Editing Electromotive force (section)

===Contact potentials===
{{See also|Volta potential|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''.<ref name=Trigg>{{cite book |title=Landmark experiments in twentieth century physics |first=George L.|last=Trigg |page=138 ''ff'' |url=https://books.google.com/books?id=YOQ9fi5yQ4sC&pg=PA138 |isbn=978-0-486-28526-9 |year=1995 |publisher=Courier Dover |edition=Reprint of Crane, Russak & Co 1975}}</ref> 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 level]]s in the two solids when they are at charge neutrality, where the Fermi level (a name for the [[chemical potential]] of an electron system<ref name=Rockett>{{cite book |title=Materials science of semiconductors |first=Angus|last=Rockett |chapter=Diffusion and drift of carriers |page=74 ''ff'' |chapter-url=https://books.google.com/books?id=n5zMiMfw6ZUC&pg=PA74 |isbn=978-0-387-25653-5 |year=2007 |publisher=Springer Science |location=New York, NY}}</ref><ref name=Kittel>{{cite book |title=Elementary Statistical Physics |first=Charles|last=Kittel |chapter-url=https://books.google.com/books?id=5sd9SAoRjgQC&pg=PA67 |chapter= Chemical potential in external fields |page=67 |isbn=978-0-486-43514-5 |publisher=Courier Dover |year=2004 |edition=Reprint of Wiley 1958}}
</ref>) describes the energy necessary to remove an electron from the body to some common point (such as ground).<ref name=Hanson>{{cite book |title=Fundamentals of Nanoelectronics |first=George W.|last=Hanson |page=100 |url=https://books.google.com/books?id=L7AUi7ltCksC&pg=PA100 |isbn=978-0-13-195708-4 |year=2007 |publisher=Prentice Hall}}</ref> 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 level]]s 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.<ref name=Sato>{{cite book |title=Electrochemistry at metal and semiconductor electrodes |first=Norio|last=Sato |page=110 ''ff'' |chapter-url=https://books.google.com/books?id=olQzaXNgM74C&pg=PA110 |isbn=978-0-444-82806-4 |year=1998 |publisher=Elsevier |edition=2nd |chapter= Semiconductor photoelectrodes}}</ref>
The contact potential cannot 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 circuit laws|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.<ref name=Quimby>{{cite book |title=Photonics and lasers |first=Richard S.|last=Quimby |page=176 |url=https://books.google.com/books?id=82f-gIvtC7wC&pg=PA176 |isbn=978-0-471-71974-8 |publisher=Wiley |year=2006}}</ref><ref name=Neamen>{{cite book |title=Semiconductor physics and devices |first=Donald A.|last=Neamen |url=https://archive.org/details/semiconductorphy00neam |url-access=registration |page=[https://archive.org/details/semiconductorphy00neam/page/240 240] |year=2002 |isbn=978-0-07-232107-4 |publisher=McGraw-Hill Professional |edition=3rd}}</ref>