The occupied states are inside the Fermi sphere in k-space as shown below; the radius is Fermi h? ( 37N 2/3 wave number kp. EF EF 2m V 2m. From these two equations, kp can be found as, Fermi Surface E = EF 1/3 2 KF k V The surface of the Fermi sphere represents the boundary between occupied & unoccupied k states at T = 0 for the free electron gas.