Il corso di inglese più bello del mondo si trova qui

  ••••• login ELINGUE Contatti: Tel. 02-36553040

Scegli qui la tua risorsa!

      HomeIl Metodo | Grammatica | Inglese con noi | Multiblog | INSEGNARE AGLI ADULTI INSEGNARE AI BAMBINIAudioBooks | VIDEO DIDATTICI | Inglese sfizioso | Articoli | TipsTesti paralleli 






- Great Painters
- Accounting
- Fundamentals of Law
- Marketing
- Shorthand
- Concept Cars
- Videogames
- The World of Sports

- Blogs
- Free Software
- Google
- My Computer

- PHP Language and Applications
- Wikipedia
- Windows Vista

- Education
- Masterpieces of English Literature
- American English

- English Dictionaries
- The English Language

- Medical Emergencies
- The Theory of Memory
- The Beatles
- Dances
- Microphones
- Musical Notation
- Music Instruments
- Batteries
- Nanotechnology
- Cosmetics
- Diets
- Vegetarianism and Veganism
- Christmas Traditions
- Animals

- Fruits And Vegetables


  1. AAAA battery
  2. AAA battery
  3. AA battery
  4. A battery
  5. Absorbent glass mat
  6. Alessandro Volta
  7. Alkaline battery
  8. Alkaline fuel cell
  9. Aluminium battery
  10. Ampere
  11. Atomic battery
  12. Backup battery
  13. Baghdad Battery
  14. Batteries
  15. Battery charger
  16. B battery
  17. Bernard S. Baker
  18. Beta-alumina solid electrolyte
  19. Betavoltaics
  20. Bio-nano generator
  21. Blue energy
  22. Bunsen cell
  23. Car battery
  24. C battery
  25. Clark cell
  26. Concentration cell
  27. Coulomb
  28. 2CR5
  29. Daniell cell
  30. Direct borohydride fuel cell
  31. Direct-ethanol fuel cell
  32. Direct methanol fuel cell
  33. Dry cell
  34. Dry pile
  35. Duracell
  36. Duracell Bunny
  37. Earth battery
  38. Electric charge
  39. Electric current
  40. Electricity
  41. Electrochemical cell
  42. Electrochemical potential
  43. Electro-galvanic fuel cell
  44. Electrolysis
  45. Electrolyte
  46. Electrolytic cell
  47. Electromagnetism
  48. Electromotive force
  49. Energizer Bunny
  50. Energy
  51. Energy density
  52. Energy storage
  53. Flashlight
  54. Float charging
  55. Flow Battery
  56. Formic acid fuel cell
  57. Fuel cell
  58. Fuel cell bus trial
  59. Galvanic cell
  60. Gel battery
  61. Grove cell
  62. Half cell
  63. History of the battery
  64. Hybrid vehicle
  65. Lead-acid battery
  66. Leclanché cell
  67. Lemon battery
  68. List of battery sizes
  69. List of battery types
  70. List of fuel cell vehicles
  71. Lithium battery
  72. Lithium ion batteries
  73. Lithium iron phosphate battery
  74. Lithium polymer cell
  75. LR44 battery
  76. Luigi Galvani
  77. Manganese dioxide
  78. Memory effect
  79. Mercury battery
  80. Metal hydride fuel cell
  81. Methane reformer
  82. Methanol reformer
  83. Michael Faraday
  84. Microbial fuel cell
  85. Molten carbonate fuel cell
  86. Molten salt battery
  87. Nickel-cadmium battery
  88. Nickel-iron battery
  89. Nickel metal hydride
  90. Nickel-zinc battery
  91. Open-circuit voltage
  92. Optoelectric nuclear battery
  93. Organic radical battery
  94. Oxyride battery
  95. Panasonic EV Energy Co
  96. Peukert's law
  97. Phosphoric acid fuel cell
  98. Photoelectrochemical cell
  99. Polymer-based battery
  100. Power density
  101. Power management
  102. Power outage
  103. PP3 battery
  104. Primary cell
  105. Prius
  106. Proton exchange membrane
  107. Proton exchange membrane fuel cell
  108. Protonic ceramic fuel cell
  109. Radioisotope piezoelectric generator
  110. Ragone chart
  111. RCR-V3
  112. Rechargeable alkaline battery
  113. Reverse charging
  114. Reversible fuel cell
  115. Searchlight
  116. Secondary cell
  117. Short circuit
  118. Silver-oxide battery
  119. Smart Battery Data
  120. Smart battery system
  121. Sodium-sulfur battery
  122. Solid oxide fuel cell
  123. Super iron battery
  124. Thermionic converter
  125. Trickle charging
  126. Vanadium redox battery
  127. Volt
  128. Voltage
  129. Voltaic pile
  130. Watch battery
  131. Water-activated battery
  132. Weston cell
  133. Wet cell
  134. Zinc-air battery
  135. Zinc-bromine flow battery
  136. Zinc-carbon battery

This article is from:

All text is available under the terms of the GNU Free Documentation License: 

Electromotive force

From Wikipedia, the free encyclopedia


Electromotive force (emf) is the amount of energy gained per unit charge that passes through a device in the opposite direction to the electric field existing across that device. It is measured in volts.

Sources and unit of measurement

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).

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).


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]

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]

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]

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.

If the vector field f is the force per unit charge on a charge carrier, the emf around a circuit C is

\mathcal{E}=\oint_C\mathbf{f}\cdot d\mathbf{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]

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.

\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.

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.

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

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

where i is the current and L is the inductance of the circuit.

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

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.


  • Griffiths, David (1999). Introduction to Electrodynamics, 3e, Prentice-Hall. ISBN 0-13-805326-X. 
  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)

See also

  • Electric potential
  • Electrochemical potential
  • Faraday paradox
  • Magnetomotive force
  • Potentiometer
  • Thermoelectric effect

Further reading

  • 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)
Retrieved from ""



L'utente può utilizzare il nostro sito solo se comprende e accetta quanto segue:

  • Le risorse linguistiche gratuite presentate in questo sito si possono utilizzare esclusivamente per uso personale e non commerciale con tassativa esclusione di ogni condivisione comunque effettuata. Tutti i diritti sono riservati. La riproduzione anche parziale è vietata senza autorizzazione scritta.
  • Il nome del sito EnglishGratis è esclusivamente un marchio e un nome di dominio internet che fa riferimento alla disponibilità sul sito di un numero molto elevato di risorse gratuite e non implica dunque alcuna promessa di gratuità relativamente a prodotti e servizi nostri o di terze parti pubblicizzati a mezzo banner e link, o contrassegnati chiaramente come prodotti a pagamento (anche ma non solo con la menzione "Annuncio pubblicitario"), o comunque menzionati nelle pagine del sito ma non disponibili sulle pagine pubbliche, non protette da password, del sito stesso.
  • La pubblicità di terze parti è in questo momento affidata al servizio Google AdSense che sceglie secondo automatismi di carattere algoritmico gli annunci di terze parti che compariranno sul nostro sito e sui quali non abbiamo alcun modo di influire. Non siamo quindi responsabili del contenuto di questi annunci e delle eventuali affermazioni o promesse che in essi vengono fatte!
  • Coloro che si iscrivono alla nostra newsletter (iscrizione caratterizzatalla da procedura double opt-in) accettano di ricevere saltuariamente delle comunicazioni di carattere informativo sulle novità del sito e, occasionalmente, delle offerte speciali relative a prodotti linguistici a pagamento sia nostri che di altre aziende. In ogni caso chiunque può disiscriversi semplicemente cliccando sulla scritta Cancella l'iscrizione che si trova in fondo alla newsletter, non è quindi necessario scriverci per chiedere esplicitamente la cancellazione dell'iscrizione.
  • L'utente, inoltre, accetta di tenere Casiraghi Jones Publishing SRL indenne da qualsiasi tipo di responsabilità per l'uso - ed eventuali conseguenze di esso - degli esercizi e delle informazioni linguistiche e grammaticali contenute sul siti. Le risposte grammaticali sono infatti improntate ad un criterio di praticità e pragmaticità più che ad una completezza ed esaustività che finirebbe per frastornare, per l'eccesso di informazione fornita, il nostro utente.


    ENGLISHGRATIS.COM è un sito di Casiraghi Jones Publishing SRL
    Piazzale Cadorna 10 - 20123 Milano - Italia
    Tel. 02- - email:
    Iscritta al Registro Imprese di MILANO - C.F. e PARTITA IVA: 11603360154
    Iscritta al R.E.A. di Milano n.1478561 • Capitale Sociale
    10.400 interamente versato

    Roberto Casiraghi                                                                                Crystal Jones