Electric current is the flow (movement) of electric charge. The SI unit of electric current is the ampere. Electric current is measured using an ammeter.

The electric charge may be either electrons or ions. The nature of the electric current is basically the same for either type.Fact: date=September 2008

Current in a metal wire

The current I in amperes can be calculated with the following equation:

I = {Q \over t}

where

Q \!\ is the electric charge in coulombs (ampere seconds)

t \!\ is the time in seconds

It follows that:

Q=It \!\ and t = {Q \over I}

More generally, electric current can be represented as the time rate of change of charge, or

I = \frac{dQ}{dt}.

Current density

main: Current density Current density is a measure of the density of electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units, the current density is measured in amperes per square meter.

The drift speed of electric charges

The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. In order for there to be a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation:

I=nAvQ \!\

where

I \!\ is the electric current

n \!\ is number of charged particles per unit volume

A \!\ is the cross-sectional area of the conductor

v \!\ is the drift velocity, and

Q \!\ is the charge on each particle.

Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light.

Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwell's Equations, and is therefore many times faster than the drift velocity of the electrons. For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance.