Electromotive force
Energy per charge driving electric current, not a physical force.
Electromotive force (emf, or EMF) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical transducers provide an emf by converting other forms of energy into electrical energy. Batteries, for instance, convert chemical energy, while generators convert mechanical energy. This conversion is achieved by physical forces applying work on electric charges, but emf itself is not a physical force; the term has been deprecated by ISO/IEC standards in favor of "source voltage" or "source tension." An electronic–hydraulic analogy compares emf to the mechanical work done by a pump on water, resulting in a pressure difference analogous to voltage. In electromagnetic induction, emf can be defined around a closed loop of a conductor as the electromagnetic work done on an elementary electric charge, such as an electron, if it travels once around the loop. For two-terminal devices modeled as a Thévenin equivalent circuit, an equivalent emf is measured as the open-circuit voltage between the terminals; this emf can drive a current when an external circuit is attached, making the device a voltage source. Devices providing emf include electrochemical cells, thermoelectric devices, solar cells, photodiodes, electrical generators, inductors, transformers, and Van de Graaff generators. In nature, emf is generated when magnetic field fluctuations occur through a surface, such as during geomagnetic storms that induce currents in electrical grids. In a battery, charge separation creating a potential difference is accomplished by chemical reactions at the electrodes, converting chemical potential energy into electromagnetic potential energy. In an electrical generator, a time-varying magnetic field creates an electric field via electromagnetic induction, producing a potential difference between terminals. The general principle governing emf in such machines is Faraday's law of induction. Historically, Alessandro Volta introduced the term "force motrice électrique" in 1801 to describe the active agent of a battery. Around 1830, Michael Faraday established that chemical reactions at each electrode–electrolyte interface provide the "seat of emf" for the voltaic cell, correcting Volta’s earlier incorrect opinion that contact alone was the origin. Emf is typically denoted by the symbol ℰ and measured in volts, equivalent to a j
- symbol
- ℰ (script E)
- alternative_names
- source voltage, source tension (denoted Uₛ)
- SI_unit
- volt (V)
- also_measured_in
- statvolt (cgs system)
- key_principle
- energy per unit charge (dW/dq)
- related_law
- Faraday's law of induction
Lore & Background
In an electrical generator, a time-varying magnetic field inside the device creates an electric field through the process of electromagnetic induction, which in turn produces a potential difference between the generator's terminals. This potential difference is the electromotive force. Within the generator, charge separation occurs as electrons move away from one terminal toward the other. In an open-circuit condition, this separation continues until the resulting electric field becomes strong enough to prevent further charge separation. The electromotive force is then counterbalanced by the electrical voltage arising from this charge separation. If an external load is attached to the terminals, this voltage can drive an electric current. The general principle governing the generation of electromotive force in such electrical machines is Faraday's law of induction. Historically, the term "electromotive force" was introduced by Alessandro Volta in 1801 to describe the active agent of a battery. Around 1830, Michael Faraday established that chemical reactions at each of the two electrode–electrolyte interfaces provide the "seat of emf" for a voltaic cell, driving the current. Earlier, Volta had incorrectly believed that mere contact between metals was the origin of the emf, ignoring the chemical reaction. The electromotive force of a cell is independent of its size but depends on the nature of the electrolyte used.
Reader's Guide
Electromotive force is a fundamental concept in electromagnetism and electronics, representing the energy provided per unit charge by a source such as a battery or generator. Although it gives rise to voltage and can be measured as such, emf is distinct from potential difference. The ISO/IEC standards have deprecated the term 'electromotive force' in favor of 'source voltage' or 'source tension' to avoid confusion with physical force. Devices that provide emf include electrochemical cells, thermoelectric devices, solar cells, photodiodes, electrical generators, inductors, transformers, and Van de Graaff generators. In nature, emf is generated by magnetic field fluctuations, such as during geomagnetic storms inducing currents in electrical grids. The concept is essential for understanding how energy conversion occurs in circuits, with the emf defined as work done per charge (dW/dq).
Did You Know?
- Michael Faraday established that chemical reactions at electrode–electrolyte interfaces provide the seat of emf.
- ISO/IEC standards have deprecated 'electromotive force' in favor of 'source voltage' or 'source tension'.
- In nature, emf is generated when magnetic field fluctuations occur through a surface, such as during geomagnetic storms.
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