Two particles of charges 4 nC and $Q$ are kept in air with a separation of 10 cm between them. If the electrostatic potential energy of the system is $1.8 \mu \mathrm{~J}$, then $Q=$

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?