The ampere (symbol: A) is the SI unit of electric current^{[1]} (symbol: I) and is one of the seven^{[2]} SI base units. It is named after AndréMarie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. In practice, its name is often shortened to amp.
In practical terms, the ampere is a measure of the amount of electric charge passing a point per unit time. Around 6.241 × 10^{18} electrons, or one coulomb, passing a given point each second constitutes one ampere.^{[3]}
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Definition
Ampère's force law^{[4]}^{[5]} states that there is an attractive 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 which will produce an attractive force of 2 × 10^{–7} 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".^{[1]}^{[6]}^{[7]}
In terms of Ampère's force law,
so
The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere."^{[8]} Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
That is, in general, charge Q is determined by steady current I flowing for a time t as Q = It.
History
The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current which generates a force of two dynes per centimetre of length between two wires one centimetre apart.^{[9]} The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.
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