When a voltage is generated by an energy source, this
generated voltage was traditionally knkown as the "electromotive force" or emf. The emf
represents energy per unit charge (voltage) which has been made available by the
generating mechanism and thus is not actually a "force". The term emf has retained for
historical reasons. It is useful to distinguish voltage sources from other voltage
drops, e.g., in a resistor.
Formally, emf is the external
work expended per unit of charge to produce an electric potential difference across two
terminals in an open circuit. The electric potential difference is created by the
electric force field created by separating positive and negative charges. The created
electrical potential difference drives current flow if a circuit is attached to the
source of emf.
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