electric current

An electric current is a flow of electric charges through an electrical conductor. In wires, the charges are carried by electrons. In fluids, the charges may be carried by both electrons and ions (positive or negative). Current is measured in amperes (amps) and is represented by the symbol I.


When an electric current moves continuously in one direction it is called a direct current (DC). When the current fluctuates rapidly back and forth it is called an alternating current (AC). Alternating current is used in almost all household wiring worldwide today. Direct current is commonly seen in battery-operated devices.


Electric current and current density

The electric current, I, through any cross-sectional area of a conductor is defined as the net charge that is transferred through it in one second, that is


    I = dq/dt


This transfer may result from the motion of either positive or negative charges, or both. The mks unit of current, the ampere, represents the passage of 1 coulomb per second. In such a current, if each charge carrier contains a single elementary charge, the number passing the observation surface is 6.25 × 1018.


Direction of a current

Although current is a scalar quantity, we often talk about the "direction" of a current. For historical reasons, the direction referred to is, by convention, the direction of motion of positive charges. So, for example, it is a convention to say a direct current flows from the positive terminal of a battery to the negative, even though, in this case, we know that the current actually consists of electrons flowing from the negative terminal to the positive (the convention of negative and positive was set up long before the discovery of electrons).


By convention, then, the direction of an electric current is from regions of higher potential to regions of lower potential. When the current is caused by the motion of electrons (as in metallic conductors), the direction of motion of the charge is opposite to the direction of the current. If all currents were caused by electron flow, a change in the conventional direction of current might be called for. As it is, however, some currents (e.g., in electrolytic solutions) result from the motion of positive ions, and so complete harmony between the direction of current and charge motion cannot be achieved.