|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||6.241509074e×10 18|
The coulomb (symbol: C) is the International System of Units (SI) unit of electric charge. Under the 2019 redefinition of the SI base units, which took effect on 20 May 2019, the coulomb is exactly 1/ elementary charges. The same number of electrons has the same magnitude but opposite sign of charge, that is, a charge of -1 C.
The coulomb is named after Charles-Augustin de Coulomb. As with every SI unit named for a person, its symbol starts with an upper case letter (C), but when written in full it follows the rules for capitalisation of a common noun; i.e., "coulomb" becomes capitalised at the beginning of a sentence and in titles, but is otherwise in lower case.
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 numerical values of those quantities are the multiplicative inverses of each other). The ampere is defined by taking the fixed numerical value of the elementary charge e to be coulombs.
Thus, one coulomb is the charge of approximately , where the number is the reciprocal of . It is impossible to realize exactly 1 C of charge, since the number of elementary charges is not an integer.
By 1878, 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 electric 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.
|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.