An electric dipole is situated in a uniform electric field `vecE` such that its moment `vecp` is aligned in the direction of the field. Is the equilibrium of the dipole stable or unstable? If `vecp` and `vecE` are in opposite directions, what will be the nature of the equilibrium?

Explanation

When the electric dipole moment `vecp` is aligned in the direction of the field `vecE`, the dipole is in stable equilibrium. Because if the dipole is deflected through angle `theta`, the torque `pE sin theta` acting on it brings it back along the field, When `vecp` and `vecE` are in opposite directions `(theta=180^(@))`, the torque acting is the dipole is zero and the dipole is zero and the dipole is again is equilibrium. But the equilibrium is unstable, because if the dipole is deflected from its present position, the torque acting on it now will rotate is dipole to set it in the direction of the field.