Electromotive force

An electromotive force (emf) is the "force", measured in volts, that is produced by a changing magnetic field. Since the word "force" now has a very specific meaning in physics, and emf is not a force in this sense, the expansion of the acronym is considered obsolete; or at least, an embarassing historical artifact.

The emf describes the electrical effect of a changing magnetic field. In the presence of a magnetic field, the electric potential and hence the potential difference (commonly known as voltage) is undefined (see the former) – hence the need for distinct concepts of emf and potential difference. Given this emf and the resistance of the circuit, the current can be computed with Ohm's Law.

Emf is often used as a synonym for potential difference, where the potential difference is induced by an energy source such as a battery. This usage is considered obsolete.