Editing Electromotive force (section)

==Overview==
Devices that can provide emf include [[electrochemical cell]]s, [[Thermoelectric effect|thermoelectric device]]s, [[solar cells]], [[photodiode]]s, [[electrical generator]]s, [[inductor]]s, [[electrical transformer|transformer]]s and even [[Van de Graaff generator]]s.<ref name="Lerner">{{cite book |first=Lawrence M. |last=Lerner |url=https://books.google.com/books?id=Nv5GAyAdijoC&pg=PA727 |title=Physics for scientists and engineers |publisher=Jones & Bartlett Publishers |year=1997 |isbn=978-0-7637-0460-5 |pages=724–727}}</ref><ref name=Tipler>{{cite book
 | title=Physics for Scientists and Engineers
 |first1=Paul A.|last1=Tipler |first2=Gene|last2=Mosca | url=https://books.google.com/books?id=BMVR37-8Jh0C&pg=PA850
 | page=850
 | isbn=978-1-4292-0124-7
 | year=2007
 | edition=6 
 | publisher=Macmillan
 }}</ref> In nature, emf is generated when [[magnetic field]] fluctuations occur through a surface. For example, the shifting of the [[Earth's magnetic field]] during a [[geomagnetic storm]] induces currents in an [[electrical grid]] as the lines of the magnetic field are shifted about and cut across the conductors.

In a battery, the charge separation that gives rise to a potential difference ([[voltage]]) between the terminals is accomplished by [[chemical reaction]]s at the [[electrode]]s that convert chemical [[potential energy]] into electromagnetic potential energy.<ref name=Schaum>{{cite book
 |title=Schaum's outline of theory and problems of beginning physics II
 |first1=Alvin M.|last1=Halpern |first2=Erich|last2=Erlbach |url=https://books.google.com/books?id=vN2chIay624C&pg=PA138
 |page=138 |year=1998
 |publisher=McGraw-Hill Professional
 |isbn=978-0-07-025707-8
 }}</ref><ref>{{cite book
 | title = Physics the easy way
 | first = Robert L.|last=Lehrman
 | publisher = Barron's Educational Series
 | year = 1998
 | isbn = 978-0-7641-0236-3
 | page = [https://archive.org/details/physicseasyway00lehr_0/page/274 274]
 | url = https://archive.org/details/physicseasyway00lehr_0
 | url-access = registration
 | quote = emf  separated charge reaction potential.
 }}</ref> A [[Voltaic Cell|voltaic cell]] can be thought of as having a "charge pump" of atomic dimensions at each electrode, that is:
{{Blockquote
|text=A (chemical) source of emf can be thought of as a kind of ''charge pump'' that acts to move positive charges 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 <math display="inline">\mathit dW</math> on that charge to move it to the high-potential terminal. The emf <math display="inline">\mathcal{E}</math> of the source is defined as the work <math display="inline">\mathit dW</math> done per charge <math display="inline">dq</math>. <math display="inline">\mathcal{E} = \frac{\mathit dW}{\mathit dq}</math>.<ref name="Singh">{{cite book|chapter-url=https://books.google.com/books?id=oS_vSI-3yuwC|title=Basic Physics|first=Kongbam Chandramani|publisher=Prentice Hall India|year=2009|isbn=978-81-203-3708-4|page=152|chapter=§3.16 EMF of a source|last=Singh}}
</ref>
}}

In an electrical generator, a time-varying magnetic field inside the generator creates an [[electric field]] via [[electromagnetic induction]], which creates a potential difference between the generator terminals. Charge separation takes place within the generator because electrons flow away from one terminal toward the other, until, in the open-circuit case, an electric field is developed that makes further charge separation impossible. The emf is countered by the electrical voltage due to charge separation. If a [[Electrical load|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]].