Editing Fermi energy (section)

==Context==
{{Main|Fermi gas}}
In [[quantum mechanics]], a group of particles known as [[fermion]]s (for example, [[electron]]s, [[proton]]s and [[neutron]]s) obey the [[Pauli exclusion principle]]. This states that two fermions cannot occupy the same [[quantum state]]. Since an idealized non-interacting Fermi gas can be analyzed in terms of single-particle [[stationary state]]s, we can thus say that two fermions cannot occupy the same stationary state. These stationary states will typically be distinct in energy. To find the ground state of the whole system, we start with an empty system, and add particles one at a time, consecutively filling up the unoccupied stationary states with the lowest energy. When all the particles have been put in, the '''Fermi energy''' is the kinetic energy of the highest occupied state.

As a consequence, even if we have extracted all possible energy from a Fermi gas by cooling it to near [[absolute zero]] temperature, the fermions are still moving around at a high speed. The fastest ones are moving at a velocity corresponding to a kinetic energy equal to the Fermi energy. This speed is known as the '''Fermi velocity'''. Only when the temperature exceeds the related '''Fermi temperature''', do the particles begin to move significantly faster than at absolute zero.

The Fermi energy is an important concept in the [[solid state physics]] of metals and [[superconductor]]s. It is also a very important quantity in the physics of [[Superfluid|quantum liquid]]s like low temperature [[helium]] (both normal and superfluid <sup>3</sup>He), and it is quite important to [[nuclear physics]] and to understanding the stability of [[White dwarf|white dwarf stars]] against [[gravitational collapse]].