(i) Derivation of expression for the electric potential due to an electric dipole at a point on the axial line. (ii) Depiction of equipotential surfaces due to an electric dipole. Potential due to charge at A, $V_A=\;\frac{1}{4\pi\varepsilon_0}\;\frac{-q}{r+a}$ Potential due to charge at $B$, $V_B=\;\frac{1}{4\pi\varepsilon_0}\;\frac{+q}{r-a}$ $\;\therefore\;\text{ Potential at point }\;P,$ $\;V=V_B+V_A$ $\;\therefore\;\text{ Net Potential at }\;P$ $\;=\;\frac{q}{4\pi\varepsilon_0}\left[\;\frac{-1}{r+a}+\;\frac{1}{r-a}\right]$