Current

Electric current is by definition the flow of electric charge. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second. more...

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Definition

The magnitude of an electric current is defined as the time derivative of electric charge:

Formally this is written as:

The quantity of charge Q flowing per unit of time t is I, standing for the intensity of the current.

Current in a metal wire

In solid conductive metal, with no external forces applied, there exists a random motion of "mobile" or "free" electrons created by the thermal energy which the electrons have gained from the surrounding medium. When a metal atom contributes a free electron, it acquires a net positive charge. The population of freed electrons form a "charge-gas" or "sea of charge" which remains bound to the ion lattice by attraction, while the metal as a whole remains neutral. Metal atoms typically contribute either one or two electrons to the "sea." Free electrons can move amongst these positive ions, while the positive ions can only oscillate about their mean fixed positions. Electrons move, but the net flow of charge remains zero: given an imaginary plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is exactly equal to the number passing in the opposite direction.

When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electron is therefore the current carrier in a typical solid conductor. For an electric current of 1 ampere rate, 1 coulomb of electric charge (which consists of about 6.242 × 10¹⁸ electrons) drifts every second through the imaginary plane through which the conductor passes.

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

where

It follows that:

Current density

Current density is a measure of the density of electrical 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.

Read more at Wikipedia.org