Alternating current is the most common form for transmitting electricity from the power plant to homes and businesses. It is convenient because the voltage can be changed easily with transformers. High voltage is good for long-distance transmission of electricity because it means less current has to flow for the same amount of power, and power is lost in the resistance of wires in proportion to the square of the current. High voltage is good for distribution, but it is disastrously unsafe inside the home, so it has to be transformed down to something safer.
Alternating current is generated at the power station by rotating a coil of wire inside a big magnet. The energy needed to make the coil turn is provided by burning fuels such as coal, oil, natural gas, or by harnessing air, water, or nuclear power to do it. Faraday's law of induction relates the change in the voltage around the coil of wire to the rate of change of the amount of magnetic flux through the wire. If the wire turns around in circles, the voltage will be positive, then negative, then postitive again. Because the voltage swings back and forth between positive and negative values, at some times it crosses zero.
Direct current is very useful in powering small things like flashlights and radios and toys. Batteries are the most common source of direct current -- the voltage of a battery is constant. Things that need lots of power in the form of direct current, like computers, have power supplies inside which transform alternating current from the wall plug to direct current, so they don't run out of batteries.
Thomas Edison orignally proposed distributing electricity using direct current, but Charles Steinmetz invented the idea of distributing AC electricity, seeing its advantages.
(republished on 07/13/06)