) is the SI
unit of electric charge
. It is named after Charles-Augustin de Coulomb
1 coulomb is the amount of electric charge transported by a current of 1 ampere in 1 second.
It can also be expressed in terms of capacitance and voltage, where one coulomb is equal to one farad of capacitance times one volt of electric potential difference:
In principle, the coulomb could be defined in terms of the charge of an electron or elementary charge. 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 (KJ ≡ 4.835 979 Hz/V and RK ≡ 2.581 280 7 Ω), it is possible to combine these values to form an alternative (not yet official) definition of the coulomb. A coulomb is then equal to exactly 6.241 509 629 152 65 elementary charges. Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit.
In everyday situations, positive and negative charges are usually balanced out. According to Coulomb's Law, two point charges of +1 C, one meter apart, would experience a repulsive force of 9 N, roughly the equivalent of 900,000 metric tons of weight.
The ampere was historically a derived unit—being defined as 1 coulomb per second. Therefore the coulomb, rather than the ampere, was the SI base electrical unit.
In 1960 the SI made the ampere the base unit.
- The electrical charge of one mole of electrons (approximately 6.022, or Avogadro's number) is known as a faraday (actually –1 faraday, since electrons are negatively charged). One faraday equals 96485.3399 coulombs (the Faraday constant). In terms of Avogadro's number (NA), one coulomb is equal to approximately 1.036 × NA elementary charges.
- one ampere-hour = 3600 C
- The elementary charge is 1.602176487 C
- One statcoulomb (statC), the CGS electrostatic unit of charge (esu), is approximately 3.3356 C or about 1/3 nC.
- 1 coulomb is the amount of electrical charge in 6.241506 electrons or other elementary charged particles.
1. Kowalski, Ludwik, "A Short History of the SI Units in Electricity", pp. 97-99 vol 24, The Physics Teacher, Feb 1986