. Why is direct current so much better than alternating current?
It depends on the situation. You cannot use a transformer with direct current, so in that sense, alternating current is better. But many electronic devices need direct current because they require a steady flow of charges that always head in the same direction. So there are times when you need DC and times when you need AC.
. 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 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.
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.
. I have an old car that has a generator instead of an alternator, so I assume it runs DC. What about newer cars? They still use a DC battery right? So what about the alternator? Doesn't that produce AC current? How does that work in a DC circuit?
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.
. If you connect two direct current motors so that the current flowing through one also flows through the other, then turning one motor will cause the other motor to turn as well. If you reverse the direction of rotation, the other motor will also reverse its direction of rotation. Why does this happen?
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.
. 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.
. When flashbulbs were used with cameras, was there a coil in the camera and a magnet, or how did they get it to light? Also, how are flashes used on cameras today different than flashbulbs?
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.
. Why does a moving magnet excite charges?
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.