Basic properties of charges

Three basic properties define electric charge. (i) Electric charge: like mass, electric charge is an intrinsic property of the fundamental particles that make up matter -- protons carry charge +e and electrons carry charge -e, while neutrons are uncharged; the net charge on any object is the algebraic (signed) sum of all the charges it contains, so charge adds like ordinary numbers (this is charge's additivity). The SI unit of charge is the coulomb (C). (ii) Conservation of charges: Benjamin Franklin argued that rubbing does not create charge, it only transfers already-existing charge from one object to another -- before rubbing, both objects are neutral, and rubbing simply moves some charge (electrons) from one to the other, so that whatever positive charge appears on one object is exactly matched by an equal negative charge on the other. This is stated as a general law: the total electric charge of an isolated system, and of the universe as a whole, is constant -- charge can neither be created nor destroyed, only transferred, redistributed, or converted between carriers. It is one of the fundamental conservation laws of physics, on par with conservation of energy and momentum. (iii) Quantisation of charges: Robert Millikan's oil-drop experiment established that charge in nature always appears as an integer multiple of a smallest indivisible unit e = 1.6 x 10^-19 C (the magnitude of charge on a single electron or proton), so that the charge q on any object obeys q = ne for some integer n = 0, +-1, …