In the Thomas–Fermi approximation, named after Llewellyn Thomas and Enrico Fermi, the system is maintained at a constant electron chemical potential (Fermi level) and at low temperature. The former condition corresponds, in a real experiment, to keeping the metal/fluid in electrical contact with a fixed potential difference with ground. The chemical potential μ is, by definition, the energy of adding an extra electron to the fluid. This energy may be decomposed into a kinetic energy T part and the potential energy − eφ part. Since the chemical potential is kept constant, Δ Δ μ μ = Δ Δ T − − e Δ Δ ϕ ϕ = 0.