(a) Four point charges of $1 \mu \mathrm{C},-2 \mu \mathrm{C}, 1 \mu \mathrm{C}$ and $-2 \mu \mathrm{C}$ are placed at the corners $\mathrm{A}, \mathrm{B}, \mathrm{C}$ and D respectively, of a square of side 30 cm . Find the net force acting on a charge of $4 \mu \mathrm{C}$ placed at the centre of the square.

OR

(b) Three point charges, 1 pC each, are kept at the vertices of an equilateral triangle of side 10 cm . Find the net electric field at the centroid of triangle.

Depict the orientation of an electric dipole in (a) stable and (b) unstable equilibrium in an external uniform electric field. Write the potential energy of the dipole in each case.