Basic properties of electric charge

A charged body whose size is very small compared with the distances involved is treated as a point charge, with all its charge placed at one point.

Additivity: charge is a scalar, so a system's net charge is found by adding its charges as signed numbers. Unlike mass, charge can be negative.

Conservation: the total charge of an isolated system never changes. Charge only moves from one body to another; charged particles may be created or destroyed, but always in equal and opposite amounts, as when a neutron turns into a proton and an electron.

Quantisation: every free charge is a whole-number multiple of the basic charge e, q = ne with n = 0, ±1, ±2, …. An electron carries −e and a proton +e.

Quantisation was first suggested by Faraday's laws of electrolysis and shown experimentally by Millikan in 1912.

The SI unit of charge is the coulomb (C), the charge carried by a current of 1 A in 1 s. The basic charge is e = 1.602192 × 10⁻¹⁹ C, so −1 C is carried by about 6 × 10¹⁸ electrons.

Because 1 μC already holds about 10¹³ electronic charges, quantisation has no practical effect on large-scale charges and they can be treated as continuous; it matters only for charges of a few tens or hundreds of e.