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.

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 contact 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: Contact transferred negatively charged electrons off the silk and onto the Teflon. It also transferred negatively charged electrons off the acrylic rod and onto the silk, leaving the acrylic rod with a net positive charge.

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.

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.

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.

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.

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.

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.

Description: You put opposite charges on the two plates of a capacitor. As you separate the two plates, an electroscope attached to one of the plates shows a rise in its charge, indicating a rise in the voltage of the capacitor plate.

Purpose: To show that separating opposite charges increases the voltage of the positive charges and lowers the voltage of the negative charges.

Procedure: Connect one plate of the capacitor to the electroscope. Remove all charge from the capacitor's plates and move them close together. Put negative charge on one plate and positive charge on the other plate. The electroscope should not deflect very much. Now gradually move the capacitor plates apart. The electroscope should indicate an increase in charge.

Explanation: While the two oppositely charged plates were close together, their opposite charges had little electrostatic potential energy. The positively charge plate had a modest positive voltage and the negatively charged plate had a modest negative voltage. But as you separated the plates and did work on the charges, the positively charged plate's voltage rose dramatically and the negatively charged plate's voltage dropped dramatically. The high voltage (whether positive or negative) pushed additional charge (whether positive or negative) onto the electroscope and it indicated that charge increase.

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.

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.

Description: A van de Graaff 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 de Graaff generator.

Procedure: First examine the components of the van de Graaff 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 de Graaff 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 de Graaff generator accumulates negative charge, so it reaches a very large negative voltage.)

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.

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.

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.

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.

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).

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.

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.

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.

Description: You put positive charge on both a pith ball and a rod, and the two objects repel one another. You then consider whether the rod is repelling the pith ball directly or whether there is something around the rod that is acting to push the pith ball away from the rod -- an electric field.

Purpose: To examine the concept of an electric field.

Procedure: Rub the silk on the acrylic rod and it becomes positively charged. Touch the acrylic rod to the pith ball to put positive charge on that ball. Recharge the acrylic rod. Now bring the rod near the pith ball and they'll repel one another. Discuss the grade-school explanation for this behavior--that the positive charge on the rod is repelling the positive charge on the pith ball. Now block everyone's view of the rod with the piece of cardboard, but all them to see the pith ball. You can still make the pith ball move, even though they can't see what's making it move. Discuss the idea that there is something in space that is pushing on the pith ball, something that may or may not be created by charge and that influences charges that are immersed in it. That something is an electric field.

Explanation: The positive charge on the acrylic rod produces an electric field and that field, in turn, pushes on the pith ball's positive charges.

Description: Felt sprinkled onto a bath of oil forms lines along the electric field direction created by probes from a static generator.

Purpose: To show that electric fields are real.

Procedure: Insert the electrodes into the oil bath, separated by sufficient distance that no sparks will occur when you operate the static generator. Sprinkle a light dusting of felt dust onto the oil. Then operate the static generator. The felt should move to form lines of electric flux between the two electrodes.

Explanation: The electric field polarizes the felt particles so that they cling together positive end to negative end in long, visible chains. Since the polarization is along the field direction, the chains are also along the field direction.

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.

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. Viewed in terms of an electric field, the ball acquires a charge from the plate it touched last and then the electric field (a voltage gradient) pushes it toward the other plate.

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.

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 and the electric field is strong enough, the candle flame will become severely distorted and will probably extinguish itself.

Explanation: The charged particles in the flame are pushed by the electric field 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.

Description: A sharp needle attached to one side of a static generator exhibits a white-blue glow in a dark room.

Purpose: To show that a corona discharge occurs when a sharp object is maintained at high voltage.

Procedure: Tape the pin to one ball of the Wimshurst static generator so that the tip of that needle points toward the other ball, a couple of centimeters away. Darken the lights and crank the static generator. A faint glow will develop around the needle tip as charge passes from it to the air as a corona discharge.

Explanation: The high-voltage needle initiates a corona discharge and sprays charges into the air. The charges acquire enough energy as they move through the high electric field around the tip to cause the air molecules to glow.

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.

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.

Description: A cadmium-sulfide photoconductive cell measures the amount of light reaching its surface.

Purpose: To show how light can convert a photoconductor from an insulator to a conductor.

Procedure: Connect the CdS cell to the ohm meter. Show that the darkened CdS cell is basically an insulator. Now expose the CdS cell to light and show that it becomes electrically conducting.

Explanation: Light promotes electrons from the filled valence levels in the CdS cell to the empty conduction levels in the CdS cell. This shifting of charges allows the CdS to transport electric charge. The ohm meter applies a modest electric field across the CdS cell and, when light is present, electric charge begins to flow through the CdS cell.

Description: Charge is deposited on an insulating surface with an array of sharp, electrically charged points. The charge in some areas of the surface is erased with your finger. Finally, felt dust is sprinkled on the surface and it sticks to those areas that are still charged.

Purpose: To illustrate the xerographic process, although without the photoconductive aspect.

Procedure: Attach or glue the clear plastic sheet to the surface of the metal sheet. Make sure that the entire surface of the metal sheet is covered by plastic. Mount the sandwich on the support and ground the metal sheet with one wire. Attach the metal screening to the stick and connect it to sphere of the van der Graaf generator with the other wire. Turn on the van der Graaf generator and brush the metal screening across the plastic coated surface. This action will cover the plastic with charges. Turn off the van der Graaf generator. With you finger, rub an identifiable mark on the plastic surface. While you won't see anything, you will have removed charge from part of the plastic surface. Now sprinkle the felt dust on the plastic surface and blow away any excess. You should see the mark you made as a light region in an otherwise relatively dark background on the plastic sheet.

Explanation: The charged metal screening deposited electric charge on the plastic surface. When you touched parts of the plastic surface, you provided a way for the charge to escape from the surface and erased those parts of the surface. In a real xerographic copier this erasure is done by light, which turns the photoconductor (here the plastic layer) into a conductor so that the charge can escape into the metal sheet. When you then sprinkle dust onto the sheet, the dust is attracted to any charged portions of the sheet. (In a real xerographic copier, the toner particles are charged by their carrier system. Here, the felt dust isn't explicitly charged and is held in place largely by polarization effects.)

Follow-up: I plan to build a metal sheet with a real photoconductor surface—one that will be sensitive to blue light but insensitive to red light. I will be able to work with it in class under red illumination and expose it to a pattern of blue light to form a charge image. When I have a version that works, I will post information about it on the demonstration web site.

Section 10.3 Flashlights