The flux of an electric field E through a surface area ΔS is given by Δϕ=E⋅ΔS , where Δϕ is the flux. Therefore, Δϕ=EΔSCosθ . Here, θ is the angle between the electric field E and the normal to the surface area.Thus, for the flux to be maximum, cosθ should be maximum. Thus, for θ=0, the flux is maximum, i.e. the electric field lines are perpendicular to the surface area.The flux is minimum if θ=90 . Thus, cosθ=0 and, hence, flux is also 0 . Thus, if the electric field lines are parallel to the surface area, the flux is minimum.