Coulomb (unit)
The SI unit of electric charge. One coulomb equals the charge carried by 6.24 × 10¹⁸ protons, or the charge that flows past a point in one second when the current is one ampere.
Definition
The coulomb (symbol C) is the SI unit of electric charge. Since the 2019 SI redefinition it has been defined exactly: one coulomb is the charge of 1/(1.602176634 × 10⁻¹⁹) elementary charges — roughly 6.24 quintillion protons' worth. Equivalently, one coulomb is the charge that passes through a cross-section of a conductor in one second when the current is one ampere.
A coulomb is an enormous amount of charge by human standards. Two one-coulomb charges placed one metre apart would repel each other with about nine billion newtons of force — roughly the weight of a million tonnes. That is why you never encounter "a coulomb" in electrostatics: a lightning bolt transfers only about 5–20 coulombs during its entire stroke, and a well-charged van de Graaff generator carries microcoulombs at most. In circuits, however, coulombs move effortlessly: a 1 A current moves one coulomb per second, so a household appliance drawing 10 A pushes tens of coulombs through its wires each second.
The unit honours Charles-Augustin de Coulomb, who first quantified how charges interact. It is the practical bridge between the microscopic elementary charge and the macroscopic currents that run civilisation.