The minimum work needed to extract electrons from the Fermi level of a metal M across a surface carrying no net charge. It is equal to the sum of the potential energy and the kinetic Fermi energy taken with the reverse sign: \[\mathit{\Phi} ^{M} = -(V_{\text{e}}+ɛ_{\text{e}}^{F})\] where V e is the potential energy for electrons in metals and ɛe^F is the kinetic energy of electrons at the Fermi level.