Coulomb

The coulomb (symbol: C) is the SI unit of electric charge. It is named after Charles-Augustin de Coulomb.

Definition
1 coulomb is the amount of electric charge transported by a current of 1 ampere in 1 second.


 * $$ 1 \ \mathrm{C} = 1 \ \mathrm{A} \cdot 1 \ \mathrm{s}$$

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:


 * $$1 \ \mathrm{C} = 1 \ \mathrm{F} \cdot 1 \ \mathrm{V}$$

Explanation
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.

Historical note
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 system made the ampere the base unit.

Conversions

 * 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 96.485 341 5 kC (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.