Author: Lou Bloomfield

What is the principle behind adding salt to water to keep the boiling temperatur…

Lou Bloomfield Leave a comment

What is the principle behind adding salt to water to keep the boiling temperature lower? Do other substances have the same effect?

Actually, it’s the other way around! Adding salt or sugar (or anything else that dissolves in water and that doesn’t boil easily itself) to water actually raises the water’s boiling temperature! That’s because the salt or sugar molecules interfere with the evaporation of water molecules and boiling is just a special type of evaporation.

Boiling occurs when the evaporation of water molecules becomes so rapid that bubbles of evaporating water molecules form inside the body of the water itself and are able to grow larger and larger, despite the crushing pressure of the surrounding atmosphere. Below water’s boiling temperature, any bubble of water vapor that forms inside the body of the water will be smashed almost instantly. But at water’s boiling temperature, the pressure of water vapor inside each bubble is high enough to keep the bubble from being crushed. However, adding sugar or salt to the water makes it harder for water molecules to enter one of these water vapor bubbles because the water molecules in the water cling to the salt or sugar molecules and thus don’t evaporate as often. With fewer water molecules entering a water vapor bubble, that bubble can’t sustain itself and is crushed. Only when you heat the salty or sugary water above the boiling temperature of pure water is there enough evaporation into each water vapor bubble to support it against atmospheric pressure.

Why do people put salt on icy sidewalks in the winter?

Lou Bloomfield Leave a comment

Why do people put salt on icy sidewalks in the winter?

Whenever a molecule dissolves in water, the water molecules bind to that molecule and surround it, forming a shell of water molecules around the impurity. Salt water is filled with these tiny balls of water, each one surrounding a single salt ion (either a sodium positive ion or a chlorine negative ion). These little water balls can’t crystallize into ice because ice can’t fit a sodium ion or a chlorine ion into its orderly structure. As a result, the presence of salt in the water makes it harder for the water to crystallize into ice. The water has to exclude the salt from the crystals that form as it freezes and this difficult process requires that the salt water be cooled below the freezing temperature of pure water before it will freeze. The more salt the water contains, the lower the temperature at which that salt water will freeze. This effect even works when you just sprinkle salt on ice. As long as the temperature of the ice isn’t too cold, the salt will begin to dissolve in the water molecules of the ice and ice’s crystalline structure will begin to break down. The result will be a puddle of cold salty water. That’s why people use salt to melt the ice on sidewalks. But if the ice is too cold, the salt will remain separate and the ice will stay pure ice. That’s why salting only works when the temperature isn’t too far below freezing.

How does Styrofoam work?

Lou Bloomfield Leave a comment

How does Styrofoam work?

Styrofoam is a rigid foam consisting of gas trapped in the closed bubbles of polystyrene. Polystyrene itself is a clear plastic that’s used in many disposable food containers. It’s a stiff, amorphous solid at temperatures below 100° C, where amorphous means that it has none of the long-range order associated with crystalline solids. The long, chain-like polystyrene molecules are arranged like a tangled bowl of spaghetti noodles. Amorphous plastics tend to be clear because they’re very homogeneous (uniform) internally and let light passes through them without being deflected or reflected. Plastics that are partially crystalline tend to be white. I think that items bearing the #5 recycling label are made of polystyrene.

But when air or another gas is injected into melted polystyrene and the mixture is beaten to a froth, it forms a stiff white solid when it cools. The whiteness comes about because of inhomogenieties—the gas spoils the uniformity of the plastic so that light is deflected and reflected as it passes through the material. The Styrofoam retains the rigidity of the polystyrene plastic below 100° C, so that it’s suitable for beverage containers for liquids that are no hotter than boiling water. At one time, one of the gases used to make polystyrene foams was Freon, but I believe that Freon is no longer used for this purpose.

How are some light emitting diodes able to emit more than one color? Can light e…

Lou Bloomfield Leave a comment

How are some light emitting diodes able to emit more than one color? Can light emitting diodes emit different amounts of light or can they only be on or off?

Light emitting diodes (LEDs) that emit more than one color are actually two different LEDs connected to a single circuit in opposite directions. When current flows in one direction around that circuit, one of the LEDs emits light. When the current reverses directions, the other LED emits light. And when the current reverses directions rapidly, both LEDs emit light alternately. If one LED emits red light and the other green light, then the overall device will appear yellow or orange when they are both operating alternately in rapid sequence. The amount of light that an LED emits depends on the current flowing through it—the more electrons that are falling into holes in the p-type semiconductor, the more light that’s being emitted. However, many devices that use LEDs just turn them on or off because that’s easier than controlling the current flowing through them. Some day, flat panel displays may use three colors of LEDs—red, green, and blue—in order to present full color images like those on a current television screen. For that scheme to work, the LEDs must be able to emit different brightnesses, so the current flowing through each one must be adjustable.

How do light emitting diodes work and what is responsible for their different co…

Lou Bloomfield Leave a comment

How do light emitting diodes work and what is responsible for their different colors?

Light emitting diodes are diodes that have been specially designed to emit light rather than heat during their operations. Whenever current is flowing through a diode, electrons are moving from the n-type semiconductor on one side of the diode’s p-n junction to the p-type semiconductor on the other side of the junction. Once an electron (which is negatively charged) arrives in the p-type semiconductor, it’s attracted toward an electron hole (which is positively charged) and the two move together. The electron soon fills the hole and it releases a small amount of energy when it does. In a normal diode, electrons lose energy at a rate of 0.6 joules of energy per coulomb of charge as they recombine with the electron holes. That means that the current flowing through the normal diode loses 0.6 volts as it flows through the diode. The missing energy becomes thermal energy or heat.

But in a light emitting diode (an LED), each electron that arrives in the p-type semiconductor after crossing the p-n junction recombines with an electron hole in a remarkable way. It gives up its extra energy as light! Each time an electron and an electron hole recombine, they emit one particle of light, a photon, and the frequency, wavelength, and color of that light depends on the amount of energy given up by the electron as it falls into the electron hole. The semiconductor material from which an LED is made has a characteristic called its band gap. This band gap measures the energy needed to pull an electron away from an electron hole in the material. If this band gap is small, the LED will emit infrared light. If this band gap is larger, the LED will emit red, orange, yellow, green, or even blue light (the farther to the right in that list, the more energy is required). Because each electron loses more energy in recombining with an electron hole in an LED than it would in a normal diode, the current flowing through an LED loses more voltage (typically 2 volts for red LEDs and as much as 4 volts for blue LEDs) than does the current flowing through a regular diode (typically 0.6 volts).

Physicists, chemists, materials scientists, and engineers have been working for years to perfect the materials used in LEDs, making them more and more efficient at turning the electrons’ energies into light. Until recently, there were no suitable materials from which to build blue LEDs, but recent developments of large band gap semiconductors have made blue LEDs possible. In fact, even blue laser diodes are now being made. A laser diode is a specially designed LED in which all of the photons are copies of one another rather than being emitted independently by the individual electrons as they drop into their respective electron holes.

One final note: it’s now possible to obtain a “white” LED! This device is actually a blue LED, combined with a fluorescent phosphor that converts the blue light into white light.

How does the wattage of a candle compare to the wattage of a light bulb?

Lou Bloomfield Leave a comment

How does the wattage of a candle compare to the wattage of a light bulb?

A 60 watt light bulb emits about 6 watts of visible light while wasting the remaining 54 watts of electric power as other forms of thermal energy. A candle probably also consumes about 60 watts of chemical energy (the paraffin wax) but emits much less than 3 watts of visible light. The light bulb is clearly not very efficient at converting electric power into visible light but the candle is even less efficient. That’s because the candle flame operates at a lower temperature (about 1700° C) than the filament of the light bulb (about 2500° C) and the spectrum of light emitted by a hot object depends strongly on its temperature. The cooler flame emits relatively more infrared light and less visible light (particularly blue light) than the hotter filament.

How does a magnet work and is there a way that I can determine which end of the …

Lou Bloomfield Leave a comment

How does a magnet work and is there a way that I can determine which end of the magnet is north and which end is south?

The magnetic fields that are responsible for the interesting behaviors of magnets can be created either by (1) moving electric charge or (2) changing electric fields. We can ignore the second process because it has very little to do with permanent magnets. Instead, let’s focus our attention on the first process: moving electric charge producing magnetic fields. Whenever electric charges flow through a wire, a phenomenon that we call an electric current, they create magnetism. Many appliances use electricity and electric currents to create magnetism, notably televisions, motors, and audio speakers. But a permanent magnet doesn’t use an obvious electric current to create its magnetic field. Instead, it uses the spinning character of the electrons inside the material from which that magnet is made. Electrons are electrically charged and they have an intrinsic spinning character. A simplistic view of an electron is as a spinning, electrically charged ball. Since its charge is in motion, an electron acts as a magnet and has both a north pole and a south pole. In most materials, the magnetic electrons are turned in opposite directions, canceling out one another’s magnetism so that the overall material is non-magnetic. But in a few special materials, including most steels, the cancellation is imperfect and some magnetism remains. In a permanent magnet, this remaining magnetism is particularly apparent. The material is, in effect, a big collection of magnetic electrons that all work together to create a large magnet.

To determine which end of a permanent magnet is its north pole and which is its south, take a compass and hold it a reasonable distance from one end of the magnet. If the north end (often the red end) of the compass needle points toward this end of the magnet, you know that this end of the magnet is a south pole! That’s because opposite poles attract and the “north” end of the compass needle, a north pole, is attracted to south poles. Interestingly enough, the magnetic pole near the earth’s geographic north pole is actually a south magnetic pole. That’s why the north pole of the compass needle points toward the earth’s north geographic pole. When you use a compass to detect which pole of the magnet is north, be careful not to bring the compass needle too close to the permanent magnet. A strong permanent magnet can remagnetize the compass needle and reverse its poles. To make sure that this hasn’t occurred, check to see whether the compass still points toward the north pole after you bring it near strong permanent magnets.

How do power lines work and what is the purpose of all the electrical things you…

Lou Bloomfield Leave a comment

How do power lines work and what is the purpose of all the electrical things you see behind the fences with signs saying “Warning: High Voltage”?

Electric power is distributed over long distance using high voltages and relatively low currents. Since the amount of power that flows through a wire is equal to the product of its voltage (the amount of energy carried by each unit of electric charge) and its current (the number of units of electric charge that flow through the wire each second), the electric company can distribute its power either as a large current at low voltages or a small current at high voltages. But it turns out that the amount of power that’s wasted by electricity as it flows through a wire is proportional to the square of the current in that wire. Thus the more current that flows through a wire, the more power that wire turns into thermal energy (or heat). To minimize this energy loss, the power company uses transformers to convert the electricity to small currents at very high voltages for transmission cross country. Near each community, there is then a power substation at which this very high voltage power is converted to lower voltage forms. Even in neighborhoods, they use medium currents at moderately high voltages to avoid power wastage. Only in the vicinity of your home is the electricity finally converted by transformers to a large current at low voltage for safe delivery to your appliances. You’ve probably seen those final transformers as the gray oil-drum sized units on utility poles or the green boxes on front lawns. But despite all this effort to minimize power loss, something like 6% of the electric power generated in this country is lost in the delivery process.

How does magnetic recording work?

Lou Bloomfield Leave a comment

How does magnetic recording work?

During the recording process, an electromagnet in the recording head magnetizes the surface of a specially coated tape. This tape is coated with a thin layer of plastic that’s impregnated with tiny cigar-shaped magnetic particles. As the tape moves past the recording head, the head magnetizes these particles back and forth to a certain depth, according to the audio signal reaching the recorder from the microphone. The higher the pitch of the sound, the more frequently the direction of magnetization reverses. The louder the volume of the sound, the deeper the magnetization extends into the layer. During playback, this magnetized layer moves past the playback head and induces electric currents in it. These currents are then amplified and used to reproduce the sound. A much more detailed discussion of this process appears in my book.

How were tape recorders invented?

Lou Bloomfield Leave a comment

How were tape recorders invented?

Magnetic recording dates to 1898, when Danish engineer Valdemar Poulsen developed a method for recording sound on a steel wire. He stretched this wire across his laboratory and put the recording apparatus on a trolley that traveled along that wire. He would run along with the moving trolley, talking into its microphone to record sound on the wire. To play back this sound, he would roll a second trolley containing the playback equipment along the wire and it would reproduce the sound. Having proven the principle of magnetic recording, Poulsen and others began to develop wire recorders. In these devices, a wire rolling from one drum to another was used to record and play back sound. In 1927, American inventor J. A. O’Neill replaced the wire with a magnetically coated ribbon and since then magnetic tape recorders have dominated the recording industry.