Alternating current (AC) is electric current that regularly reverses its direction of flow. Produced by AC generators, AC is used in most homes and businesses (see Electric generator ). By contrast, direct current (DC) does not alternate. It always flows in the same direction and is produced by batteries and DC generators.
Each time an alternating current completes two changes of direction, it goes through one cycle. The number of cycles per second is called the frequency of the AC. Frequency is measured in units called hertz (Hz). The United States generates power at 60 Hz. Many other countries use 50 Hz.
Direct current operates automobile electric systems, locomotives, and some types of motors used in industry. Radios, televisions, computers, and other electronic devices use AC as their power source, but they need DC to operate their internal circuits. Devices called rectifiers change AC into DC.
Alternating current is generated at electric power plants and sent out through power lines. Power plants generate AC because it is both easier to produce and easier to boost to higher voltages compared with DC. Power companies boost voltage before transmitting current through power lines to limit energy loss. Energy loss occurs because current traveling through a wire heats the wire. To transmit a given amount of power, high voltage allows the current to be much smaller with less energy loss.
However, the high voltages used in power lines are not safe to use in homes. Devices called transformers decrease the voltage before the current enters the home. See Transformer .
See also Electric current ; Electric power (Transmission) ; Electricity .
