The work done by some source in separating electrical charges to produce a potential difference capable of driving current round a circuit; often abbreviated emf. The term force is a misnomer; generally, emf is a property of the source, whereas potential difference depends on both source and current flow. A source of emf transfers energy to the circuit by doing work in raising potential. For example, a battery is a source of emf in which chemical energy moves charges to the terminals, making one positive, the other negative. The emf is work done on the charges to bring about this separation: an emf of one volt means that the battery expends one joule of energy to bring about the separation of one coulomb of charge.
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Electromotive force (emf) is defined as the amount of energy gained per unit charge that passes through a device in the opposite direction to the electric field produced by that device.
Sources of electromotive force include electric generators (both alternating current and continuous current types), batteries, and thermocouples (in a heat gradient).
Electromotive force is measured in volts (in the International System of Units equal in amount to a joule per coulomb of electric charge). Electromotive force in electrostatic units is the statvolt (in the centimeter gram second system of units equal in amount to an erg per electrostatic unit of charge). Maxwell's 1865 explication of what are now called Maxwell's equations used the term "electromotive force" for what is now called the electric field strength.
Electromotive force has been stated to be the force that has the disposition to produce a circuit's electric current and is, under normal conditions, called voltage.
In physics, the unit of emf is the "energy per unit electric charge", so the "force" term of "electromotive force" is misleading to a degree.
Explanation of electromotive force
In electrodynamics, a measure of electromotance indicates the tendency for electric charge to flow around a circuit or other closed curve. The electromotive force of a device is defined to be the amount of energy gained per unit charge that passes through it in the "uphill" direction.
If the vector field f is the force per unit charge on a charge carrier, the emf around a circuit C is
Like the electric potential at a point and the voltage between two points, the emf around a loop is measured in volts. Unlike the first two quantities, the emf is sensitive to non-electrostatic forces, since the force f can include magnetic, chemical, mechanical, and gravitational components.
Electromotive force in thermodynamics
When multiplied by an amount of charge de the emf ℰ yields a thermodynamic work term ℰde that is used in the formulism for the change in Gibbs free energy when charge is passed in a battery:
dG = -SdT + VdP + ℰdeThe combination ℰ.e is an example of a conjugate pair of variables.
Electromotive force and potential difference
If no external circuit is connected to a source of emf, an electric current cannot exist.
Electromotive force generation
Commonly, electromotive force is generated by electrochemical reaction (e.g., a fuel cell). The electromotive force generated in this way is often referred to as motional electromotive force. The current at any instant t is then given by
where E is the electromotive force of the source, i is the instantaneous current, and R is the resistance of the resistor connected in series with the inductor, in the circuit. Carhart, "Thermo-electromotive force in electric cells, the thermo-electromotive force between a metal and a solution of one of its salts".
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