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10 Most Recent Questions and Answers (out of 34)

1451. How can I make an electric generator from scratch? — OD
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Generators and motors are very closely related and many motors that contain permanent magnets can also act as generators. If you move a permanent magnet past a coil of wire that is part of an electric circuit, you will cause current to flow through that coil and circuit. That's because a changing magnetic field, such as that near a moving magnet, is always accompanied in nature by an electric field. While magnetic fields push on magnetic poles, electric fields push on electric charges. With a coil of wire near the moving magnet, the moving magnet's electric field pushes charges through the coil and eventually through the entire circuit.

A convenient arrangement for generating electricity endlessly is to mount a permanent magnet on a spindle and to place a coil of wire nearby. Then as the magnet spins, it will turn past the coil of wire and propel currents through that coil. With a little more engineering, you'll have a system that looks remarkably like the guts of a typical permanent magnet based motor. In fact, if you take a common DC motor out of a toy and connect its two electrical terminals to a 1.5 V light bulb or a light emitting diode (try both directions with an LED because it can only carry current in one direction), you'll probably be able to light that bulb or LED by spinning the motor's shaft rapidly. A DC motor has a special switching system that converts the AC produced in the motor's coils into DC for delivery to the motor's terminals, but it's still a generator. So the easiest answer to your question is: "find a nice DC motor and turn its shaft".


1403. How does a light-detecting diode create voltage when light hits it? — T
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Diodes are one-way devices for electric current and are thus capable of separating positive charges from negative charges and keeping them apart. Those charges can separate by moving away from one another in the diode's allowed direction and then can't get back together because doing so would require them to move through the diode in the forbidden direction. Given a diode's ability to keep separated charges apart, all that's needed to start collecting separated charges is a source of energy. This energy is required to drive the positive and negative charges apart in the first place. One such energy source is a particle of light—a photon. When a photon with the right amount of energy is absorbed near the one-way junction of the diode, it can produce an electron-hole pair (a hole is a positively charged quasiparticle that is actually nothing more than a missing electron). The junction will allow only one of these charged particles to cross it and, having crossed, that particle cannot return. Thus when the diode is exposed to light, separated charge begins to accumulate on its two ends and a voltage difference appears between those ends.

1396. If I pinch a sheet of aluminized Mylar between two concentric circular rings and weight the middle of the sheet with water so that it sags into a curved shape, like a parabola, is there an adhesive such as fiberglass which I can adhere to the back surface to stiffen it so that I can make a giant reflective surface to serve as a solar collector? — AM, Weldon, CA
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What a great idea! Mylar is DuPont's brand of PET film, where "PET" is Poly(ethylene terephthalate)—the same plastic used in most plastic beverage containers (look for "PET" or "PETE" in the recycling triangle on the bottom). PET isn't a particularly inert plastic and you shouldn't have any trouble gluing to it. To form a rigid structure, you need either a glassy plastic backing (one that is stiff and brittle at room temperature) or a stiff composite backing. I'd go with fiberglass—mount the Mylar in a large quilting or needlepoint frame, coat the back of the Mylar with the glass and epoxy mixture, invert it, weight it with water, and let it harden. Mylar doesn't stretch easily, so you'll get a very shallow curve and a very long focal length mirror. While the mirror will probably have some imperfections and a non-parabolic shape, it should still do a decent job of concentrating sunlight.

1236. If you wrap a three-phase power cord into a coil and allow it to deliver power to equipment, will the coil develop magnetic fields and, as a consequence exhibit both an inductive reactance and a voltage drop? — JH
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If any current reaching the equipment through the three-phase power cord returns through that same power cord, then the net current in the cord is always exactly zero. Despite the complicated voltage and current relationships between the three power wires, one simple fact remains: the equipment can't store electric charge. As a result, any current that flows toward the equipment must be balanced by a current flowing away from the equipment, and if both flows are in the same power cord, they'll cancel perfectly. Since there is no net current flowing through the power cord, it develops no magnetic field and exhibits no inductive reactance or voltage drop.

1235. Does a moving magnet use up its energy when it generates electricity? Does this mean that the term "permanent magnet" is a misnomer because its magnetism can be used up? — MT, San Antonio, TX
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When a moving magnet generates electricity, it does transfer energy to the electric current. However, that energy comes from either the magnet's kinetic energy (its energy of motion) or from whatever is pushing the magnet forward. The magnet's magnetism is basically unchanged by this process.

Nonetheless, a large permanent magnet isn't really permanent. The random fluctuations of thermal energy and the influences of passing magnetic fields gradually demagnetize large permanent magnets. However, good permanent magnets demagnetize so slowly that the changes are completely undetectable. You might have to wait a billion years to detect any significant weakening in the magnetic field around such a magnet.


1224. How much current can a power generator produce and how does that current vary as you introduce more load onto the generator?
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There is no fundamental limit to how much current a generator can handle, however, the characteristics of the generator's wiring, its magnetic fields, and the machinery turning it all tend to limit its current capacity. A generator's wires aren't perfect and, as the current passing through the generator increases, its wires waste more and more power. Like any wiring, a generator's wires convert electric power into thermal power in proportion to the square of the current. Thus if you double the current in the generator, you quadruple the power loss. While this power loss and the resulting heat are trivial at low currents, they become serious problems at high currents.

Increasing the current in the generator also affects its magnetic fields because currents are magnetic. At a low current, the current's magnetism can be ignored. But when a generator is handling a very large current, the magnetic fields associated with that current are no longer small perturbations on the generator's normal magnetic fields and the generator may not perform properly any more.

Finally, a generator's job is to transfer energy from a mechanical system to the electric current passing through it. As the amount of current in the generator increases, the amount of work that the mechanical system provides must also increase—the generator becomes harder to turn. There will always be a limit to how much torque an engine or crank can exert on the generator to keep it spinning and thus there will be a limit to how much current the generator can handle.

As for how the current varies with load: the more current the load permits to pass through it, the more current will pass through the generator. Assuming that the generator is well built and has very little electric resistance, the load will serve to limit the current. The generator will then deliver just as much current as the load will permit. If the load permits more current, the generator will deliver more. As a result, the wires in the generator will waste more power as heat, the magnetic fields in the generator will become more complicated, and the device powering the generator will have to work harder to keep the generator turning.


1198. How does the tachometer in a new car work? It looks like a magnet wrapped with wire that's located very near a saw-toothed wheel that spins as the engine turns. — TR, Provo, UT
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The device you describe is essentially an electric generator. The toothed wheel is made of pure iron so that its teeth can become temporarily magnetized while they are close to the permanent magnet. When a tooth becomes magnetized as it approaches the permanent magnet, or demagnetized as it moves away from the permanent magnet, it changes the shape and strength of the magnetic field around the permanent magnet. Since changing magnetic fields produce electric fields, the tooth's movement causes an electric field to appear around the magnet. This electric field pushes on mobile electric charges in the wire coil wrapped around the magnet and generates electricity. The current in the coil flows one way as a tooth approaches the magnet and reverses when that tooth moves away from the magnet. Also, the faster the tooth moves, the stronger the change in the magnetic field and the higher the voltage generated in the coil. The tachometer can tell how fast the engine is turning by how frequently the current in the coil reverses directions or by how much voltage the coil generates.

1191. During a recent ice storm, I was standing in my front doorway before dawn and the entire southern sky turned brilliant blue-green for about five seconds or more. What caused this effect? People who missed it tell me it was just a transformer "blowing up" but I've seen one blow up on our street and there is no comparison. The light I saw virtually filled the entire horizon.
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You probably saw a sustained high-voltage arc between high-tension wires and/or the ground. I would guess that the ice pulled down one of the wires or caused a tree to fall across them. While transformer explosions often involve hundreds of kilowatts of electric power being turned into light and heat, most of that light is hidden from view inside the transformer. Such an explosion can be dramatic, with some nice sparks and flashes, but it's usually not very bright. However, when a high-tension wire arcs, a significant fraction of the many megawatts of power flowing through the arc is converted directly into light. In effect, a high-pressure arc lamp forms right in the air and it looks like a camera flash that just keeps going until something stops the arc or the power is shut off. The blue-green color you saw comes from characteristics of the air and metal wires involved in the arc. As you saw, a couple of million watts of light are enough to light up the predawn sky quite effectively!

There is, however, an alternative explanation: you may have seen the "green flash" that occasionally appears just as the sun reaches the horizon at sunrise or sunset. This flash is a refraction effect in the atmosphere in which only blue-green light from the sun reaches the viewer's eyes for a second or two while the sun is just below the horizon. However, this green flash should appear in the eastern sky just before dawn, not the southern sky.


1185. With the amount of wind that's produced by high-speed traffic on expressways, why don't electric companies put wind-powered generators in the center lanes? Using this (wasted) wind to generate electricity would be cheaper, safer, and environmentally friendlier than the power plants that they are running now. — DJA
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While wind generators are being used experimentally to charge batteries in roadway equipment that can't be reached with power lines, there are at least three reasons why such generators aren't in large scale use. First, wind generators that connect to the AC power grid work most efficiently when they turn at a steady rate—the generator itself must remain in synch with the cyclic alternating current in the electric power lines. The intermittent and sporadic winds produced by passing cars and trucks aren't really suitable for such wind generators.

Second, to make efficient use of the wind created by traffic, hundreds of wind generators would have to be installed on each mile of expressway. Since wind generators are expensive, it's much more cost effective to put them on windy ridges out in the country or by the seashore.

Third, the wind generators you propose would actually extract energy from the cars and trucks and reduce their gas mileages! That fact might surprise you, since it would seem that extracting energy from the wind wouldn't have any effect on the cars and trucks that created that wind. But the wind and the vehicles continue to interact as they move along the expressway—each vehicle drags a pocket of air with it and interfering with this air pocket has the effect of interfering with the vehicle! The vehicle uses energy to maintain this moving air pocket and it burns additional fuel. An aerodynamically well-designed vehicle has a relatively small air pocket, but there is a limit to what can be done. To reduce the energy cost of maintaining the air pocket, the vehicle's driver can steer it into the air pocket behind another vehicle so that the two vehicles share a single air pocket. The lead vehicle then provides most of the energy needed to keep the air pocket moving. This technique of sharing an air pocket is called "drafting" and is frequently used by bicycle racers. But while drafting makes it easier for many vehicles to keep their air pockets moving, the wind generators that you propose would make it harder—they would steal energy from the air pockets of every passing vehicle and make those vehicles fight harder to keep their air pockets moving.

A better way to save energy would be to encourage large-scale drafting in some safe way. Having chains of independent cars tailgate one another would be energy efficient, but would cause horrific accidents. However, assembling those cars into a tightly coupled "train" may someday become possible with advances in technology and computer controls.


1180. If I measure current from a photocell, am I indirectly measuring power as well? — MR
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As long as current is free to flow from one end of the photocell to the other, the amount of current flowing through that circuit is almost exactly proportional to the number of light particles (photons) striking the photocell each second. Since the rate at which photons strike a photocell is generally proportional to the light power striking that photocell, you can use a measurement of current to make a measurement of light power. While there are a few subtle details that you must be careful about, particularly changes in the light spectrum and unanticipated impediments to the free flow of current through the circuit, this relationship between the current and the light power is very useful. For example, most camera light meters use photocells to determine exposures.

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