coulomb
Electric chargeexact by definition
The coulomb is the SI unit of electric charge, defined as the charge transported by a current of one ampere in one second. Since the 2019 revision fixed the elementary charge at exactly 1.602176634e-19 coulombs, one coulomb is exactly 1/1.602176634e-19 elementary charges, about 6.241509074 quintillion of them. That makes the coulomb exact by definition and no longer tied to any measured quantity.
| 1 C | 1 C |
Named for Charles-Augustin de Coulomb, who in the 1780s used a torsion balance to establish the inverse-square law of force between charges.
About the coulomb
A coulomb is an enormous amount of charge to have in one place and an ordinary amount to have moving. One ampere for one second delivers a coulomb through a wire without incident, because the wire holds an equal countervailing charge in its lattice. Separate that same coulomb from its partner and put the two a metre apart, and Coulomb's law gives a force of about \(9 \ imes 10^9\) newtons, roughly the weight of a million tonnes. This is why static electricity involves microcoulombs at most and why bulk matter is neutral to extraordinary precision.
Charge is conserved and quantised, which are two facts of very different character. Conservation is a symmetry principle with no known exception. Quantisation means every observed free charge is an integer multiple of \(e\), a fact Millikan's oil-drop experiment established experimentally around 1909 and which the 2019 SI now takes as a defining constant. Quarks carry thirds of \(e\), but they are never found free, so the practical quantum of charge remains \(e\).