The electric potential of a line charge λ λ located at ( ρ ρ ′ , θ θ ′ ) is given by Φ Φ ( ρ ρ , θ θ ) = − − λ λ 2 π π ϵ ϵ ln ⁡ ⁡ R = − − λ λ 4 π π ϵ ϵ ln ⁡ ⁡ | ρ ρ 2 + ( ρ ρ ′ ) 2 − − 2 ρ ρ ρ ρ ′ cos ⁡ ⁡ ( θ θ − − θ θ ′ ) | where R is the shortest distance between the line charge and the observation point.