Electric charge

Electric charge is the property of matter that makes it respond to electric and magnetic fields. It comes in two signs — positive and negative — and the combined charge of any isolated system never changes. You cannot create net charge out of nothing; you can only move it around or produce pairs of opposite charges that sum to zero.

At the microscopic level, ordinary matter is made of protons (charge +e), electrons (charge −e), and neutrons (neutral), where e ≈ 1.602 × 10⁻¹⁹ coulombs is the elementary charge. Everything electric we see in daily life — lightning, static cling, the current in every wire — traces back to electrons being shuffled from one place to another. A charged balloon has gained or lost perhaps 10¹² extra electrons out of the roughly 10²⁴ it contains; the imbalance is tiny, and yet the forces it produces can lift hair off a scalp.

Charge is conserved with more rigour than almost any other physical quantity. Nuclear reactions, chemical reactions, particle collisions at the LHC: the total charge before always equals the total charge after. This conservation law is the deep reason Kirchhoff's current law holds at every junction in a circuit, and the deep reason antimatter must be produced in charge-neutral pairs.

Benjamin Franklin proposed the sign convention (positive and negative) in the 1740s as part of his single-fluid theory of electricity. He picked the sign of glass-rubbed-with-silk as positive — which, after the electron was identified in 1897, turned out to mean that conventional "current" flows opposite to actual electron motion. Physics has lived with that accident ever since.