Inserting this into the above equation for Δ μ yields e Δ Δ ρ ρ ≃ ≃ ε ε 0 k 0 2 Δ Δ ϕ ϕ where k 0 = d e f 3 e 2 ρ ρ 2 ε ε 0 E F = m e 2 k F ε ε 0 π π 2 ℏ ℏ 2 is called the Thomas–Fermi screening wave vector.