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Suppose a potential difference is established between two points, and some charges are released. These charges will be acted on by the electrical force and start to move. If we measure the amount of charge that flows by a given point in a certain time interval, we can define the current (i.e. the charge flowing through that point per unit time) via

\fbox{\parbox{4.5in}{\vspace*{7pt}Current = Charge / time\vspace*{7pt}}}

The units of charge are Coulombs / second, or Ampères (A).

Physical currents in electrical circuits consist of electrons in the wire moving from lower to higher electrical potential. Since electrons by convention havenegative charge, the net effect is equivalent to having a positive current flow from higher to lower electrical potential.

There are two distinct types of currents we encounter in everyday life. A direct current (DC), like that coming from a flashlight battery, is a current which always flows in one particular direction. An alternating current (AC), like that coming from a household plug, is one which alternately flows in one direction and then the other.