David Gerstenfeld, age 11, of Staten Island, New York, for his question:

What's the difference between AC and DC?

AC and DC, as all of us know, are different kinds of electrical energy. In both cases, the "C" stands for current, electric current. The "A" stands for alternating, the "D" stands for direct. Obviously alternating and direct current differ because appliances and other electric gadgets are designed to run on either AC or DC, but not on both.

Before the energy crisis, many of us assumed that an unlimited amount of electric current flowed through the wires and would continue forever. Now we realize that most electric generators are run by fossil fuels, and that these supplies are limited. This makes us curious about electricity in general. And when we get right down to the problem, we learn that our limited electric power never did flow through the wires.

This fact is related to the difference between alternating and direct current. Any electric current is the combined energy of zillions of electrons harnessed to move together in formation, like soldiers on parade. Electrons are the negatively charged particles that swarm around the nucleus of the atom. All these negative particles are alike. They have been weighed, which gives some idea of their size. Two electrons weigh one millionth millionth millionth millionth part of a pound. Normally they are attached to atoms, mainly by the opposite positive charge of the nucleus. But stray electrons in outer lanes tend to leave home. An electric generator uses a pushing power called voltage to control these free electrons. When zillions of them move in formation through a wire circuit, their combined, energy creates electric current.

In direct current, voltage pressure pushes free electrons to move in the same direction. The voltage power surges through the wire circuit at almost the speed of light. But the moving electrons merely inch along    hence the current does not flow through the wires.

In alternating current, the moving electrons are forced to move in a fancy two step. Together, they jog forward, then backward in formation. The voltage power surges forward to a peak, subsides and reverses the parade to the opposite direction. This forward and backward motion is repeated many times a second.

In technical terms, each jog to and fro is called a cycle. The number of cycles per second is called frequency. Electrical appliances have tags to show the frequencies they need. If the tag says 60 cycles, it means that the required AC must jog back and forth 60 times per second. And to keep a reading lamp glowing, more than six billion billion electrons must stay on the job.

An AC generator can equal the output of 100 DC generators and very high voltage is needed to send current over long distances. This high power must be reduced for safe use in homes and factories. AC current can be stepped down by transformers which have no moving parts and use no energy. An energy consuming motor is needed to modify DC power. Hence, early in the 1900s, it was decided to generate AC for nationwide power systems.