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The coulomb (named after Charles-Augustin de Coulomb, unit symbol: C) is a fundamental unit of electrical charge, and is also the SI derived unit of electric charge (symbol: Q or q). It is equal to the charge of approximately 6.241×10^{18} electrons.
Its SI definition is the charge transported by a constant current of one ampere in one second:
One coulomb is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt:
This SI unit is named after Charles-Augustin de Coulomb. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (C). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (coulomb), except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that “degree Celsius” conforms to this rule because the “d” is lowercase.— Based on The International System of Units, section 5.2.^{[2]}
In the SI system, the coulomb is defined in terms of the ampere and second: 1 C = 1 A × 1 s.^{[3]} The second is defined in terms of a frequency which is naturally emitted by caesium atoms.^{[4]} The ampere is defined using Ampère's force law;^{[5]} the definition relies in part on the mass of the international prototype kilogram, a metal cylinder housed in France.^{[6]} In practice, the watt balance is used to measure amperes with the highest possible accuracy.^{[6]}
Since the charge of one electron is known to be about 1.60217657×10^^{−19} coulombs,^{[1]} a coulomb can also be considered to be the charge of roughly 6.241509324×10^^{18} electrons (or protons), the reciprocal of 1.60217657×10^^{−19}.
See also SI prefix.
The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately 1.602176487(40)×10^^{−19} C.^{[1]} In SI, the elementary charge in coulombs is an approximate value: no experiment can be infinitely accurate. However, in other unit systems, the elementary charge has an exact value by definition, and other charges are ultimately measured relative to the elementary charge.^{[7]} For example, in conventional electrical units, the values of the Josephson constant K_{J} and von Klitzing constant R_{K} are exact defined values (written K_{J-90} and R_{K-90}), and it follows that the elementary charge e =2/(K_{J}R_{K}) is also an exact defined value in this unit system.^{[7]} Specifically, e_{90} = (2×10^{−9})/(25812.807 × 483597.9) C exactly.^{[7]} SI itself may someday change its definitions in a similar way.^{[7]} For example, one possible proposed redefinition is "the ampere...is [defined] such that the value of the elementary charge e (charge on a proton) is exactly 1.602176487×10^^{−19} coulombs"^{[8]} This proposal is not yet accepted as part of the SI; the SI definitions are unlikely to change until at least 2015.^{[9]}
International System of Units, Electric current, Metre, Watt, Second
Litre, Ampere, Force, Pressure, Mass
Proton, Electric charge, Electron, Quasiparticle, Silicon
Proton, Electron, Electric current, Electromagnetism, Glass
Electromagnetism, Maxwell's equations, Electric charge, Volume, Electron
Si, Gaussian units, Coulomb, Electric displacement field, Units of measurement
Ampere, Second, Alessandro Volta, Metre, Watt