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".

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.

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.

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.

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.

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.

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.

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.

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.

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.

A photocell is actually a large diode--a one-way device for electric current. Like most diodes, the photocell consists of two different layers of chemically altered or "doped" semiconductors, the anode layer and the cathode layer, and the junction between these two layers has the peculiar property that it normally allows electrons to cross it in only one direction. There is what's called a "depletion region" at the junction, a very thin insulating layer with two electrically charged surfaces--the surface on the cathode side is positively charged and the surface on the anode side is negatively charged.

When an electron, which is negatively charged, approaches the depletion region from the anode side, it first encounters the depletion region's negatively charged surface and is repelled. But when the electron approaches from the cathode side, it first encounters the depletion region's positively charged surface and is attracted. If it has enough energy when it approaches the depletion region from the cathode side, the electron can cross the depletion region to reach the anode layer. Thus electrons can move relatively easily from the photocell's cathode layer to its anode layer but they can't go back.

When a photocell is exposed to light, some of the light particles (photons) are absorbed in the diode's cathode layer. When such an absorption occurs, the photon's energy may be transferred to an electron in the cathode, giving that electron the energy it needs to cross the depletion region and reach the anode. But once the electron has arrived at the anode it can't return to the cathode directly across the depletion region. Instead, it must flow through an external circuit in order to return to the cathode. As that electron flows through the external circuit, it can give up some of its energy, obtained from the light photon, to devices in that circuit. In that manner, light energy has provided energy to an electrically powered device.

Since not all of the light power absorbed by a photocell is converted into electric power, a photocell that's exposed to too much light will overheat. High temperatures are disastrous for all semiconductor devices, including computer chips and photocells. If a semiconductor device overheats slightly, the excessive thermal energy will change the electronic properties of the semiconductor layers so that these layers won't behave as they were chemically prepared to do. In an overheated photocell, charge will be allowed to flow backward so that the photocell will become less energy efficient. But if a semiconductor device overheats seriously, the semiconductor layers will change permanently--atoms, molecules, and entire structures will migrate and rearrange, and the device will never work properly again.

By itself, an overheated photocell won't fail dramatically; it will just stop working. If you've overheated it severely, it will remain broken from then on. But if the photocell is part of a larger collection of power generating elements that continues to produce power, that photocell may suddenly consume all of the power from the other elements. In that case, the photocell may explode as its temperature skyrockets.

In general, metal detectors find metal objects by looking for their electromagnetic responses. For example, you can tell when an iron or steel object is nearby by waving a magnet around. If you feel something attracting the magnet, you can be pretty sure that there is a piece of iron or steel nearby. Similarly, if you wave a strong magnet rapidly across an aluminum or copper surface, you'll feel a drag effect as the moving magnet causes electric currents to flow in the metal surface--electric currents are themselves magnetic.

Of course, a real metal detector is much more sensitive than your hands are, but it's using similar principles to detect nearby metal. Most often, a metal detector uses a coil of wire with an alternating current in it to create a rapidly changing magnetic field around the coil. If that changing magnetic field enters a piece of nearby metal, the metal responds. If the metal is ferromagnetic--meaning that it has intrinsic magnetic order like iron or steel--it will respond strongly with its own magnetic field. If the metal is non-ferromagnetic--meaning that it doesn't have the appropriate intrinsic magnetic order--it will respond more weakly with magnetic fields that are caused by electric currents that begin to flow through it.

In a short range metal detector, the detector looks for the direct interaction of its magnetic field and a nearby piece of metal. That nearby metal changes the characteristics of the detector's wire coil in a way that's relatively easy to detect. But in a longer-range metal detector, the electromagnetic coil must actually radiate an electromagnetic wave and then look for the reflection of this electromagnetic wave from a more distant piece of metal. That's because the magnetic field of the coil doesn't extend outward forever--it dies away a few diameters of the coil away from the coil itself. For the metal detector to look for metal farther away, it needs help carrying the magnetic field through space. By combining an electric field with the magnetic field, the long-range metal detector creates an electromagnetic wave--a radio wave--that travels independently through space. Electromagnetic waves reflect from many things, particularly objects that conduct electricity. So the long-range metal detector launches an electromagnetic wave and then looks for the reflection of that wave. This wave reflection technique is the basis for sonar (sound waves) and radar (radio waves), and it can be used to find metals deep in the ground. Unfortunately, the ground itself conducts electricity to some extent, so it becomes harder and harder to distinguish the reflections from metal from the reflections from other things in the ground.

Windmills extract energy from the wind by rotating as the wind twists them. Whenever an object rotates in the same direction as the torque (the twist) being exerted on it, mechanical work is done on that object. In this case, wind exerts a torque on the windmill's blades and they rotating in the direction of that torque, so the wind is doing work on the blades. Work is the mechanical transfer of energy, so the wind is transferring some of its energy to the blades.

The blades don't keep this newly acquired energy. Instead, they do work on a generator. The generator, which consists of a rotating magnet that spins within stationary coils of wire, uses this energy to generate electricity. The amount of power that a windmill generates depends on the wind speed and the windmill's size, but large windmills can generate in excess of a million watts of electric power.

First, it doesn't matter when the magnet moves past the coils or the coils past the magnet; a generator will work the same way in either case. The voltage produced by the generator is determined by the number of turns in its coils, the strength of its magnet, and the rate at which its magnet turns. The more turns in the coils, the more work the generator does on each charge that passes through those coils and the more voltage the charges have when they leave the generator. The current that the generator can handle is limited by the power of its engine and by the wire's ability to handle the current without wasting too much power. In general, a generator's wire gauge is chosen to minimize power loss while keeping the coils reasonably small and light. If you try to send too much current through the generator, its engine may stall or its wires may overheat.

First, you would need to put enough solar panels in series to develop a voltage greater than that of your battery. For example, to recharge a 1.5 volt battery, you would probably have to attach three or four simple solar cells in series because each one only provides a current passing through it with about 0.5 volts of voltage rise. Having assembled enough solar cells, you should then attach the positive output terminal of the solar cell chain to the positive terminal of your battery and attach the negative output terminal of the solar cell chain to the negative terminal of your battery. When you put the solar cells in the light, they will begin to push electric current backward through the battery and the battery will recharge. Whenever you send current backward through a battery, its electrochemical reactions can run backward and it can recharge to some extent. Unfortunately, some batteries recharge more effectively than others--the bad ones just turn the recharging energy into thermal energy. The only real subtlety in this business is in stopping the charging when the battery is fully recharged. You should check the battery voltage periodically and when it's close to the voltage of a new battery, it probably can't take any more charging.

Solar cells are made in the same way that semiconductor diodes are made. Two different types of semiconductor, p-type and n-type, are joined together to form a diode--a one-way device for electric current. When light energy is absorbed in the n-type portion of the diode, it can propel an electron across the p-n junction between the materials and into the p-type material. Since the electron can't return across the p-n junction to its original location, it must flow through an external circuit to get back. Since it obtains energy from the light that sent it across the junction, the electron can provide that energy to the circuit. The solar cell is thus a source of electric power.

Those three items, electric fields, magnetic fields, and currents, are strongly interrelated. Here are some of those relationships: (1) currents cause magnetic fields, (2) currents that change with time cause magnetic fields that change with time, (3) magnetic fields that change with time cause electric fields, (4) electric fields cause currents to flow in electric conductors. From these relationships, you can see that any time you have a changing current through one circuit, you can end up with a current flowing through another nearby circuit. Power moves from the first circuit to the second circuit with the help of a magnetic field and an electric field. A moving magnet also produces a magnetic field that changes with time and it can send a current through a nearby circuit, too.

If you are trying to light a 60 watt bulb, you must deliver 60 watts of electric power to it (unless you are willing to have it glow relatively dimly). So the answers to your questions are 60 watts of waterpower and 60 watts of windpower. But you are probably more interested in how much water or wind is needed to run those power sources. An efficient water generator that produces 60 watts of power lowers about 6 liters (or one and a half gallons) of water about 1 meter (or 3 feet) each second. An efficient wind generator that produces 60 watts of power stops about 1 cubic meter (or 32 cubic feet) of air moving at 36 km/h (or 21 mph) each second. Finally, a solar powered vehicle needs at least several hundred watts of power to operate. Since solar panels are only about 20% energy efficient and artificial light sources are also only about 10 to 50% energy efficient, it would take thousands of watts of artificial lighting to operate a solar powered car. Not very practical.

In a steam generating plant, water is boiled in a confined container (a "boiler") to produce very high-pressure steam. This steam is allowed to flow through a turbine to the low-pressure region beyond the turbine. A turbine resembles a fan, but one that is turned by the gas that flows through it rather than by a motor. The steam flows through the blades of the turbine and exerts forces on those blades to keep the turbine rotating. The steam loses energy as it twists the turbine around in a circle and this energy is transferred to the rotating turbine. The low-pressure steam is recovered from the end of the turbine. It is then condensed back into liquid water with the help of a cooling tower and then returned to the boiler for reuse.

The rotating turbine is connected to the rotating portion of a generator. This rotating component is an electromagnet and, as it spins, its magnetic field passes across a set of stationary wire coils. Whenever the magnetic field through a coil of wire changes, any current flowing through that coil experiences forces that may add or subtract energy from it. In this case, the rotating magnet transfers energy to the current passing through the wire coils and "generates" electricity. The current in these stationary wires carries away energy from the generator and it is this energy that eventually arrives in your home through the power lines. Overall, the energy flows from the boiler, to the steam, to the turbine, to the generator, to the current, and to your home.

You can make electricity by moving a magnet past a wire. The magnet has a magnetic field around it--something that exerts forces on magnetic poles. If you move the magnet and its magnetic field, you create an electric field--something that exerts forces on electric charges. That's because whenever a magnetic field changes with time, it creates an electric field. This electric field will push on the mobile electrons in a wire. So when you move a magnet past a wire, you are producing a changing magnetic field in the wire. This changing magnetic field produces an electric field and the electric field makes the electrons in the wire accelerate. The moving electrons are electricity. Generators move magnets past wires (or wires past magnets) to produce electricity.

By "waterpower" I assume that you mean hydroelectric power. In that case, water from an elevated source enters a pipe and travels downhill to a generating plant. As the water descends, its gravitational potential energy (the stored energy associated with height and the earth's gravity) becomes pressure potential energy (the stored energy associated with pressure) and kinetic energy (the energy of motion). By the time the water reaches the generating plant, it has enormous pressure and a modest speed.

This moving, high-pressure water is then sent through a fan-like turbine. As the water moves toward the low pressure beyond the turbine, it does work on the turbine's rotating blades and its energy is transferred to those blades. The water gives up its energy and the turbine takes away this energy in its rotary motion. The turbine is attached to an electric generator, which uses moving magnets and wire coils to turn the turbine's rotary energy into electric energy. The electric energy is carried away on wire to be used elsewhere. Overall, the water's gravitational potential energy has become electric energy.

An alternator is a device that uses rotary motion to generate electricity. As the car engine turns, it spins a magnet (the rotor) in the alternator and this spinning magnet induces electric currents in a set of stationary wire coils (the stator). The alternator's ability to generate electric currents by spinning a magnet past stationary wires is an example of electromagnetic induction. Induction is a general phenomenon in which a moving or changing magnetic field creates an electric field, which in turn pushes electric charges through a conducting material. Overall, some of the engine's mechanical energy is converted into electric energy.

The amount of energy given to each electric charge that flows through the wires in the stator depends on the speed with which the magnet turns and the strength of that magnet. Whether it's internal or external, the voltage regulator monitors this energy per charge--also known as the voltage--to make sure that it's correct. If not, it adjusts the strength of the alternator's magnet. It can do this because the alternator's magnet is actually an electromagnet and its strength depends on how much current is flowing through its wire coils. The voltage regulator carefully adjusts the current flowing through the electromagnet in order to obtain the proper output voltage from the alternator. Actually, the alternator itself produces alternating current, so a set of solid-state diodes converts this alternating current into direct current. A car's electric system, particularly its battery, operates on direct current. Since the alternator's operation is the same whether the voltage regulator is inside it or external to it, neither version should be better than the other.

The answer is yes, but the method may not be what you had in mind. While it's possible to make a battery by inserting two dissimilar metal strips into the fruit, the battery that results is really powered by the metals themselves. The fruit juice just acts as an "electrolyte"--an electrically conductive liquid that facilitates the movement of electric charges. Claiming that the fruit is responsible for the energy is like claiming that the stone in "stone soup" (an old tale about a beggar who tricks the villagers in a community into contributing vegetables to spice up the soup that he's making with his magic stone) is really the basis for the soup.

The best way to obtain energy from the fruit is to eat it! The sugars and starches in the fruit have plenty of chemical potential energy that's released when those chemicals are oxidized in your body. This released energy is what allows you to live, work, and play.

There are several ways to measure their efficiencies. One way is to compare the energy these devices extract from sunlight or from the wind to the electric energy they produce. By that measure, solar cells are roughly 15% efficient and windmills are roughly 50% efficient. However, you're probably most interested in their cost efficiency--in how much power these devices can produce for a given operating cost. By that measure, both devices are somewhat more expensive to build and operate than conventional fossil-fuel power plants. As a result, the United States continues to rely on fossil-fuel plants because they cost less for each kilowatt-hour of electric energy produced. Nonetheless, solar cells are gradually becoming cheaper and they may become cost effective in the next decade or two. Windmills are already cost effective in some countries that rely entirely on imported fossil fuels. Denmark, for example, uses windmills extensively for electric power. While windmill power plants do exist in the United States, they are largely the results of regulation rather than market forces. But that, too, may change in the next decade or two.

A moving magnet, which carries with it a magnetic field, creates an electric field. That's just the way our universe works. Changing magnetic fields create electric fields. Since an electric field exerts a force on any electrically charged particle, the charges in a wire are pushed around whenever a magnet moves past them.

Flashbulbs contain a wad of very fine magnesium wire that burns almost instantly in a gas of pure oxygen. The wire is ignited by a small piece of gunpowder-like primer material that is itself ignited by the camera. There are/were three techniques for igniting the primer: impact (a little lever smacked the side of a tube containing the primer and it burst into flame, just like a cap), electric current (a thin filament inside the bulb overheated when current ran through it), and spark (a spark jumped between two wires and ignited the primer). A camera that uses/used the current-ignited bulbs has a battery in it and taking a picture closes a circuit that then sends current through the bulb. A camera that uses/used the spark-ignited bulbs used a piezoelectric spark igniter, like the ones in outdoor gas grills. A camera that uses/used the impact-ignited bulbs just hit the primer itself. Modern cameras uses gas discharges to produce light. Since the flashlamp isn't burned up during a flash, it can be used many times.

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.

DC motors 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 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.

Generators can produce either DC or AC power, depending on how they're arranged. A car generator was one that produced DC power. An alternator produces AC power. Since all cars operate on DC power (they use a battery, after all), the AC power is always converted to DC power. In modern cars, this is done with electronic devices, similar to those used in electronic equipment such as stereos and televisions. Converting DC to AC or vice versa is no big deal anymore. In the old days, it was harder and they used DC generators.

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.

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.

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.

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.