The property of a source of electrical energy that causes a current to flow in a circuit. The algebraic sum of the potential differences in a circuit equals the e.m.f., which is measured by the energy liberated when unit electric charge passes completely round the circuit. A battery of e.m.f. E will supply a current I to an external resistance R:

where r is the internal resistance of the battery. The term ‘electromotive force’ strictly applies to a source of electrical energy but is sometimes misused as being equivalent to potential difference.