The coulomb (symbol: C) is the International System of Units (SI) unit of electric charge. It is the charge (symbol: Q or q) transported by a constant current of one ampere in one second:
- {\displaystyle 1~{\text{C}}=1~{\text{A}}\times 1~{\text{s}}}
Thus, it is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt:
- {\displaystyle 1~{\text{C}}=1~{\text{F}}\times 1~{\text{V}}}
Under the 2019 redefinition of the SI base units, which took effect on 20 May 2019, the elementary charge (the charge of the proton and of the electron, but also of other fundamental particles) is exactly 1.602176634×10−19 coulombs. Thus the coulomb is the charge of exactly 1/(1.602176634×10−19) elementary charges, which is approximately 6.2415090744×1018 elementary charges (1.036×10−5 mol). The same number of electrons has the same magnitude but opposite sign of charge, that is, a charge of −1 C.