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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]}
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