thermal energy, the
monomers are consumed and long molecules are formed.
368.
Epoxy
Description: You mix two liquids and stir them together. About 5 minutes later, you
have a solid material.
Purpose: To demonstrate that polymerizations are common in high performance adhesives.
(Students are remarkably unaware of any glues besides superglue and white glue.
They don't understand that superglue polymerizes and doesn't simply dry the way
white glue does.)
Supplies:
1 single-use
pouch of 5 minute epoxy
1 stirring stick
1 piece of
cardboard
Procedure: Tear off the end of the 5 minute epoxy pouch and squeeze both liquids onto
the cardboard. Stir the mixture until it's uniform and leave the stick in it.
About 5 minutes later, it will be completely hard. Discuss the fact that
the glue has not "dried," that it has polymerized into a clear
plastic. All of the atoms that were in the package are still present, but the
molecules have joined together into giant chains that are no longer mobile. The
plastic is in the glassy regime.
Explanation: During polymerization the epoxy rings that are present in the resin molecules
open and link together, forming long chain molecules that are a glassy solid at
room temperature.
369.
Superglue
Description: You squeeze a few drops of superglue (cyanoacrylate monomer) onto a smooth
metal surface and press a second smooth metal surface against it. About
1 minute later, it's difficult to separate those surfaces—they are joined
by long polymer molecules.
Purpose: To demonstrate a polymerization that proceeds simply in the presence of
moisture.
Supplies:
2 pieces of
smooth, flat metal
1 tube of
cyanoacrylate glue (superglue)
Procedure: Squeeze a few drops of the glue onto one of the metal pieces and press the
second metal piece on top. Rub the pieces against one another to distribute the
glue. Leave them pressed against one another for about a minute and then show
that they have bonded together.
Explanation: The cyanoacrylate monomer in this polymerization is quite similar to the methyl
methacrylate monomer used to form Plexiglas. However, cyanoacrylate will polymerize
just in the presence of moisture. Since moisture is everywhere, all you need to
do is squeeze it out onto a surface and it will begin to polymerize. Like
Plexiglas, this cyanoacrylate plastic is glassy at room temperature.
370.
Plastics Fail by Tearing - Piercing a Balloon
Description: To show that plastics fail when a tear propagates through them, you carefully
insert a sharpened knitting needle all the way through a balloon. You carefully
work the needle between the molecules of rubber, so that you don't start a
tear, and the needle don't pop the balloon.
Purpose: To show that polymers break by tearing.
Supplies:
1 good-quality
latex rubber balloon
1 sharpened
knitting needle (or another needle-sharp thin rod with smooth polished edges)
oil
Procedure: Inflate the balloon and tie it off. Place a drop of oil near the nipple portion
of the balloon (where the stresses on its surface aren't as high as elsewhere
and the rubber is relatively thick). Carefully insert the needle through the oil
drop and into the rubber. Twisting the needle helps it find its way between the
rubber molecules. Once you have the needle inside the balloon, aim it at the
bump at the other end (another region of relatively low stress). Carefully push
the needle through that area of the balloon so that it comes out the other
side. You will then have a balloon with a knitting needle passing all the way
through it.
Explanation: As long as you don't start a tear, the rubber will tolerate the insertion of
the needle between its molecules.
371.
Nylon
Description: You pour a solution of adipic chloride in cylcohexane onto a solution of
1,6-hexanediamine in water. A film forms at the interface between the two and
you catch this film with a copper wire. You then pull out the film as a long,
continuous piece of Nylon–6,6.
Purpose: To show how nylon is made from two monomers that form a copolymer.
Supplies:
5% solution of
1,6-hexanediamine in water
5% adipic chloride in
cyclohexane
20% sodium hydroxide in
water
1 clean 100 ml
beaker
1 clean copper
wire
rubber gloves
safety glasses
Procedure: Put about 20 ml of the 1,6-hexanediamine solution in the beaker and add about
20 drops of the sodium hydroxide solution. Now carefully pour about 20 ml of
the adipic chloride solution down the inside of the beaker so that it floats
neatly on top of the other liquid. A film of nylon will appear at the interface
between the two layers. Bend the copper wire into a hook and lift that film out
of the liquid. You'll be able to pull out a continuous strand of nylon almost
indefinitely. Having fresh solutions helps because pure chemicals gives the
longest and strongest nylon molecules.
Explanation: 1,6-hexanediamine is a two-ended base while adipic chloride is essentially a
two-ended acid. The base and acid ends join in a nearly endless chain molecule
when the two chemical are brought together.
372.
Polyurethane Foam
Description: You mix two liquids together in a cup and then wait. In a few moments, a dark
foam rises up in the cup and pours over its sides. In a minute, you have a hard
mushroom of polyurethane foam.
Purpose: To show how polyurethane foam is made.
Supplies:
polyurethane foam
kit (two chemicals that mix to form polyurethane foam—from a hardware or hobby
store)
1 paper or plastic
cup
1 stirring stick
Procedure: Pour equal quantities of the two chemicals into the paper cup (or follow the
directions on the kit). The paper cup should be about ¼ full when you're done.
Stir. In about 20 seconds, the mixture will foam up and begin to overflow the
cup. A minute later, it will be hard to the touch.
Explanation: During its polymerization reaction, the chemicals release carbon dioxide gas.
This gas inflates the hardening plastic and turns it into a foam. Polyurethane
is glassy at room temperature, giving the foam a firm character.
373.
High-Strength Polymers
Description: You step into a loop at the end of normal plastic rope that's attached to the
ceiling and the rope stretches considerably as it begins to support your
weight. You then step into a loop at the end of a high-strength rope that's
also attached to the ceiling. It doesn't stretch noticeably.
Purpose: To show how straight-chain polymers (either liquid crystal materials like
Kevlar or artificially oriented materials like Spectra polyethylene) have
enormous tensile strengths and barely stretch at all.
Supplies:
1 polypropylene
rope (about a quarter inch diameter)
1 Spectra or
Kevlar rope of the same diameter
Procedure: Suspend both ropes from the ceiling and tie loops in them near the ground. When
you step into the loop of the polypropylene rope, the rope will stretch
considerably. But when you step in the loop of the Spectra or Kevlar rope, it
won't stretch. You'll feel like you just stepped onto a steel cable. You can
bounce all you like, but nothing will happen.
Explanation: The molecules in Spectra and Kevlar are all aligned straight so they all work
together to support your weight. Moreover, because they are already aligned
straight, the rope can't stretch without actually stretching or breaking the
molecules. Normal ropes stretch because the molecules are bent or coiled and
they can unwind to give the rope some additional length.
