Electric Charge and Its Properties

What is electric charge? Charge is a fundamental, intrinsic property of certain elementary particles (such as the electron and the proton), just as mass is. Rubbing two different materials together does not create charge out of nothing; it transfers electrons from one material's surface to the other, leaving one object with a surplus of electrons (net negative charge) and the other with a deficit (net positive charge). Benjamin Franklin's choice of which kind to call "positive" (the charge acquired by glass rubbed with silk) and which to call "negative" (the charge acquired by ebonite rubbed with fur) is the sign convention used to this day. Charges of the same sign repel each other; charges of opposite sign attract.

Three properties, verified in every experiment ever performed, define how charge behaves:

1. Additivity. If a body carries several charges $q_1, q_2, q_3, \ldots$ at different points, its total charge is simply their algebraic (scalar) sum, $Q = q_1+q_2+q_3+\cdots$, with the sign of each charge included. This is unlike, say, adding two forces, which must be added as vectors; charge has no direction, so ordinary arithmetic addition is enough. A body with $+5\ \mu\text{C}$ in one region and $-2\ \mu\text{C}$ in another carries a net charge of exactly $+3\ \mu\text{C}$.

2. Conservation. The total electric charge of an isolated system never changes: charge can be transferred from one part of the system to another, or redistributed between two bodies that touch, but it can never be created or destroyed. When a plastic rod is rubbed with wool, the rod does not "produce" negative charge from nothing; electrons already present in the wool's atoms simply move onto the rod, so the wool is left positively charged by exactly the same amount that the rod is left negatively charged -- the total charge on the rod-plus-wool system, taken together, is exactly zero, both before and after the rubbing. This conservation law holds even in the most violent particle-physics processes: when a neutron decays into a proton, an electron and an antineutrino, the neutron's charge of $0$ is exactly matched by the proton's $+e$ and the electron's $-e$ adding to zero.

3. Quantisation. Electric charge does not take on any arbitrary value; it always comes in whole-number multiples of a smallest indivisible amount, the elementary charge $e = 1.6\times10^{-19}\ \text{C}$, so that any observable charge $q$ obeys

$q = ne, \qquad n = 0, \pm1, \pm2, \pm3, \ldots$ …