An EPRI White Paper
DC Power Production, Delivery and Utilization
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.
Figure 1. Direct current versus alternating current
A direct current
An alternating current
Note: Irms = root mean squared, a DC equivalent for same power output