Two charges 5 nC and -2 nC are placed at points $( 5 cm\mathrm{~}$ $0,0)$ and $(23 \mathrm{~cm}, 0,0)$ in a region of space where there is no other external field. The electrostatic potential energy of this charge system is

A neutral ammonia $\mathrm{NH}_3$ in its vapour state has electric dipole moment of magnitude $5 \times 10^{-30} \mathrm{C}-\mathrm{m}$. How far apart are the molecules centres of positive and negative charge?

If four charges $q_1=+1 \times 10^{-8} \mathrm{C}, q_2=-2 \times 10^{-8} \mathrm{C}$, $q_3=+3 \times 10^{-8} \mathrm{C}$ and $q_4=+2 \times 10^{-8} \mathrm{C}$ are kept at the four corners of a square of side 1 m , then the electric potential at the centre of the square is