Coulomb


The coulomb is the International System of Units unit of electric charge. The unit is the amount of electric charge transported by a constant electric current of one ampere in one second:
Thus, it is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt:
Under the 2019 redefinition of the SI base units, which took effect on 20 May 2019, the elementary charge is exactly coulombs. Thus the coulomb is the charge of exactly 1/ elementary charges, which is approximately elementary charges. The same number of electrons has the same magnitude but opposite sign of charge, that is, a charge of −1 C.

Name and notation

Definition

The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s. The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the elementary charge when expressed in coulombs, and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge. The ampere is defined by taking the fixed numerical value of the elementary charge e to be coulomb.
Thus, one coulomb is the charge of, where the number is the reciprocal of
By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb. In 1881, the International Electrical Congress, now the International Electrotechnical Commission, approved the volt as the unit for electromotive force, the ampere as the unit for electric current, and the coulomb as the unit of electric charge.
At that time, the volt was defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.
The coulomb was part of the EMU system of units. The "international coulomb" based on laboratory specifications for its measurement was introduced by the IEC in 1908. The entire set of "reproducible units" was abandoned in 1948 and the "international coulomb" became the modern Coulomb.

SI prefixes

See also Metric prefix.

Conversions