Electromotive force (SEM) - definition
Electromotive force (SEM) - definition
Electromotive force (SEM, or electromotive force - EMF) is a physical quantity describing the ability of an electrical energy source to induce a flow of charge in a circuit. Although it is expressed in volts (V), like voltage, SEM does not represent the voltage between two points in a purely potential sense, but the total energy supplied by a source per unit charge to overcome the internal and external resistance of a circuit. It is therefore a measure of the work done by the internal mechanism of the source - e.g. a chemical reaction in an electrochemical cell or magnetic induction in a generator - to move an electrical charge.
In the context of Kirchhoff's laws and electrical circuit analysis, the SEM represents the primary energy introduced into the circuit, regardless of losses due to the resistance of the Cables or components. In galvanic cells the electromotive force results from the electrochemical potential difference between electrodes, while in current generators it results from the Lorentz force on charges moving in an alternating magnetic field. SEM also appears in inductive phenomena, e.g. in Coils, where an alternating magnetic field generates a potential difference according to Faraday's law.
From the thermodynamic definition, the electromotive force is the averaged value of the electric field along the closed path taken by the charge in the source. Its value depends on the nature of the source and its operating conditions, such as temperature, chemical composition, magnetic field intensity or frequency of field changes. Despite the name suggesting 'force', it is not a force in the sense of classical mechanics, but a concept corresponding to the difference of potential energy, relevant to the description and design of power systems and electronic devices.
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