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Unit system SI derived unit
Unit of Electric charge
Named after Charles-Augustin de Coulomb
1 C in ...... is equal to ...
    SI base units     As
    CGS units    2997924580 statC
    Atomic units    6.241509074 e ×10^18 [1]

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, [2] the coulomb is exactly 1/(1.602176634×10−19) 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

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. [3]


The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s. [4] 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 1.602176634×10−19 coulombs. [5]

Thus, one coulomb is the charge of approximately 6241509074460762607.776 elementary charges, where the number is the reciprocal of 1.602176634×10−19 C. [6] It is impossible to realize exactly 1 C of charge, since it is not an integer number of elementary charges.

By 1878, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb. [7] 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. [8] 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. [9]

SI prefixes

SI multiples of coulomb (C)
ValueSI symbolNameValueSI symbolName
10−1 CdCdecicoulomb101 CdaCdecacoulomb
10−2 CcCcenticoulomb102 ChChectocoulomb
10−3 CmCmillicoulomb103 CkCkilocoulomb
10−6 CµCmicrocoulomb106 CMCmegacoulomb
10−9 CnCnanocoulomb109 CGCgigacoulomb
10−12 CpCpicocoulomb1012 CTCteracoulomb
10−15 CfCfemtocoulomb1015 CPCpetacoulomb
10−18 CaCattocoulomb1018 CECexacoulomb
10−21 CzCzeptocoulomb1021 CZCzettacoulomb
10−24 CyCyoctocoulomb1024 CYCyottacoulomb
Common multiples are in bold face.

See also Metric prefix.


In everyday terms

See also

Notes and references

  1. 6.241509126(38)×1018 is the reciprocal of the 2014 CODATA recommended value 1.6021766208(98)×10−19 for the elementary charge in coulomb.
  2. "SI Brochure (2019)" (PDF). SI Brochure. BIPM. p. 127. Retrieved May 23, 2019.
  3. "SI Brochure, Appendix 1" (PDF). BIPM. p. 144.
  4. "SI brochure (2019)" (PDF). SI Brochure. BIPM. p. 130. Retrieved May 23, 2019.
  5. "SI brochure (2019)" (PDF). SI Brochure. BIPM. p. 132. Retrieved May 23, 2019.
  6. "2018 CODATA Value: elementary charge". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
  7. W. Thomson, et al. (1873) "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units," Report of the 43rd Meeting of the British Association for the Advancement of Science (Bradford, September 1873), pp. 222–225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 109 C.G.S. units of resistance; the "volt" is approximately 108 C.G.S. units of electromotive force; and the "farad" is approximately 1/109 of the C.G.S. unit of capacity."
  8. (Anon.) (September 24, 1881) "The Electrical Congress," The Electrician, 7 .
  9. Donald Fenna, A Dictionary of Weights, Measures, and Units, OUP (2002), 51f.
  10. "2018 CODATA Value: Faraday constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
  11. Martin Karl W. Pohl. "Physics: Principles with Applications" (PDF). DESY. Archived from the original (PDF) on 2011-07-18.
  12. Hasbrouck, Richard. Mitigating Lightning Hazards Archived 2013-10-05 at the Wayback Machine , Science & Technology Review May 1996. Retrieved on 2009-04-26.
  13. How to do everything with digital photography – David Huss , p. 23, at Google Books, "The capacity range of an AA battery is typically from 1100–2200 mAh."

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