In electronics, the work function is the energy (usually measured in electron volts) needed to remove an electron from the Fermi level in a metal to a point at infinite distance away outside the surface. The work function is important in the theory of thermionic emission. In that case, as for example that of an electron escaping from the headed negatively-charged filament of a vacuum tube, the work function may be called the thermionic work function. Photoelectric emission has a corresponding work function.
See also free energy for the Helmholtz free energy equation, which is the thermodynamic work function.