electromotive force

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electromotive force,

abbr. emf, difference in electric potential, or voltage, between the terminals of a source of electricity, e.g., a battery from which no current is being drawn. When current is drawn, the potential difference drops below the emf value. Electromotive force is usually measured in voltsvolt
[for Alessandro Volta], abbr. V, unit of electric potential and electromotive force. It is defined as the difference of electric potential existing across the ends of a conductor carrying a constant current of 1 ampere when the power dissipated is 1 watt.
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Electromotive force (emf)

A measure of the strength of a source of electrical energy. The term is often shortened to emf. It is not a force in the usual mechanical sense (and for this reason has sometimes been called electromotance), but it is a conveniently descriptive term for the agency which drives current through an electric circuit. In the simple case of a direct current I (measured in amperes) flowing through a resistor R (in ohms), Ohm's law states that there will be a voltage drop (or potential difference) of V = IR (in volts) across the resistor. To cause this current to flow requires a source with emf (also measured in volts) E = V. More generally, Kirchhoff's voltage law states that the sum of the source emf's taken around any closed path in an electric circuit is equal to the sum of the voltage drops. This is equivalent to the statement that the total emf in a closed circuit is equal to the line integral of the electric field strength around the circuit. See Electric current, Electric field, Electrical resistance, Ohm's law

An emf may be steady (direct), as for a battery, or time-varying, as for a charged capacitor discharging through a resistor. Emf's may be generated by a variety of physical, chemical, and biological processes. Some of the more important are:

1. Electrochemical reactions, as used in direct-current (dc) batteries, in which the emf results from the reactions between electrolyte and electrodes.

2. Electromagnetic induction, in which the emf results from a change in the magnetic flux linking the circuit. This finds application in alternating-current rotary generators and transformers, providing the basis for the electricity supply industry. See Electromagnetic induction, Faraday's law of induction

3. Thermoelectric effects, in which a temperature difference between different parts of a circuit produces an emf. The main use is for the measurement of temperature by means of thermocouples; there are some applications to electric power generation. See Thermocouple, Thermoelectricity

4. The photovoltaic effect, in which the absorption of light (or, more generally, electromagnetic radiation) in a semiconductor produces an emf. This is widely used for scientific purposes in radiation detectors and also, increasingly, for the generation of electric power from the Sun's radiation. See Photovoltaic effect, Radiometry

5. The piezoelectric effect, in which the application of mechanical stress to certain types of crystal generates an emf. There are applications in sound recording, in ultrasonics, and in various types of measurement transducer. See Kirchhoff's laws of electric circuits, Piezoelectricity, Ultrasonics

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Electromotive Force

(emf), the physical quantity that characterizes the effect of external (nonpotential) forces in DC or AC sources; in a closed conducting circuit; it is equal to the work done by the forces in carrying a unit positive charge around the circuit. If Eext denotes the field strength of the external forces, then the emf in a closed circuit L is ℰ = ∮Ed1, where d1 is a line element of the circuit.

The potential forces of an electrostatic, or stationary, field cannot support a steady current in a circuit, since the work they perform over a closed path is equal to zero. The passage of current through conductors, however, involves the liberation of energy through the heating of the conductors. External forces cause the charged particles to move within such current sources as generators, primary cells, and storage batteries. The external forces may originate in various ways. In generators they are an aspect of the rotational electric field, which is developed by time variation of a magnetic field, or a Lorentz force, which is the effect of a magnetic field on the electrons in a moving conductor; in primary cells and storage batteries they are chemical forces.

The emf determines the current intensity in a circuit for a given resistance. Like a voltage, it is measured in volts.

REFERENCES

Kalashnikov, S. G. Elektrichestvo, 4th ed. (Obshchii kurs fiziki.) Moscow, 1977.
Tamm, I. E. Osnovy teorii elektrichestva, 9th ed. Moscow, 1976.

G. IA. MIAKISHEV

electromotive force

[i¦lek·trə′mōd·iv ′fōrs]
(physical chemistry)
The difference in electric potential that exists between two dissimilar electrodes immersed in the same electrolyte or otherwise connected by ionic conductors.
The resultant of the relative electrode potential of the two dissimilar electrodes at which electrochemical reactions occur. Abbreviated emf. Also known as electromotance.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Electromotive force (emf)

A measure of the strength of a source of electrical energy. The term is often shortened to emf. It is not a force in the usual mechanical sense (and for this reason has sometimes been called electromotance), but it is a conveniently descriptive term for the agency which drives current through an electric circuit. In the simple case of a direct current I (measured in amperes) flowing through a resistor R (in ohms), Ohm's law states that there will be a voltage drop (or potential difference) of V = IR (in volts) across the resistor. To cause this current to flow requires a source with emf (also measured in volts) E = V. More generally, Kirchhoff's voltage law states that the sum of the source emf's taken around any closed path in an electric circuit is equal to the sum of the voltage drops. This is equivalent to the statement that the total emf in a closed circuit is equal to the line integral of the electric field strength around the circuit. See Ohm's law

An emf may be steady (direct), as for a battery, or time-varying, as for a charged capacitor discharging through a resistor. Emf's may be generated by a variety of physical, chemical, and biological processes. Some of the more important are:

1. Electrochemical reactions, as used in direct-current (dc) batteries, in which the emf results from the reactions between electrolyte and electrodes.

2. Electromagnetic induction, in which the emf results from a change in the magnetic flux linking the circuit. This finds application in alternating-current rotary generators and transformers, providing the basis for the electricity supply industry. See Alternating-current generator, Electromagnetic induction, Transformer

3. Thermoelectric effects, in which a temperature difference between different parts of a circuit produces an emf. The main use is for the measurement of temperature by means of thermocouples; there are some applications to electric power generation. See Thermocouple, Thermoelectricity

4. The photovoltaic effect, in which the absorption of light (or, more generally, electromagnetic radiation) in a semiconductor produces an emf. This is widely used for scientific purposes in radiation detectors and also, increasingly, for the generation of electric power from the Sun's radiation. See Solar cell

5. The piezoelectric effect, in which the application of mechanical stress to certain types of crystal generates an emf. There are applications in sound recording, in ultrasonics, and in various types of measurement transducer. See Direct-current motor, Kirchhoff's laws of electric circuits, Transducer

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.

electromotive force

The force which causes (or tends to cause) the movement of electricity in a conductor; the difference in potential between the terminals of an electric source.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

electromotive force

Physics
a. a source of energy that can cause a current to flow in an electrical circuit or device
b. the rate at which energy is drawn from this source when unit current flows through the circuit or device, measured in volts.
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

electromotive force

Electrical energy measured in volts. See volt.
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