The basic forms of electric power, alternating current (AC) and direct current (DC), have significant differences. The understanding and use of DC dates to about 1790, with the development of simple chemical batteries. Alternating current theory was developed in the 1880s and was found to be better for sending electric power over long distances. Today, engineers use both AC and DC, applying them to where their advantages best fit a situation's needs.
Engineers call electricity that flows in one direction direct current. Both the current and voltage are steady and predictable. DC power sources have polarity; that is, they have positive and negative terminals. The current flows from the positive side to the negative side through a conductor. If you measure a DC source with a voltmeter, the voltage may drop over time but its polarity will never change. Batteries produce DC, as do solar cells and the adapters used to charge cell phones or mp3 players.
The electricity that comes from a household wall outlet is alternating current. Unlike DC, the voltage and current exist in a smooth rippled shape called a sine wave. The voltage and current increase to a maximum, decrease to 0, reverse polarity, reach a negative maximum, and reverse again. For electricity generated in the United States, these reversals happen 60 times per second. Because of this, AC has no polarity. Mechanical generators and electronic circuits produce AC current.
Voltage and Power
A DC voltage measurement is simple and easy. The voltage reading you make with a voltmeter gives you a true value, within the meter's accuracy. Because AC inherently changes, you usually interpret a voltage measurement as a peak or maximum value.
Though it's a more complex form of electricity, AC has advantages for generation and distribution. Electric companies use high voltages to send electricity over long distances, as less power is lost in the wires along the way. Transformers, which only work with AC, allow the producer to increase or decrease the voltage efficiently and easily.
Electronic gadgets need DC to run. Though your computer plugs into the wall outlet, which is AC, a circuit inside the computer converts the AC to lower-voltage DC which runs the rest of the system. Electronic circuits need a precisely regulated source of voltage in order to work properly. Electrical noise, such as excess ripple from AC, causes sensitive circuits to lose stability. Direct current, such as from batteries or an AC-to-DC converter, provides stable voltages for electronics.