Volta Potential Between Two Metals
When two metals are electrically isolated from each other, an arbitrary potential difference may exist between them. However, when two different neutrally charged metals are brought into electrical contact (even indirectly, say, through a long wire), electrons will flow from the metal with a higher Fermi level to the metal with the lower Fermi level until the Fermi levels in the both phases are equal. Once this has occurred, the metals are in thermodynamic equilibrium with each other (the actual number of electrons that passes between the two phases is usually small). Just because the Fermi levels are equal, however, does not mean that the electric potentials are equal. The electric potential outside each material is controlled by its work function, and so dissimilar metals can show an electric potential difference even at equilibrium.
The Volta potential is not an intrinsic property of the two bulk metals under consideration, but rather is determined by work function differences between the metals' surfaces. Just like the work function, the Volta potential depends sensitively on surface state, contamination, and so on.
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