Electric current is flow of electric charge. Suppose a collection of charges is moving perpendicular to a surface of area A, as show in figure.
Charges moving through a cross section
Te electric current is defined to be the rate at which charges flow across any cross-sectional area. If an amount of charge Q passes through a surface in a time interval t, then the average current lavg is given by
The SI unit of current is ampere (A), with 1 A = 1 coulomb/s. Common currents range from mega-amperes in lightning to nano-amperes in your nerves. In the limit t 0, the instantaneous current l may be defined as
Since flow has a direction, we have implicitly introduced a convention that the direction of current corresponds to the direction in which positive charges are following. The flowing charges inside wires are negatively charges electrons that move in the opposite direction of the current. Electric currents flow in conductors: solids (metals, semiconductors), liquids (electrolytes, ionized) and gases (ionized), but the flow is impeded in non-conductors or insulators.