emf voltaic oint 82806 electromotive boldsymbol mathcal i_ kirchhoff anode electrochemistry norio ohmic electrode ell diode electrodes cathode 86094 electrolyte 11985 licoo j.r.w 7898 fogiel resistances r.j.d ideality sadamichi araújo
emf (electro-motive force) is a voltage
Electromotive force - wikipedia Electromotive force Not to be confused with Electromagnetic field . Part of a series of articles about Electromagnetism Electricity Magnetism Electrostatics ( show ) Electric charge Static electricity Electric field Conductor Insulator Triboelectricity Electrostatic discharge Induction Coulomb 's law Gauss 's law Electric flux / potential energy Electric dipole moment Polarization density Magnetostatics ( show ) Ampère 's law Magnetic field Magnetization Magnetic flux Biot -- Savart law Magnetic dipole moment Gauss 's law for magnetism Electrodynamics ( show ) Lorentz force law Electromagnetic induction Faraday 's law Lenz 's law Displacement current Magnetic potential Maxwell 's equations Electromagnetic field Electromagnetic pulse Electromagnetic radiation Maxwell tensor Poynting vector Liénard -- Wiechert potential Jefimenko 's equations Eddy current London equations Mathematical descriptions of the electromagnetic field Electrical network ( hide ) Electric current Electric potential Voltage Resistance Ohm 's law Series circuit Parallel circuit Direct current Alternating current Electromotive force Capacitance Inductance Impedance Resonant cavities Waveguides Covariant formulation ( show ) Electromagnetic tensor ( stress -- energy tensor ) Four - current Electromagnetic four - potential Scientists ( show ) Ampère Coulomb Faraday Gauss Heaviside Henry Hertz Lorentz Maxwell Tesla Volta Weber Ørsted Electromotive force , abbreviated emf ( denoted E ( \ displaystyle ( \ mathcal ( E ) ) ) and measured in volts ) , is the electrical intensity or `` pressure '' developed by a source of electrical energy such as a battery or generator . A device that converts other forms of energy into electrical energy ( a `` transducer '' ) provides an emf as its output . ( The word `` force '' in this case is not used to mean mechanical force , as may be measured in pounds or newtons . ) In electromagnetic induction , emf can be defined around a closed loop of conductor as the electromagnetic work that would be done on an electric charge ( an electron in this instance ) if it travels once around the loop . For a time - varying magnetic flux linking a loop , the electric potential scalar field is not defined due to a circulating electric vector field , but an emf nevertheless does work that can be measured as a virtual electric potential around the loop . ( While electrical charges travel around the loop , their energy is typically converted into thermal energy due to the resistance of the conductor comprising the loop . ) In the case of a two - terminal device ( such as an electrochemical cell ) which is modeled as a Thévenin 's equivalent circuit , the equivalent emf can be measured as the open - circuit potential difference or `` voltage '' between the two terminals . This potential difference can drive an electric current if an external circuit is attached to the terminals . Contents 1 Overview 2 History 3 Notation and units of measurement 4 Formal definitions 5 In thermodynamics 6 Voltage difference 7 Generation 7.1 Chemical sources 7.1. 1 Voltaic cells 7.1. 2 Of cells 7.2 Electromagnetic induction 7.3 Contact potentials 7.4 Solar cell 8 See also 9 References 10 Further reading 11 External links Overview ( edit ) Devices that can provide emf include electrochemical cells , thermoelectric devices , solar cells , photodiodes , electrical generators , transformer and even Van de Graaff generators . In nature , emf is generated whenever magnetic field fluctuations occur through a surface . The shifting of the Earth 's magnetic field during a geomagnetic storm induces currents in the electrical grid as the lines of the magnetic field are shifted about and cut across the conductors . In the case of a battery , the charge separation that gives rise to a voltage difference between the terminals is accomplished by chemical reactions at the electrodes that convert chemical potential energy into electromagnetic potential energy . A voltaic cell can be thought of as having a `` charge pump '' of atomic dimensions at each electrode , that is : A source of emf can be thought of as a kind of charge pump that acts to move positive charge from a point of low potential through its interior to a point of high potential. ... By chemical , mechanical or other means , the source of emf performs work dW on that charge to move it to the high potential terminal . The emf E of the source is defined as the work dW done per charge dq : E = dW / dq . In the case of an electrical generator , a time - varying magnetic field inside the generator creates an electric field via electromagnetic induction , which in turn creates a voltage difference between the generator terminals . Charge separation takes place within the generator , with electrons flowing away from one terminal and toward the other , until , in the open - circuit case , sufficient electric field builds up to make further charge separation impossible . Again , the emf is countered by the electrical voltage due to charge separation . If a load is attached , this voltage can drive a current . The general principle governing the emf in such electrical machines is Faraday 's law of induction . History ( edit ) Around 1830 , Michael Faraday established that the reactions at each of the two electrode -- electrolyte interfaces provide the `` seat of emf '' for the voltaic cell , that is , these reactions drive the current and are not an endless source of energy as was initially thought . In the open - circuit case , charge separation continues until the electrical field from the separated charges is sufficient to arrest the reactions . Years earlier , Alessandro Volta , who had measured a contact potential difference at the metal -- metal ( electrode -- electrode ) interface of his cells , had held the incorrect opinion that contact alone ( without taking into account a chemical reaction ) was the origin of the emf . Notation and units of measurement ( edit ) Electromotive force is often denoted by E ( \ displaystyle ( \ mathcal ( E ) ) ) or E ( script capital E , Unicode U + 2130 ) . In a device without internal resistance , if an electric charge Q passes through that device , and gains an energy W , the net emf for that device is the energy gained per unit charge , or W / Q. Like other measures of energy per charge , emf uses the SI unit volt , which is equivalent to a joule per coulomb . 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 ) . Formal definitions ( edit ) Inside a source of emf that is open - circuited , the conservative electrostatic field created by separation of charge exactly cancels the forces producing the emf . Thus , the emf has the same value but opposite sign as the integral of the electric field aligned with an internal path between two terminals A and B of a source of emf in open - circuit condition ( the path is taken from the negative terminal to the positive terminal to yield a positive emf , indicating work done on the electrons moving in the circuit ) . Mathematically : E = − ∫ A B E c s ⋅ d l , ( \ displaystyle ( \ mathcal ( E ) ) = - \ int _ ( A ) ^ ( B ) ( \ boldsymbol ( E ) ) _ ( \ mathrm ( cs ) ) \ cdot \ mathrm ( d ) ( \ boldsymbol ( \ ell ) ) \ , ) where E is the conservative electrostatic field created by the charge separation associated with the emf , dl is an element of the path from terminal A to terminal B , and ' ' denotes the vector dot product . This equation applies only to locations A and B that are terminals , and does not apply to paths between points A and B with portions outside the source of emf . This equation involves the electrostatic electric field due to charge separation E and does not involve ( for example ) any non-conservative component of electric field due to Faraday 's law of induction . In the case of a closed path in the presence of a varying magnetic field , the integral of the electric field around a closed loop may be nonzero ; one common application of the concept of emf , known as `` induced emf '' is the voltage induced in such a loop . The `` induced emf '' around a stationary closed path C is : E = ∮ C ⁡ E ⋅ d l , ( \ displaystyle ( \ mathcal ( E ) ) = \ oint _ ( C ) ( \ boldsymbol ( E ) ) \ cdot \ mathrm ( d ) ( \ boldsymbol ( \ ell ) ) \ , ) where now E is the entire electric field , conservative and non-conservative , and the integral is around an arbitrary but stationary closed curve C through which there is a varying magnetic field . The electrostatic field does not contribute to the net emf around a circuit because the electrostatic portion of the electric field is conservative ( that is , the work done against the field around a closed path is zero ) . This definition can be extended to arbitrary sources of emf and moving paths C : E = ∮ C ⁡ ( E + v × B ) ⋅ d l ( \ displaystyle ( \ mathcal ( E ) ) = \ oint _ ( C ) \ left ( ( \ boldsymbol ( E ) ) + ( \ boldsymbol ( v ) ) \ times ( \ boldsymbol ( B ) ) \ right ) \ cdot \ mathrm ( d ) ( \ boldsymbol ( \ ell ) ) \ ) + 1 q ∮ C ⁡ E f f e c t i v e c h e m i c a l f o r c e s ⋅ d l ( \ displaystyle + ( \ frac ( 1 ) ( q ) ) \ oint _ ( C ) \ mathrm ( Effective \ chemical \ forces \ \ cdot ) \ \ mathrm ( d ) ( \ boldsymbol ( \ ell ) ) \ ) + 1 q ∮ C ⁡ E f f e c t i v e t h e r m a l f o r c e s ⋅ d l , ( \ displaystyle + ( \ frac ( 1 ) ( q ) ) \ oint _ ( C ) \ mathrm ( Effective \ thermal \ forces \ \ cdot ) \ \ mathrm ( d ) ( \ boldsymbol ( \ ell ) ) \ , ) which is a conceptual equation mainly , because the determination of the `` effective forces '' is difficult . In thermodynamics ( edit ) When multiplied by an amount of charge dQ the emf E yields a thermodynamic work term EdQ that is used in the formalism for the change in Gibbs energy when charge is passed in a battery : d G = − S d T + V d P + E d Q , ( \ displaystyle dG = - SdT + VdP+ ( \ mathcal ( E ) ) dQ \ , ) where G is the Gibb 's free energy , S is the entropy , V is the system volume , P is its pressure and T is its absolute temperature . The combination ( E , Q ) 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 T ( a measurable quantity ) to the change in entropy S when charge is passed isothermally and isobarically . The latter is closely related to the reaction entropy of the electrochemical reaction that lends the battery its power . This Maxwell relation is : ( ∂ E ∂ T ) Q = − ( ∂ S ∂ Q ) T ( \ displaystyle \ left ( ( \ frac ( \ partial ( \ mathcal ( E ) ) ) ( \ partial T ) ) \ right ) _ ( Q ) = - \ left ( ( \ frac ( \ partial S ) ( \ partial Q ) ) \ right ) _ ( T ) ) If a mole of ions goes into solution ( for example , in a Daniell cell , as discussed below ) the charge through the external circuit is : Δ Q = − n 0 F 0 , ( \ displaystyle \ Delta Q = - n_ ( 0 ) F_ ( 0 ) \ , ) where n is the number of electrons / ion , and F is the Faraday constant and the minus sign indicates discharge of the cell . Assuming constant pressure and volume , the thermodynamic properties of the cell are related strictly to the behavior of its emf by : Δ H = − n 0 F 0 ( E − T d E d T ) , ( \ displaystyle \ Delta H = - n_ ( 0 ) F_ ( 0 ) \ left ( ( \ mathcal ( E ) ) - T ( \ frac ( d ( \ mathcal ( E ) ) ) ( dT ) ) \ right ) \ , ) where ΔH is the enthalpy of reaction . The quantities on the right are all directly measurable . Voltage difference ( edit ) An electrical voltage difference is sometimes called an emf . The points below illustrate the more formal usage , in terms of the distinction between emf and the voltage it generates : For a circuit as a whole , such as one containing a resistor in series with a voltaic cell , electrical voltage does not contribute to the overall emf , because the voltage difference on going around a circuit is zero . ( The ohmic IR voltage drop plus the applied electrical voltage sum to zero . See Kirchhoff 's Law ) . The emf is due solely to the chemistry in the battery that causes charge separation , which in turn creates an electrical voltage that drives the current . For a circuit consisting of an electrical generator that drives current through a resistor , the emf is due solely to a time - varying magnetic field within the generator that generates an electrical voltage that in turn drives the current . ( The ohmic IR drop plus the applied electrical voltage again is zero . See Kirchhoff 's Law ) A transformer coupling two circuits may be considered a source of emf for one of the circuits , just as if it were caused by an electrical generator ; this example illustrates the origin of the term `` transformer emf '' . A photodiode or solar cell may be considered as a source of emf , similar to a battery , resulting in an electrical voltage generated by charge separation driven by light rather than chemical reaction . Other devices that produce emf are fuel cells , thermocouples , and thermopiles . In the case of an open circuit , the electric charge that has been separated by the mechanism generating the emf creates an electric field opposing the separation mechanism . For example , the chemical reaction in a voltaic cell stops when the opposing electric field at each electrode is strong enough to arrest the reactions . A larger opposing field can reverse the reactions in what are called reversible cells . The electric charge that has been separated creates an electric potential difference that can be measured with a voltmeter between the terminals of the device . The magnitude of the emf for the battery ( or other source ) is the value of this ' open circuit ' voltage . When the battery is charging or discharging , the emf itself can not be measured directly using the external voltage because some voltage is lost inside the source . It can , however , be inferred from a measurement of the current I and voltage difference V , provided that the internal resistance r already has been measured : E = V + Ir . Generation ( edit ) Chemical sources ( edit ) Main article : Electrochemical cell A typical reaction path requires the initial reactants to cross an energy barrier , enter an intermediate state and finally emerge in a lower energy configuration . If charge separation is involved , this energy difference can result in an emf . See Bergmann et al. and Transition state . Galvanic cell using a salt bridge The question of how batteries ( galvanic cells ) generate an emf is one that occupied scientists for most of the 19th century . The `` seat of the electromotive force '' was eventually determined by Walther Nernst to be primarily at the interfaces between the electrodes and the electrolyte . Molecules are groups of atoms held together by chemical bonds , and these bonds consist of electrical forces between electrons ( negative ) and protons ( positive ) . The molecule in isolation is a stable entity , but when different molecules are brought together , some types of molecules are able to steal electrons from others , resulting in charge separation . This redistribution of charge is accompanied by a change in energy of the system , and a reconfiguration of the atoms in the molecules . The gain of an electron is termed `` reduction '' and the loss of an electron is termed `` oxidation '' . Reactions in which such electron exchange occurs ( which are the basis for batteries ) are called reduction - oxidation reactions or redox reactions . In a battery , one electrode is composed of material that gains electrons from the solute , and the other electrode loses electrons , because of these fundamental molecular attributes . The same behavior can be seen in atoms themselves , and their ability to steal electrons is referred to as their electronegativity . As an example , a Daniell cell consists of a zinc anode ( an electron collector ) , is oxidized as it dissolves into a zinc sulfate solution , the dissolving zinc leaving behind its electrons in the electrode according to the oxidation reaction ( s = solid electrode ; aq = aqueous solution ) : Z n ( s ) → Z n ( a q ) 2 + + 2 e − ( \ displaystyle \ mathrm ( Zn_ ( ( s ) ) \ rightarrow Zn_ ( ( aq ) ) ^ ( 2 + ) + 2e ^ ( - ) \ ) ) The zinc sulfate is the electrolyte in that half cell . It is a solution which contains zinc cations Z n 2 + ( \ displaystyle \ mathrm ( Zn ) _ ( ) ^ ( 2 + ) ) , and sulfate anions S O 4 2 − ( \ displaystyle \ mathrm ( SO ) _ ( 4 ) ^ ( 2 - ) \ ) with charges that balance to zero . In the other half cell , the copper cations in a copper sulfate electrolyte are drawn to the copper cathode to which they attach themselves as they adopt electrons from the copper electrode by the reduction reaction : C u ( a q ) 2 + + 2 e − → C u ( s ) ( \ displaystyle \ mathrm ( Cu_ ( ( aq ) ) ^ ( 2 + ) + 2e ^ ( - ) \ rightarrow Cu_ ( ( s ) ) \ ) ) in effect leaving a deficit of electrons on the copper cathode . The difference of excess electrons on the anode and deficit of electrons on the cathode creates an electrical potential between the two electrodes . ( A detailed discussion of the microscopic process of electron transfer between an electrode and the ions in an electrolyte may be found in Conway . ) If the cathode and anode are connected by an external conductor , electrons would pass through that external circuit ( light bulb in figure ) , while the ions pass through the salt bridge to maintain charge balance until such a time as the anode and cathode reach electrical equilibrium of zero volts as chemical equilibrium is reached in the cell . In the process the zinc anode is dissolved while the copper electrode is plated with copper . The so - called `` salt bridge '' is not made of salt but could be made of material able to wick the cations and anions ( salts ) in the solutions , where the flow of positively charged cations along the `` bridge '' amounts to the same number of negative charges flowing in the opposite direction . If the light bulb is removed ( open circuit ) the emf between the electrodes is opposed by the electric field due to charge separation , and the reactions stop . For this particular cell chemistry , at 298 K ( room temperature ) , the emf E = 1.0934 V , with a temperature coefficient of dE / dT = − 4.53 × 10 V / K . Voltaic cells ( edit ) Volta developed the voltaic cell about 1792 , and presented his work March 20 , 1800 . Volta correctly identified the role of dissimilar electrodes in producing the voltage , but incorrectly dismissed any role for the electrolyte . Volta ordered the metals in a ' tension series ' , `` that is to say in an order such that any one in the list becomes positive when in contact with any one that succeeds , but negative by contact with any one that precedes it . '' A typical symbolic convention in a schematic of this circuit ( -- -- ) would have a long electrode 1 and a short electrode 2 , to indicate that electrode 1 dominates . Volta 's law about opposing electrode emfs implies that , given ten electrodes ( for example , zinc and nine other materials ) , 45 unique combinations of voltaic cells ( 10 × 9 / 2 ) can be created . Of cells ( edit ) The electromotive force produced by primary ( single - use ) and secondary ( rechargeable ) cells is usually of the order of a few volts . The figures quoted below are nominal , because emf varies according to the size of the load and the state of exhaustion of the cell . EMF <Th_colspan="3"> Cell chemistry Common name Anode Solvent , electrolyte Cathode 1.2 V Cadmium Water , potassium hydroxide NiO ( OH ) nickel - cadmium 1.2 V Mischmetal ( hydrogen absorbing ) Water , potassium hydroxide Nickel nickel -- metal hydride 1.5 V Zinc Water , ammonium or zinc chloride Carbon , manganese dioxide Zinc carbon 2.1 V Lead Water , sulfuric acid Lead dioxide Lead -- acid 3.6 V to 3.7 V Graphite Organic solvent , Li salts LiCoO Lithium - ion 1.35 V Zinc Water , sodium or potassium hydroxide HgO Mercury cell Electromagnetic induction ( edit ) Main article : Faraday 's law of induction The principle of electromagnetic induction , noted above , states that a time - dependent magnetic field produces a circulating electric field . A time - dependent magnetic field can be produced either by motion of a magnet relative to a circuit , by motion of a circuit relative to another circuit ( at least one of these must be carrying a current ) , or by changing the current in a fixed circuit . The effect on the circuit itself , of changing the current , is known as self - induction ; the effect on another circuit is known as mutual induction . For a given circuit , the electromagnetically induced emf is determined purely by the rate of change of the magnetic flux through the circuit according to Faraday 's law of induction . An emf is induced in a coil or conductor whenever there is change in the flux linkages . Depending on the way in which the changes are brought about , there are two types : When the conductor is moved in a stationary magnetic field to procure a change in the flux linkage , the emf is statically induced . The electromotive force generated by motion is often referred to as motional emf . When the change in flux linkage arises from a change in the magnetic field around the stationary conductor , the emf is dynamically induced . The electromotive force generated by a time - varying magnetic field is often referred to as transformer emf . Contact potentials ( edit ) See also : Volta potential and Electrochemical potential When solids of two different materials are in contact , thermodynamic equilibrium requires that one of the solids assume a higher electrical potential than the other . This is called the contact potential . Dissimilar metals in contact produce what is known also as a contact electromotive force or Galvani potential . The magnitude of this potential difference is often expressed as a difference in Fermi levels in the two solids when they are at charge neutrality , where the Fermi level ( a name for the chemical potential of an electron system ) describes the energy necessary to remove an electron from the body to some common point ( such as ground ) . If there is an energy advantage in taking an electron from one body to the other , such a transfer will occur . The transfer causes a charge separation , with one body gaining electrons and the other losing electrons . This charge transfer causes a potential difference between the bodies , which partly cancels the potential originating from the contact , and eventually equilibrium is reached . At thermodynamic equilibrium , the Fermi levels are equal ( the electron removal energy is identical ) and there is now a built - in electrostatic potential between the bodies . The original difference in Fermi levels , before contact , is referred to as the emf . The contact potential can not drive steady current through a load attached to its terminals because that current would involve a charge transfer . No mechanism exists to continue such transfer and , hence , maintain a current , once equilibrium is attained . One might inquire why the contact potential does not appear in Kirchhoff 's law of voltages as one contribution to the sum of potential drops . The customary answer is that any circuit involves not only a particular diode or junction , but also all the contact potentials due to wiring and so forth around the entire circuit . The sum of all the contact potentials is zero , and so they may be ignored in Kirchhoff 's law . Solar cell ( edit ) Main article : Theory of solar cells The equivalent circuit of a solar cell ; parasitic resistances are ignored in the discussion of the text . Solar cell voltage as a function of solar cell current delivered to a load for two light - induced currents I ; currents as a ratio with reverse saturation current I. Compare with Fig. 1.4 in Nelson . Operation of a solar cell can be understood from the equivalent circuit at right . Light , of sufficient energy ( greater than the bandgap of the material ) , creates mobile electron -- hole pairs in a semiconductor . Charge separation occurs because of a pre-existing electric field associated with the p-n junction in thermal equilibrium ( a contact potential creates the field ) . This charge separation between positive holes and negative electrons across a p-n junction ( a diode ) yields a forward voltage , the photo voltage , between the illuminated diode terminals . As has been noted earlier in the terminology section , the photo voltage is sometimes referred to as the photo emf , rather than distinguishing between the effect and the cause . The charge separation causes a photo voltage that drives current through any attached load . The current available to the external circuit is limited by internal losses I = I + I : I = I L − I 0 = I L − I S H − I D ( \ displaystyle I = I_ ( L ) - I_ ( 0 ) = I_ ( L ) - I_ ( SH ) - I_ ( D ) ) Losses limit the current available to the external circuit . The light - induced charge separation eventually creates a current ( called a forward current ) I through the cell 's junction in the direction opposite that the light is driving the current . In addition , the induced voltage tends to forward bias the junction . At high enough levels , this forward bias of the junction will cause a forward current , I in the diode opposite that induced by the light . Consequently , the greatest current is obtained under short - circuit conditions , and is denoted as I ( for light - induced current ) in the equivalent circuit . Approximately , this same current is obtained for forward voltages up to the point where the diode conduction becomes significant . The current delivered by the illuminated diode , to the external circuit is : I = I L − I 0 ( e q V / ( m k T ) − 1 ) , ( \ displaystyle I = I_ ( L ) - I_ ( 0 ) \ left ( e ^ ( qV / ( mkT ) ) - 1 \ right ) \ , ) where I is the reverse saturation current . Where the two parameters that depend on the solar cell construction and to some degree upon the voltage itself are m , the ideality factor , and kT / q the thermal voltage ( about 0.026 V at room temperature ) . This relation is plotted in the figure using a fixed value m = 2 . Under open - circuit conditions ( that is , as I = 0 ) , the open - circuit voltage is the voltage at which forward bias of the junction is enough that the forward current completely balances the photocurrent . Solving the above for the voltage V and designating it the open - circuit voltage of the I -- V equation as : V oc = m k T q ln ⁡ ( I L I 0 + 1 ) , ( \ displaystyle V_ ( \ text ( oc ) ) = m \ ( \ frac ( kT ) ( q ) ) \ \ ln \ left ( ( \ frac ( I_ ( \ text ( L ) ) ) ( I_ ( 0 ) ) ) + 1 \ right ) \ , ) which is useful in indicating a logarithmic dependence of V upon the light - induced current . Typically , the open - circuit voltage is not more than about 0.5 V . When driving a load , the photo voltage is variable . As shown in the figure , for a load resistance R , the cell develops a voltage that is between the short - circuit value V = 0 , I = I and the open - circuit value V , I = 0 , a value given by Ohm 's law V = IR , where the current I is the difference between the short - circuit current and current due to forward bias of the junction , as indicated by the equivalent circuit ( neglecting the parasitic resistances ) . In contrast to the battery , at current levels delivered to the external circuit near I , the solar cell acts more like a current source rather than a voltage source ( near vertical part of the two illustrated curves ) . The current drawn is nearly fixed over a range of load voltages , to one electron per converted photon . The quantum efficiency , or probability of getting an electron of photocurrent per incident photon , depends not only upon the solar cell itself , but upon the spectrum of the light . The diode possesses a `` built - in potential '' due to the contact potential difference between the two different materials on either side of the junction . This built - in potential is established when the junction is manufactured and that voltage a by - product of thermodynamic equilibrium within the cell . Once established , this potential difference can not drive a current , however , as connecting a load does not upset this equilibrium . In contrast , the accumulation of excess electrons in one region and of excess holes in another , due to illumination , results in a photo voltage that does drive a current when a load is attached to the illuminated diode . As noted above , this photo voltage also forward biases the junction , and so reduces the pre-existing field in the depletion region . See also ( edit ) Counter-electromotive force Electric battery Electrochemical cell Electrolytic cell Galvanic cell Voltaic pile References ( edit ) Jump up ^ emf. ( 1992 ) . American Heritage Dictionary of the English Language 3rd ed . Boston : Houghton Mifflin . Jump up ^ Irving Langmuir ( 1916 ) . `` The Relation Between Contact Potentials and Electrochemical Action '' . Transactions of the American Electrochemical Society . The Society . 29 ( 14 ) : 125 -- 182 . Jump up ^ Tipler , Paul A. ( January 1976 ) . Physics . New York , NY : Worth Publishers , Inc. p. 803 . ISBN 0 - 87901 - 041 - X . Jump up ^ David M. Cook ( 2003 ) . The Theory of the Electromagnetic Field . Courier Dover . p. 157 . ISBN 978 - 0 - 486 - 42567 - 2 . ^ Jump up to : Lawrence M Lerner ( 1997 ) . Physics for scientists and engineers . Jones & Bartlett Publishers . pp. 724 -- 727 . ISBN 0 - 7637 - 0460 - 1 . Jump up ^ Paul A. Tipler ; Gene Mosca ( 2007 ) . Physics for Scientists and Engineers ( 6 ed . ) . Macmillan . p. 850 . ISBN 1 - 4292 - 0124 - X . Jump up ^ Alvin M. Halpern ; Erich Erlbach ( 1998 ) . Schaum 's outline of theory and problems of beginning physics II . McGraw - Hill Professional . p. 138 . ISBN 0 - 07 - 025707 - 8 . Jump up ^ Robert L. Lehrman ( 1998 ) . Physics the easy way . Barron 's Educational Series . p. 274 . ISBN 978 - 0 - 7641 - 0236 - 3 . Jump up ^ Singh , Kongbam Chandramani ( 2009 ) . `` § 3.16 EMF of a source '' . Basic Physics . Prentice Hall India . p. 152 . ISBN 81 - 203 - 3708 - 5 . ^ Jump up to : Florian Cajori ( 1899 ) . A History of Physics in Its Elementary Branches : Including the Evolution of Physical Laboratories . The Macmillan Company . pp. 218 -- 219 . Jump up ^ Van Valkenburgh ( 1995 ) . Basic Electricity . Cengage Learning . pp. 1 -- 46 . ISBN 978 - 0 - 7906 - 1041 - 2 . Jump up ^ David J Griffiths ( 1999 ) . Introduction to Electrodynamics ( 3rd ed . ) . Pearson / Addison - Wesley . p. 293 . ISBN 0 - 13 - 805326 - X . Jump up ^ Only the electric field due to the charge separation caused by the emf is counted . In a solar cell , for example , an electric field is present related to the contact potential that results from thermodynamic equilibrium ( discussed later ) , and this electric field component is not included in the integral . Rather , only the electric field due to the particular portion of charge separation that causes the photo voltage is included . Jump up ^ Richard P. Olenick ; Tom M. Apostol ; David L. Goodstein ( 1986 ) . Beyond the mechanical universe : from electricity to modern physics . Cambridge University Press . p. 245 . ISBN 978 - 0 - 521 - 30430 - 6 . Jump up ^ David M. Cook ( 2003 ) . The Theory of the Electromagnetic Field . Courier Dover . p. 158 . ISBN 978 - 0 - 486 - 42567 - 2 . ^ Jump up to : Colin BP Finn ( 1992 ) . Thermal Physics . CRC Press . p. 163 . ISBN 0 - 7487 - 4379 - 0 . Jump up ^ M. Fogiel ( 2002 ) . Basic Electricity . Research & Education Association . p. 76 . ISBN 0 - 87891 - 420 - X . ^ Jump up to : David Halliday ; Robert Resnick ; Jearl Walker ( 2008 ) . Fundamentals of Physics ( 6th ed . ) . Wiley . p. 638 . ISBN 978 - 0 - 471 - 75801 - 3 . Jump up ^ Roger L Freeman ( 2005 ) . Fundamentals of Telecommunications ( 2nd ed . ) . Wiley . p. 576 . ISBN 0 - 471 - 71045 - 8 . Jump up ^ Terrell Croft ( 1917 ) . Practical Electricity . McGraw - Hill . p. 533 . Jump up ^ Leonard B Loeb ( 2007 ) . Fundamentals of Electricity and Magnetism ( Reprint of Wiley 1947 3rd ed . ) . Read Books . p. 86 . ISBN 1 - 4067 - 0733 - 3 . Jump up ^ Jenny Nelson ( 2003 ) . The Physics of Solar Cells . Imperial College Press . p. 6 . ISBN 1 - 86094 - 349 - 7 . Jump up ^ John S. Rigden , ( editor in chief ) , Macmillan encyclopedia of physics . New York : Macmillan , 1996 . Jump up ^ J.R.W. Warn ; A.P.H. Peters ( 1996 ) . Concise Chemical Thermodynamics ( 2 ed . ) . CRC Press . p. 123 . ISBN 0 - 7487 - 4445 - 2 . Jump up ^ Samuel Glasstone ( 2007 ) . Thermodynamics for Chemists ( Reprint of D. Van Nostrand Co ( 1964 ) ed . ) . Read Books . p. 301 . ISBN 1 - 4067 - 7322 - 0 . Jump up ^ Nikolaus Risch ( 2002 ) . `` Molecules - bonds and reactions '' . In L Bergmann ; et al. Constituents of Matter : Atoms , Molecules , Nuclei , and Particles . CRC Press . ISBN 0 - 8493 - 1202 - 7 . Jump up ^ The brave reader can find an extensive discussion for organic electrochemistry in Christian Amatore ( 2000 ) . `` Basic concepts '' . In Henning Lund ; Ole Hammerich . Organic electrochemistry ( 4 ed . ) . CRC Press . ISBN 0 - 8247 - 0430 - 4 . Jump up ^ The idea of electronegativity has been extended to include the concept of electronegativity equalization , the notion that when molecules are brought together the electrons rearrange to achieve an equilibrium where there is no net force upon them . See , for example , Francis A. Carey ; Richard J. Sundberg ( 2007 ) . Advanced organic chemistry ( 5 ed . ) . Springer . p. 11 . ISBN 0 - 387 - 68346 - 1 . Jump up ^ BE Conway ( 1999 ) . `` Energy factors in relation to electrode potential '' . Electrochemical supercapacitors . Springer . p. 37 . ISBN 0 - 306 - 45736 - 9 . Jump up ^ R.J.D. Tilley ( 2004 ) . Understanding Solids . Wiley . p. 267 . ISBN 0 - 470 - 85275 - 5 . Jump up ^ Paul Fleury Mottelay ( 2008 ) . Bibliographical History of Electricity and Magnetism ( Reprint of 1892 ed . ) . Read Books . p. 247 . ISBN 1 - 4437 - 2844 - 6 . Jump up ^ Helge Kragh ( 2000 ) . `` Confusion and Controversy : Nineteenth - century theories of the voltaic pile '' ( PDF ) . Nuova Voltiana : Studies on Volta and his times . Università degli studi di Pavia . Archived from the original ( PDF ) on 2009 - 03 - 20 . Jump up ^ Linnaus Cumming ( 2008 ) . An Introduction to the Theory of Electricity ( Reprint of 1885 ed . ) . BiblioBazaar . p. 118 . ISBN 0 - 559 - 20742 - 5 . Jump up ^ George L. Trigg ( 1995 ) . Landmark experiments in twentieth century physics ( Reprint of Crane , Russak & Co 1975 ed . ) . Courier Dover . p. 138 ff . ISBN 0 - 486 - 28526 - X . Jump up ^ Angus Rockett ( 2007 ) . `` Diffusion and drift of carriers '' . Materials science of semiconductors . New York , NY : Springer Science . p. 74 ff . ISBN 0 - 387 - 25653 - 9 . Jump up ^ Charles Kittel ( 2004 ) . `` Chemical potential in external fields '' . Elementary Statistical Physics ( Reprint of Wiley 1958 ed . ) . Courier Dover . p. 67 . ISBN 0 - 486 - 43514 - 8 . Jump up ^ George W. Hanson ( 2007 ) . Fundamentals of Nanoelectronics . Prentice Hall . p. 100 . ISBN 0 - 13 - 195708 - 2 . Jump up ^ Norio Sato ( 1998 ) . `` Semiconductor photoelectrodes '' . Electrochemistry at metal and semiconductor electrodes ( 2nd ed . ) . Elsevier . p. 110 ff . ISBN 0 - 444 - 82806 - 0 . Jump up ^ Richard S. Quimby ( 2006 ) . Photonics and lasers . Wiley . p. 176 . ISBN 0 - 471 - 71974 - 9 . Jump up ^ Donald A. Neamen ( 2002 ) . Semiconductor physics and devices ( 3rd ed . ) . McGraw - Hill Professional . p. 240 . ISBN 0 - 07 - 232107 - 5 . ^ Jump up to : Jenny Nelson ( 2003 ) . Solar cells . Imperial College Press . p. 8 . ISBN 1 - 86094 - 349 - 7 . Jump up ^ SM Dhir ( 2000 ) . `` § 3.1 Solar cells '' . Electronic Components and Materials : Principles , Manufacture and Maintenance . Tata McGraw - Hill . ISBN 0 - 07 - 463082 - 2 . ^ Jump up to : Gerardo L. Araújo ( 1994 ) . `` § 2.5. 1 Short - circuit current and open - circuit voltage '' . In Eduardo Lorenzo . Solar Electricity : Engineering of photovoltaic systems . Progenza for Universidad Politechnica Madrid . p. 74 . ISBN 84 - 86505 - 55 - 0 . Jump up ^ In practice , at low voltages m → 2 , whereas at high voltages m → 1 . See Araújo , op . cit . ISBN 84 - 86505 - 55 - 0 . page 72 Jump up ^ Robert B. Northrop ( 2005 ) . `` § 6.3. 2 Photovoltaic Cells '' . Introduction to Instrumentation and Measurements . CRC Press . p. 176 . ISBN 0 - 8493 - 7898 - 2 . Further reading ( edit ) Andrew Gray , `` Absolute Measurements in Electricity and Magnetism '' , Electromotive force . Macmillan and co. , 1884 . John O 'M . Bockris ; Amulya K.N. Reddy ( 1973 ) . `` Electrodics '' . Modern Electrochemistry : An Introduction to an Interdisciplinary Area ( 2 ed . ) . Springer . ISBN 0 - 306 - 25002 - 0 . Roberts , Dana ( 1983 ) . `` How batteries work : A gravitational analog '' . Am. J. Phys. 51 : 829 . Bibcode : 1983AmJPh ... 51 ... 829R . doi : 10.1119 / 1.13128 . 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 20 - 20413 Hazel Rossotti , `` Chemical applications of potentiometry '' . London , Princeton , N.J. , Van Nostrand , 1969 . ISBN 0 - 442 - 07048 - 9 LCCN 69 - 11985 / / 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 07 - 3935 / / 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 . Norio Sato ( 1998 ) . `` Semiconductor photoelectrodes '' . Electrochemistry at metal and semiconductor electrodes ( 2nd ed . ) . Elsevier . p. 326 ff . ISBN 0 - 444 - 82806 - 0 . External links ( edit ) Hai , Pham Nam ; Ohya , Shinobu ; Tanaka , Masaaki ; Barnes , Stewart E. ; Maekawa , Sadamichi ( 2009 - 03 - 08 ) . `` Electromotive force and huge magnetoresistance in magnetic tunnel junctions '' . Nature . 458 ( 7237 ) : 489 -- 92 . Bibcode : 2009Natur. 458 ... 489H . doi : 10.1038 / nature07879 . PMID 19270681 . Retrieved 2009 - 03 - 10 . 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