Electromotive force (emf, ) is a term used to characterize electrical devices, such as voltaic cells, thermoelectric devices, electrical generators and transformers, and even resistors.
For a given device, if a charge Q passes through that device, and gains an energy U, the net emf for that device is the energy gained per unit charge, or U/Q.
In most circuits current is driven by a so-called "source of emf", which usually is a voltaic cell (or battery, which consists of voltaic cells in series and/or in parallel) or the power company.
Sources of electromotive force include electric generators (both alternating current and continuous current types), batteries, and thermocouples (in a heat gradient).
The electromotive force produced by primary and secondary cells is usually of the order of a few volts.