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When two metals are electrically isolated from each other, an arbitrary potential difference may exist between them. However, when two different neutral metal surfaces are brought into electrical contact (even indirectly, say, through a long electro-conductive wire), electrons will flow from the
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is the Volta potential. It follows therefore that the value of the potential can be measured by varying the capacitance between the materials by a known amount (e.g., by moving the objects further from each other) and measuring the displaced charge that flows through the wire that connects them.
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to the metal with the lower Fermi level until the Fermi levels in the two 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
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The Volta potential, however, corresponds to a real electric field in the spaces between and around the two metal objects, a field generated by the accumulation of charges at their surfaces. The total charge
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The Volta potential difference between a metal and an electrolyte can be measured in a similar fashion. The Volta potential of a metal surface can be mapped on very small scales by use of a
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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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equal, however, does not mean that the electric potentials are equal. The electric potential outside each material is controlled by its
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Kelvin probe energy diagram at flat vacuum configuration, used for measuring Volta potential between sample and probe.
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The Volta potential can be significant (of order 1 volt) but it cannot be measured directly by an ordinary
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When the two metals depicted here are in thermodynamic equilibrium with each other as shown (equal
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an intrinsic property of the two bulk metals under consideration, but rather is determined by
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between the two materials, a difference that is exactly zero at equilibrium.
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IUPAC Gold Book, definition of contact (Volta) potential difference.
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221:Measurement of Volta potential (Kelvin probe)
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320:{\displaystyle Q=C\Delta \psi }
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343:{\displaystyle \Delta \psi }
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288:{\displaystyle C}
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358:, based on
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398:(voltage)
338:ψ
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