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An EPRI White Paper

Understanding direct current and alternating current

Direct current (DC) is a continuous flow of electricity in one direc- tion through a wire or conductor. Direct current is created by gen- erators such as fuel cells or photovoltaic cells, and by static electric- ity, lightning, and batteries. It flows from a high to a low potential; for example, in a battery, from a positive to a negative pole. Any device that relies on batteries—a flashlight, a portable CD player, a laptop computer—operates on direct current. When represented graphically, DC voltage appears as a straight line, usually flat.

Alternating current (AC) is electricity that changes direction at regular intervals. It builds to a maximum voltage in one direction, decreases to zero, builds up to a maximum in the opposite direc- tion, and then returns to zero once more.This complete sequence, or cycle, repeats, and the rate at which it repeats is called the fre- quency of the current. In the U.S. the AC power provided to a home outlet has a frequency of 60 cycles per second. This is ex- pressed as 60 hertz (Hz), the hertz being a unit equal to one cycle per second.

Imax

0

Time

Amperage Imax

Irms

An alternating current

16.67 ms

33.33 ms

50.00 ms

1 cycle

2 cycles

3 cycles

• 0

Time

Note: Irms = root mean squared, a DC equivalent for same power output

June 2006

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