Current

Electric current is the flow (movement) 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. Electric current is measured using an ammeter. more...

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Definition

The amount of electric current (measured in amperes) through some surface, e.g., a section through a copper conductor, is defined as the amount of electric charge (measured in coulombs) flowing through that surface over time. If Q is the amount of charge that passed through the surface in the time t, then the average current I is:

By making the measurement time T shrink to zero, we get the instantaneous current i(t) as:

The ampere, the measure of electric current, is an SI base unit so that the coulomb, the measure of electric charge, is derived from the definition of the ampere.

Current in a metal wire

In solid conductive metal, a large population of electrons are mobile or free electrons. These electrons are bound to the metal lattice but not to any individual atom. Even without an external electric field applied, these electrons move about randomly due to thermal energy but on average, there is zero net current within the metal. 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