If the chemical potential does not vary too much, the above equation can be linearized: ρ ρ induced ( r ) ≈ ≈ − − e 2 ∂ ∂ n ∂ ∂ μ μ ϕ ϕ ( r ) where ∂ ∂ n / ∂ ∂ μ μ is evaluated at μ 0 and treated as a constant.