The angle between the electric field line and an equipotential surface is _____________.

A point $\mathrm{P}$ is $40 \mathrm{m}$ away from $20 \mathrm{\mu C}$ point charge and $20 \mathrm{m}$ from $4\mathrm{\mu C}$ point charge. The electric potential at point $\mathrm{P}$ is____$\mathrm{V}$.

($\mathrm{K}=9\times {10}^9{\mathrm{Nm}}^2{\mathrm{C}}^{-2}$)

Derive an expression for the capacitance of the parallel plate capacitor.

A $600 \mathrm{pF}$ capacitor is charged by a $200 \mathrm{V}$ supply. It is then disconnected from the supply and is connected to another uncharged $600 \mathrm{pF}$ capacitor. How much electrostatic energy is lost in the process?