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Encyclopedia > Electromotive force
Electrostatics Electromagnetism Electricity · Magnetism Electric charge Coulomb's law Electric field Gauss's law Electric potential Electric dipole moment Ampère's law Magnetic field Magnetic dipole moment Electric current Lorentz force law Electromotive force (EM) Electromagnetic induction Faraday-Lenz law Displacement current Maxwell's equations (EMF) Electromagnetic field (EM) Electromagnetic radiation Electrical conduction Electrical resistance Capacitance Inductance Impedance Resonant cavities Waveguides This box: view • talk • edit

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Sources of electromotive force include electric generators (both alternating current and continuous current types), batteries, and thermocouples (in a heat gradient). [1] Electromotive force is often denoted by $mathcal{E}$ or (script capital E). Generator redirects here. ... City lights viewed in a motion blurred exposure. ... Direct current (DC or continuous current) is the continuous flow of electric charge through a conductor such as a wire from high to low potential. ... For other uses, see battery (disambiguation). ... In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. ...

Electromotive force is measured in (V) 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). Josephson junction array chip developed by NIST as a standard volt. ... Cover of brochure The International System of Units. ... The joule (IPA pronunciation: or ) (symbol: J) is the SI unit of energy. ... The coulomb (symbol: C) is the SI unit of electric charge. ... The statcoulomb (statC) or franklin (Fr) or electrostatic unit of charge (esu) is the physical unit for electrical charge used in the centimetre-gram-second (cgs) electrostatic system of units. ... The statvolt is the unit of voltage and electrical potential used in the cgs system of units. ... This article or section is in need of attention from an expert on the subject. ... An erg is the unit of energy and mechanical work in the centimetre-gram-second (CGS) system of units, symbol erg. Its name is derived from the Greek word meaning work. The erg is a small unit, equal to a force of one dyne exerted for a distance of one... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ...

Terminology

The term origin is attributed to Alessandro Volta (1745–1827), who invented the voltaic pile. The term "electromotive force" originally referred to the 'force' with which positive and negative charges could be separated (i.e. moved, hence "electromotive"), and was also called "electromotive power" (although it is not a power in the modern sense). 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. [2] This article is about the physicist Alessandro Volta. ... A copper-zinc Voltaic pile The Voltaic pile was the first modern electric battery, invented by Alessandro Volta in 1800. ... In physics, force is an influence that may cause a body to accelerate. ... In physics, power (symbol: P) is the rate at which work is performed. ... In electromagnetism, Maxwells equations are a set of equations, developed in the latter half of the nineteenth century by James Clerk Maxwell. ... In physics, an electric field or E-field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. ...

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. [3] International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ...

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. The expansion of the acronym is considered obsolete.[citation needed] Nonetheless, it is sometimes helpful to picture emf as analogous to a force or a pressure such as when making a mechanical or liquid analogy of an electric circuit. The use of the term "emf" is in decline but it is still found in introductory and technical level texts on electricity.[citation needed] Physics (Greek: (phÃºsis), nature and (phusikÃ©), knowledge of nature) is the science concerned with the fundamental laws of the universe and their precise formulation in a mathematical framework. ...

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. An emf is also commonly used to express the strength of a compact source of electrical energy. 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. It has units of joules per coulomb, otherwise more commonly known as the volt. Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... An electrical network or electrical circuit is an interconnection of analog electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. ... For other uses, see Curve (disambiguation). ...

If the vector field f is the force per unit charge on a charge carrier, the emf around a circuit C is Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ...

$mathcal{E}=oint_Cmathbf{f}cdot dmathbf{l}.$[4]

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.[5] Electric potential is the potential energy per unit of charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts. ... Josephson junction array chip developed by NIST as a standard volt. ...

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 + ℰde

The combination ℰ.e is an example of a conjugate pair of variables. At constant pressure the above relationship produces a Maxwell relation that links the change in open cell voltage with temperature (a measurable quantity) to the change in entropy when charge is passed isothermally and isobarically. The latter is closely related to the reaction entropy ΔrS of the electrochemical reaction that lends the battery its power. Thermodynamic potentials Maxwell relations Bridgmans equations Exact differential (edit) In thermodynamics, the internal energy of a system is expressed in terms of pairs of conjugate variables such as pressure/volume or temperature/entropy. ... Maxwells relations are a set of equations in thermodynamics which are derivable from the definitions of the thermodynamic potentials. ... An isothermal process is a thermodynamic process in which the temperature of the system stays constant: Î”T = 0. ... An isobaric process is a thermodynamic process in which the pressure stays constant: . The heat transferred to the system does work but also changes the internal energy of the system: The yellow area represents the work done According to the first law of thermodynamics, where W is work done by... Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ...

$left(frac{partial mathcal{E}}{partial T}right)_e= -left(frac{partial S}{partial e}right)_T$

Electromotive force and potential difference

If no external circuit is connected to a source of emf, an electric current cannot exist (Ohm's Law). Thus, between the terminals of the source, there must exist an electric field that exactly cancels the generated emf.

The source of this field is the electric charges separated by the mechanism generating the emf [6]. For example, the chemical reaction in the battery proceeds only to the point that the electric field between the separated charges is strong enough to stop the reaction.

This electric field between the terminals of the battery creates an electric potential difference that can be measured with a voltmeter. The polarity of this measured potential difference is always opposite to that of the generated emf. The value of the emf for the battery (or other source) is the value of this 'open circuit' voltage. The emf itself cannot be measured directly. Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ... Two digital voltmeters. ...

Electromotive force generation

Commonly, electromotive force is generated by electrochemical reaction (e.g., a fuel cell). Dissimilar metals in contact also produce what is know as a contact electromotive force or contact potential (eg., the volta effect). Absorption of radiant or thermal energy (e.g., a solar cell or a thermocouple). Some other sources include thermocouples, thermopiles, and photodiodes. English chemists John Daniell (left) and Michael Faraday (right), both credited to be founders of electrochemistry as known today. ... A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i. ... It has been suggested that this article or section be merged with Contact electrification. ... It has been suggested that this article or section be merged with Contact electrification. ... The Volta effect is an obsolete name for the weak potential developed by the contact of different metals. ... Radiant energy is the energy of electromagnetic waves. ... 1. ... A thermopile is a group of thermocouples connected in series. ... A photodiode is an electronic component and a type of photodetector. ...

Electromagnetic induction is a means of converting mechanical energy, i.e., energy of motion into electrical energy. The electromotive force generated in this way is often referred to as motional electromotive force. Motional emf is ultimately due to the electrical effect of a time-varying magnetic field. In the presence of such a magnetic field, the electric potential and hence the potential difference (commonly known as voltage) is undefined (see the former) — hence the need for distinct concepts of emf and potential difference. Technically, the emf is an effective potential difference included in a circuit to make Kirchhoff's voltage law valid: it is exactly the amount from Faraday's law of induction by which the line integral of the electric field around the circuit is not zero. The emf is then given by Electromagnetic induction is the production of an electrical potential difference (or voltage) across a conductor situated in a changing magnetic flux. ... Current (I) flowing through a wire produces a magnetic field () around the wire. ... Electric potential is the potential energy per unit of charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts. ... Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ... Kirchhoffs circuit laws are a pair of laws that deal with the conservation of charge and energy in electrical circuits, and were first described in 1845 by Gustav Kirchhoff. ... Faradays law of induction (more generally, the law of electromagnetic induction) states that the induced emf (electromotive force) in a closed loop equals the negative of the time rate of change of magnetic flux through the loop. ...

$mathcal{E} = -L { di over dt }$

where i is the current and L is the inductance of the circuit. Inductance (or electric inductance) is a measure of the amount of magnetic flux produced for a given electric current. ...

Given this emf and the resistance of the circuit, the instantaneous current can be computed with Ohm's Law, for example, or more generally by solving the differential equations that arise out of Kirchhoff's laws. The current at any instant t is then given by Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... For the phase law, see Ohms Phase Law. ... Kirchhoffs circuit laws are a pair of laws that deal with the conservation of charge and energy in electrical circuits, and were first described in 1845 by Gustav Kirchhoff. ...

$i(t) = { 1 over R} left( E - L {di over dt} right)$

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. Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Resistor symbols (non-European) Resistor symbols (Europe, IEC) A pack of resistors A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohms law. ... An inductor is a passive electrical device employed in electrical circuits for its property of inductance. ...

References

General
• Griffiths, David (1999). Introduction to Electrodynamics, 3e, Prentice-Hall. ISBN 0-13-805326-X.
Citations
1. ^ John S. Rigden, (editor in chief), Macmillan encyclopedia of physics. New York : Macmillan, 1996.
2. ^ Edward J. Rothwell and Michael J. Cloud, Electromagnetics. CRC Press. Pg 22. ISBN 0-8493-1397-X
3. ^ John Markus, Neil Sclater, McGraw-Hill electronics dictionary. New York, McGraw-Hill, Edition 5th ed., international 3rd ed. c1994. ISBN 0-07-113486-7 ISBN 0-07-040434-8
4. ^ Griffiths, Introduction to Electrodynamics, p.293
5. ^ Griffiths, Introduction to Electrodynamics, p.285; "...or trained ants with tiny harnesses."
6. ^ Roberts, Dana: "How batteries work: A gravitational analog", Am. J. Phys., 51,829 (1983)

Ohm's Law (PDF in German)

Electric potential is the potential energy per unit of charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts. ... Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. ... In 1832 famous scientist Michael Faraday performed some very interesting experiments with magnets and conducting disks. ... In physics, the magnetomotive force produces magnetic flux. ... The original meaning of the term potentiometer, which is still in use, is an apparatus used to measure the potential (or voltage) in a circuit by tapping off a portion of a known voltage from a resistive slide wire and comparing it with the unknown voltage by means of a... The Peltierâ€“Seebeck effect, or thermoelectric effect, is the direct conversion of heat differentials to electric voltage and vice versa. ...

• Andrew Gray, "Absolute Measurements in Electricity and Magnetism", Electromotive force. Macmillan and co., 1884.
• Charles Albert Perkins, "Outlines of Electricity and Magnetism", Measurement of Electromotive Force. Henry Holt and co., 1896.
• John Livingston Rutgers Morgan, "The Elements of Physical Chemistry", Electromotive force. J. Wiley, 1899.
• George F. Barker, "On the measurement of electromotive force". Proceedings of the American Philosophical Society Held at Philadelphia for Promoting Useful Knowledge, American Philosophical Society. January 19, 1883.
• "Abhandlungen zur Thermodynamik, von H. Helmholtz. Hrsg. von Max Planck". (Tr. "Papers to thermodynamics, on H. Helmholtz. Hrsg. by Max Planck".) Leipzig, W. Engelmann, Of Ostwald classical author of the accurate sciences series. New consequence. No. 124, 1902.
• Nabendu S. Choudhury, "Electromotive force measurements on cells involving [beta]-alumina solid electrolyte". NASA technical note, D-7322.
• Henry S. Carhart, "Thermo-electromotive force in electric cells, the thermo-electromotive force between a metal and a solution of one of its salts". New York, D. Van Nostrand company, 1920. LCCN 20020413
• Hazel Rossotti, "Chemical applications of potentiometry". London, Princeton, N.J., Van Nostrand, 1969. ISBN 0-442-07048-9 LCCN 69011985 //r88
• Theodore William Richards and Gustavus Edward Behr, jr., "The electromotive force of iron under varying conditions, and the effect of occluded hydrogen". Carnegie Institution of Washington publication series , 1906. LCCN 07003935 //r88
• G. W. Burns, et al., "Temperature-electromotive force reference functions and tables for the letter-designated thermocouple types based on the ITS-90". Gaithersburg, MD : U.S. Dept. of Commerce, National Institute of Standards and Technology, Washington, Supt. of Docs., U.S. G.P.O., 1993.

External articles

• Doug Gingrich, "Physics lecture notes, electronics", Direct Current Circuits, Electromotive Force (EMF). University of Alberta, Department of Physics, 1999.
• Advanced Physics lecture notes, "Electromagnetism", Faraday’s Law—Electromagnetic Induction. Electromotive Force". Semiconductor Physics Group, Department of Physics, University of Cambridge, 2006. (PDF)

Results from FactBites:

 Electromotive force - Wikipedia, the free encyclopedia (855 words) The term "electromotive force" originally referred to the 'force' with which positive and negative charges could be separated (i.e. moved, hence "electromotive"), and was also called "electromotive power" (although it is not a power in the modern sense). Oxford English Dictionary, "electromotive force".) The term is attributed to Alessandro Volta.
 Electric voltage (284 words) When a voltage is generated by a battery, or by the magnetic force according to Faraday's Law, this generated voltage has been traditionally called an "electromotive force" or emf. It is useful to distinguish voltages which are generated from the voltage changes which occur in a circuit as a result of energy dissipation, e.g., in a resistor. The magnetic force exerted on the charges in a moving conductor will generate a voltage (a motional emf).
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