In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons (see electrical conduction). Positive charges may also be mobile, such as the cationic electrolyte(s) of a battery, or the mobile protons of the proton conductor of a fuel cell. In general use, the term "conductor" is interchangeable with "wire." Insulators are non-conducting materials with few mobile charges and which support only insignificant electric currents.