Two point charges −q and +q  are located at points (0,  0,  −2a)  and (0,  0,  2a)  respectively. The electric potential at a point (0,  0,  z),  where z>a is :

Potential at P due to (+q)  chargeV1=14πε0.q(z−2a)Potential at P due to (-q) chargeV2=14πε0.−q(z+2a)Total potential at P due to (AB) electric dipoleV=V1+V2=14πε0.q(z−2a)−14πε0q(z+2a)=q4πε0(z+2a−z+2a)(z−2a)(z+2a)⇒  V=4qa4πε0(z2−4a2)