Electromotive force
Electromotive force, also called emf (denoted and measured in volts), is the voltage developed by any source of electrical energy such as a battery or dynamo. It is generally defined as the electrical potential for a source in a circuit. A device that supplies electrical energy is called electromotive force or emf. Emfs convert chemical, mechanical, and other forms of energy into electrical energy. The product of such a device is also known as emf.
The word "force" in this case is not used to mean mechanical force, measured in newtons, but a potential, or energy per unit of charge, measured in volts.
In electromagnetic induction, emf can be defined around a closed loop as the electromagnetic work that would be done on a charge if it travels once around that loop. (While the charge travels around the loop, it can simultaneously lose the energy gained via resistance into thermal energy.) For a time-varying magnetic flux linking a loop, the electric potential scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop.
In the case of a two-terminal device (such as an electrochemical cell or electromagnetic generator) which is modeled as a Thévenin's equivalent circuit, the equivalent emf can be measured as the open-circuit potential difference or voltage between the two terminals. This potential difference can drive a current if an external circuit is attached to the terminals.
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