The image of an electric dipole moment p at ( 0 , 0 , a ) above an infinite grounded conducting plane in the xy -plane is a dipole moment at ( 0 , 0 , − − a ) with equal magnitude and direction rotated azimuthally by π. That is, a dipole moment with Cartesian components ( p sin ⁡ ⁡ θ θ cos ⁡ ⁡ ϕ ϕ , p sin ⁡ ⁡ θ θ sin ⁡ ⁡ ϕ ϕ , p cos ⁡ ⁡ θ θ ) will have in image dipole moment ( − − p sin ⁡ ⁡ θ θ cos ⁡ ⁡ ϕ ϕ , − − p sin ⁡ ⁡ θ θ sin ⁡ ⁡ ϕ ϕ , p cos ⁡ ⁡ θ θ ) . The dipole experiences a force in the z direction, given by