February 13, 1995
One Minute Papers - Questions and Answers
What happens to the current when it "stops"?
Current refers to moving charged particles. In most solids, the particles that do the moving are negatively charged electrons that move in the opposite direction from the way we say that current is flowing. These charged particles are the components of atoms and molecules, so they are always there inside a wire or the filament of a light bulb, even if they are not moving. Thus when the current "stops", these electrically charged particles simply stop moving. You can imagine a pipe full of water. The water can be flowing to the right or left (a current) or it can be standing still (no current). The water itself, like the charged particles, doesn't disappear when the flow stops.
In the demonstration in which two motors are connected together with wires so turning one motor causes the second motor to turn, why did reversing the direction of the first motor cause the second motor to reverse also?
The demo motors are actually DC motors: they use direct current and turn in a direction that depends on the direction of that current. If you reverse the direction of current flowing through the motor, its direction reverses, too. When you use one of these DC motors as a generator, it produces DC current! The direction of that current depends on which way you turn the motor. Thus as you turn the first motor clockwise, it generates current in a particular direction through the circuit connecting the two motors and the second motor also turns clockwise. If you then reverse the first motor, the current in the circuit reverses and so does the second motor.
How can current alternate -- why doesn't it cancel itself out.
Actually, it does cancel out on the average. When you plug a toaster into the AC power line and turn it on, current begins to flow back and forth through that toaster. At first it flows out one wire of the outlet, through the toaster, and returns into the other wire of the outlet. About 1/120th of a second later, the current has reversed direction and is now flowing out of the second wire of the outlet, through the toaster, and into the first wire. It continues flowing back and forth so that, on the average, it heads nowhere. But the toaster receives energy with every cycle of the current so that there is a net flow of power to the toaster even if there is no net flow of current through it.
How do photoconductors work?
When the atoms and molecules in a solid join together, some of their electrons may become shared between them. These electrons can travel about the solid as waves. Because they travel as waves, they can only follow paths that bring them back perfectly in phase with how they started out, like steady ripples on a pond. As a result, they can only follow certain paths and can only have certain energies. For complex and fundamental reasons, only two electrons can adopt any particular path, so the electrons take turns filling up all of these paths or "levels" from the lowest energy ones up. The electrons fill up these levels until there are no more electrons seeking a path. The behavior of the solid depends on the nature of the levels remaining after all of the electrons have found a path. The last few levels filled with electrons are called "valence levels" and the first few empty levels are called "conduction levels". If there are no more empty levels at energies near the last one filled, the material will behave as an insulator. The conduction levels are far higher in energy than the valence levels. If there are empty levels at energies near the last one filled, the material will behave as a conductor. The conduction levels and valence levels are right nearby. A photoconductor is of the former type: there are no conduction energy levels near the last one filled valence level so it is an insulator. But it becomes a conductor when exposed to light because the light can move the valence level electrons into empty conduction levels at much higher energies.
How do diodes work?
Diodes are made of semiconductors, which are essentially the same as photoconductors. These materials normally have electrons filling all of the valence levels and empty conduction levels. The empty conduction levels are at energies well above those of the valence levels so that electrons cannot easily shift from a valence level to a conduction level, a shift that is necessary for the material to conduct electricity. Thus semiconductors are normally insulating. But when the semiconductor is mixed or "doped" with other atoms, it can become conducting. A doping that removes electrons from the valence levels and leaves some of those levels empty produces "p-type" semiconductor. A doping that adds electrons to the conduction levels produces "n-type" semiconductor. Both "n-type" and "p-type" semiconductors can conduct electricity. But when the two materials touch, the form a non-conducting "depletion" region, where all of the conduction electrons in the "n-type" material near the junction have wandered into the "p-type" material to fill the empty valence levels there. This p-n junction or diode can only carry current in one direction. If you add electrons to the "n-type" side of the junction, they will push into the depletion region and can cross over into the "p-type" side. Thus electrons can flow from the "n-type" side to the "p-type" side; current can flow from the "p-type" side to the "n-type" side. But if you add electrons to the "p-type" side, they fill in empty valence levels in that "p-type" material and make the depletion region even larger. The diode cannot conduct current from the "n-type" side to the "p-type" side. Thus the diode is a one-way device for current.
How do photocells work?
A photocell is just a diode that is specialized to turn light into separated electrical charge. When light hits the "n-type" side of this diode, it adds energy to the valence level electrons there and moves them to the empty conduction levels. These electrons may even have enough energy to leap across the p-n junction into the "p-type" material. Once they get there, they cannot return because of the depletion region and the one-way effect of the diode. Instead, they are collected by wires attached to the "p-type" material, flow out through some electrical circuit, and return to the "n-type" material through another set of wires.
Are there any objects that use compressed air to create electricity?
Moving air is used to create electricity: wind-powered generators. Compressed air is usually created with electrical power, so using it to generate electricity would be inefficient. But wind-powered generators are a common sight in some parts of the country. The wind blows on the turbine blades, doing work on them and providing the mechanical power needed to turn a generator. The generator converts this mechanical work into electrical energy.
How does the flask of boiling water make itself spin?
The flask has two vanes extending from its body and they have small openings the point at right angles to those vanes. The boiling water produces steam, which accelerates out of the openings in the vanes. This acceleration requires a force, so the vanes push on the steam and the steam pushes back. The reaction forces exert torques on the vanes and flask. Since both vanes are made to exert torques in the same direction, the vanes and flask undergo angular acceleration. The whole object begins to spin faster and faster.
When the two motors are connected in a circuit and you turn one so that the other turns, why do the turn at different rates?
The motors both have friction problems. When a small DC current flows through one of those motors, the motor may not be able to overcome static friction so it may not turn. Thus a gentle rotation of the first motor may not cause the second motor to begin turning. It takes a pretty vigorous turn of the first motor to get the second motor spinning. Even then, sliding friction in the motor and its bearings wastes energy so that the second motor turns more slowly that the first motor.
Is it possible for an object attracted by a magnet (such as a paper clip) to retain the energy it gains from its attraction to the magnet?
Actually, this is a complicated question. When a paper clip approaches a magnet, it becomes magnetic. Creating the magnetic field requires energy, so the paper clip's potential energy increases. However, the paper clip's poles are opposite those of the approaching magnet, so the two objects are attracted toward one another. Their magnetic potential energy decreases as they approach. The latter decrease is larger than the former increase so that, overall, the potential energy of the system decreases! The two objects leap toward one another. It then takes work (and energy) to separate them. Sometimes a paper clip retains a bit of its magnetism even after it is pulled away from the magnet. In that case, it does retain some of the magnetic potential energy.
All of the motors you've show were run by some other power that is run by electricity (or by hand). How are we saving energy... or rather, how is it efficient to run electromagnetic generators? It all seems circular.
You are right that generators do not create energy and power, they simply convert it from one form to another. Using an electric motor to turn an electrical generator is very circular (although not completely useless). But a steam-powered generator is quite useful: it takes chemical fuel such as coil or oil and converts its chemical potential energy into electrical energy. Similarly, a bicycle generator converts mechanical energy in the wheels into electrical energy to power a light. You would have trouble making light in some other manner. Even an electric motor running and electrical generator has a place: motor-generators are used to convert AC current into DC current and vice versa. They are also used to convert DC current at one voltage into DC current at another voltage, something that transformers cannot do for DC current.