electric charge

Rub a balloon on your hair and it sticks to a wall; the same hairs stand up and push apart from each other. That tug and that push are electric charge at work. Charge is one of a small handful of basic labels we pin on every particle to say how it behaves — and electric charge is the label that decides whether a particle feels the electromagnetic force, and which way it gets pulled.

Electric charge comes in two signs, called positive and negative, and the rule is simple: like signs repel, opposite signs attract, while a particle with zero charge feels no electromagnetic pull at all. Charge is quantized, meaning it comes only in fixed steps; the electron carries exactly one unit of negative charge, the proton exactly one unit of positive charge. Quarks are the odd exception that proves the rule: they carry fractions of a unit (such as plus two-thirds or minus one-third), but they are always bound together in combinations whose total charge is a whole number, so an isolated fractional charge is never seen. Charge is also strictly conserved: in any interaction, the total charge before equals the total charge after.

Electric charge matters because it is one of the most fundamental and best-tested quantities in all of physics. Its conservation underlies all of chemistry and electronics, and the fixed size of the charge unit, written e, is one of the constants of nature. A useful caution: do not confuse the everyday word charge (as in a charged battery, meaning stored energy) with electric charge the property. A particle's charge is a permanent identity tag, not something it runs out of.