Electromotive force
From Engineering
Electromotive force (emf, 
) is a term used to characterize electrical devices, such as voltaic cells, thermoelectric devices, electrical generators and transformers, and even resistors.
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[edit] Explained
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.
[edit] Details
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.
[edit] Sources
Sources of electromotive force include electric generators (both alternating current and continuous current types), batteries, and thermocouples (in a heat gradient).
[edit] Electromotive force of cells
The electromotive force produced by primary and secondary cells is usually of the order of a few volts.
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