The coulomb, symbol C, is the SI unit of electric charge, and is defined in terms of the ampere: 1 coulomb is the amount of electric charge (quantity of electricity) carried by a current of 1 ampere flowing for 1 second. It is also about 6.241506×1018 times the charge of an electron. It is named after Charles-Augustin de Coulomb (1736-1806).
Since the values of the Josephson (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20) constants have been given conventional values, it is possible to combine these values (KJ ≡ 4.835 979×1014 Hz/V and RK ≡ 2.581 280 7×104 Ω) to form an alternate (not yet official) definition of the coulomb. Very simply, a coulomb is then equal to exactly 6.241 509 629 152 65 × 1018 elementary charges.
A wrong definition
It is sometimes incorrectly said that a coulomb is defined as one mole of electrons (approximately 6.022×1023, or Avogadro's number). This is completely wrong. One mole of electrons is known as a faraday. Just as the ampere and second were defined well before the electron's charge was known, the kilogram was defined well before the mass of the amu was known. Neither value has anything to do with the other. In terms of Avogadro's number (NA), a coulomb is equal to approximately 1.036 × NA × 10-5 elementary charges.
SI electricity units
Last updated: 07-30-2005 17:03:57