mrs-campbell | High School Teacher | (Level 1) Educator Emeritus

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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.

ndnordic | High School Teacher | (Level 2) Associate Educator

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Conceptually the electromotive force, also referred to as the voltage, can be looked at as analogous to gravitational potential energy.  To increase the gravitational potential energy of an object work is done on the object to increase its distance from some ground reference point.  When the object is released work can then be done by the object as it falls back to the ground.  In a battery there is a potential difference due to a separation of charge. When a circuit is completed, charge flows and work is done in the circuit.

crims | Student, Grade 9 | (Level 1) eNoter

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The electromotive force (EMF) is the maximum potential difference between two electrodes of a galvanic or voltaic cell. This quantity is related to the tendency for an element, a compound or an ion to acquire (i.e. gain) or release (loss) electrons.

andyluv | Student, Grade 11 | (Level 1) Honors

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emf is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. The electric potential difference is created by separating positive and negative charges, thereby generating an electric field. The created electrical potential difference drives current flow if a circuit is attached to the source of emf. When current flows, however, the voltage across the terminals of the source of emf is no longer the open-circuit value, due to voltage drops inside the device due to its internal resistance.

Devices that can provide emf include voltaic cells, thermoelectric devices, solar cells, electrical generators, transformers, and even Van de Graaff generators.

summer-song | Student, Grade 11 | (Level 1) Honors

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Electromotive force is bascially voltage. It's the force needed to 'push' electrons around the circuit. This is obtained by a cell or battery (or other source of power)