An electric dipole is held in a uniform electric field. (i) Show that the net force acting on it is zero. (ii) The dipole is aligned parallel to the field. Find the work in rotating it through an angle of 180°.

(I) Let an electric dipole be placed in a uniform electric field →E, making an angle θ with the field. Force on charge +q →F+q = +q →E (along →E). Force on charge −q →F−q = −q →E (opposite to →E). As two equal and opposite forces are acting on the dipole, therefore net force →F = 0.
(ii) We know that work done in rotating an electric dipole from θ1 to θ2 (where θ1 is the angle of the dipole with the electric field) is given by, W = pE(cosθ1 − cosθ2). Now given θ1 = 0, θ2 = 180°. Therefore, W = pE(cos0 − cos180) = 2pE