'''Electric charge''' is the [[physical property]] of [[matter]] that causes it to experience a [[force]] when placed in an [[electromagnetic field]]. There are two types of electric charges: [[proton|positive]] and [[electron|negative]]. Like charges repel and unlike attract. An object (one not made of [[antimatter]]) is negatively charged if it has an excess of [[electron]]s, and is otherwise positively charged or uncharged. The [[Systeme International|SI]] derived unit of electric charge is the [[coulomb]] (C). In electrical engineering, it is also common to use the [[ampere-hour]] (Ah), and, in [[chemistry]], it is common to use the [[elementary charge]] (''e'') as a unit. The symbol ''Q'' often denotes charge. Early knowledge of how charged substances interact is now called [[classical electrodynamics]], and is still accurate for problems that don't require consideration of [[quantum mechanics|quantum effects]].

Twentieth-century [[oil drop experiment|experiments]] demonstrated that [[charge quantization|electric charge is ''quantized'']]; that is, it comes in integer multiples of individual small units called the [[elementary charge]], ''e'', approximately equal to {{val|1.602|e=-19|u=coulombs}} (except for particles called [[quark]]s, which have charges that are integer multiples of ''e/3''). The [[proton]] has a charge of +''e'', and the [[electron]] has a charge of −''e''. The study of charged particles, and how their interactions are mediated by [[photons]], is called [[quantum electrodynamics]].