electromotive force

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 volts.

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electromotive force

Energy per unit electric charge that is imparted by an energy source, such as an electric generator or a battery. As the device does work on the electric charge being transferred within itself, energy is converted from one form to another. The work done on a unit of electric charge or the energy gained by the unit charge is the electromotive force emf (or E) and is characteristic of any energy source capable of driving electric charge around a circuit. A common unit of electromotive force is the volt V, a unit equal to the difference in electric potential between two points in a conductor carrying a current of one ampere and dissipating one watt of power between the two points.

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electromotive force

The pressure in an electric circuit measured in volts. See volt.

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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.

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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.

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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

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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 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