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[[File:Work function mismatch gold aluminum.svg|thumb|300 px|Plot of electron energy levels against position, in a gold-vacuum-aluminium system. The two metals depicted here are in complete thermodynamic equilibrium. However, the vacuum [[electrostatic potential]] {{math|''ϕ''}} is not flat due to a difference in work function.]]
In practice, one directly controls {{math|''E''<sub>F</sub>}} by the voltage applied to the material through electrodes, and the work function is generally a fixed characteristic of the surface material. Consequently, this means that when a [[voltage]] is applied to a material, the [[Electrostatic discharge materials|electrostatic]] potential {{math|''ϕ''}} produced in the vacuum will be somewhat lower than the applied voltage, the difference depending on the work function of the material surface. Rearranging the above equation, one has
:<math>\phi = V - \frac{W}{e}</math>
where {{math|''V'' {{=}} −''E''<sub>F</sub>/''e''}} is the voltage of the material (as measured by a [[voltmeter]], through an attached electrode), relative to an [[electrical ground]] that is defined as having zero Fermi level. The fact that {{math|''ϕ''}} depends on the material surface means that the space between two dissimilar conductors will have a built-in [[electric field]], when those conductors are in total equilibrium with each other (electrically shorted to each other, and with equal temperatures).
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