The ampere (SI symbol A), named after André-Marie Ampère (1775–1836) is the SI unit of electric current and is one of the seven base units.
In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time with
constituting one ampere. In this way, amperes can be viewed as a flow rate, i.e. number of particles (charged) transiting per unit time, and coulombs simply as the number of particles. We can write
where
is the charge passing a point In a time
and is equal to the number of electrons multiplied by the charge on an electron and
is a constant current.
Ampère's force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of
newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum".
The standard ampere is accurately determined using a watt balance or using Ohm's Law and is accurate to a few parts in
Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges. Since a coulomb is approximately equal to
elementary charges, one ampere is approximately equivalent toelementary charges per second. The proposed change would define
as being the current in the direction of flow of a particular number of elementary charges per second.