Electromotive force (cells)

Article By:

Archer, Mary D. Grantchester, Cambridge, United Kingdom.

Feldberg, Stephen W. Department of Applied Science, Brookhaven National Laboratory, Upton, New York.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.223300

The voltage or electric potential difference across the terminals of a cell when no current is drawn from it. The electromotive force (emf) is the sum of the electric potential differences produced by a separation of charges (electrons or ions) that can occur at each phase boundary (or interface) in the cell. The magnitude of each potential difference depends on the chemical nature of the two contacting phases. Thus, at the interface between two different metals, some electrons will have moved from the metal with a higher free energy of electrons to the metal with a lower free energy of electrons. The resultant charge separation will produce a potential difference, just as charge separation produces a voltage across a capacitor; at equilibrium this exactly opposes further electron flow. Similarly, potential differences can be produced when electrons partition across a metal|solution interface or metal|solid interface, and when ions partition across a solution| membrane|solution interface.

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