<Dd> I = V R (\ displaystyle I = (\ frac (V) (R))) </Dd> <P> where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms . More specifically, Ohm's law states that the R in this relation is constant, independent of the current . </P> <P> In alternating current (AC) systems, the movement of electric charge periodically reverses direction . AC is the form of electric power most commonly delivered to businesses and residences . The usual waveform of an AC power circuit is a sine wave . Certain applications use different waveforms, such as triangular or square waves . Audio and radio signals carried on electrical wires are also examples of alternating current . An important goal in these applications is recovery of information encoded (or modulated) onto the AC signal . </P> <P> In contrast, direct current (DC) is the unidirectional flow of electric charge, or a system in which the movement of electric charge is in one direction only . Direct current is produced by sources such as batteries, thermocouples, solar cells, and commutator - type electric machines of the dynamo type . Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams . An old name for direct current was galvanic current . </P>

The type of electrical current in which electrons flow first in one direction then in the other is