|Unit system||SI derived unit|
|Unit of||Electric charge|
|Named after||Charles-Augustin de Coulomb|
|SI base units||A⋅s|
|CGS units||2997924580 statC|
|Atomic units||6.24150934(14)e×10 18|
This SI unit is named after Charles-Augustin de Coulomb. As with every International System of Units (SI) unit named for a person, the first letter of its symbol is upper case (C). However, when an SI unit is spelled out in English, it is treated as a common noun and 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 material using title case.
The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s. The second is defined in terms of a frequency naturally emitted by caesium atoms. The ampere is defined using Ampère's force law; the definition relies in part on the mass of the international prototype kilogram, a metal cylinder housed in France. In practice, the Kibble balance is used to measure amperes with the highest possible accuracy.
Since the charge of one electron is known to be about , 1 C can also be considered the charge of roughly 6.241509×10 18 electrons or +1 C the charge of that many positrons or protons, where the number is the reciprocal of 1.602177×10 -19.
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 (IEC), approved the volt as the unit for electromotive force, the ampere as the unit for electic current and the coulomb as the unit of electric charge. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power. The coulomb (later "absolute coulomb" or "abcoulomb" for disambiguation) 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.
The proposed redefinition of the ampere and other SI base units would have the effect of fixing the numerical value of the elementary charge to an explicit constant expressed in coulombs, and therefore it would implicitly fix the value of the coulomb when expressed as a multiple of the fundamental charge (the numerical values of those quantities are the multiplicative inverses of each other).
|Value||SI symbol||Name||Value||SI symbol||Name|
|10-1 C||dC||decicoulomb||101 C||daC||decacoulomb|
|10-2 C||cC||centicoulomb||102 C||hC||hectocoulomb|
|10-3 C||mC||millicoulomb||103 C||kC||kilocoulomb|
|10-6 C||µC||microcoulomb||106 C||MC||megacoulomb|
|10-9 C||nC||nanocoulomb||109 C||GC||gigacoulomb|
|10-12 C||pC||picocoulomb||1012 C||TC||teracoulomb|
|10-15 C||fC||femtocoulomb||1015 C||PC||petacoulomb|
|10-18 C||aC||attocoulomb||1018 C||EC||exacoulomb|
|10-21 C||zC||zeptocoulomb||1021 C||ZC||zettacoulomb|
|10-24 C||yC||yoctocoulomb||1024 C||YC||yottacoulomb|
|Common multiples are in bold face.|
See also Metric prefix.
The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately . 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. For example, in conventional electrical units, the values of the Josephson constant KJ and von Klitzing constant RK are exact defined values (written KJ-90 and RK-90), and it follows that the elementary charge is also an exact defined value in this unit system. Specifically, exactly. SI itself may someday change its definitions in a similar way. 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 ", (in which the numeric value is the 2006 CODATA recommended value, since superseded). This proposal is not yet accepted as part of the SI.
2014 CODATA recommended values