question_answer

An electric dipole free to move is placed in a uniform electric field. Explain along with diagram its motion when it is placed, (a)  parallel to the field, (b) perpendicular to the field.                

Answer:

                (a) Since the line of action of the two forces passes through the same point, the net force and the net torque acting on the dipole is zero. So no motion is produced when a dipole is placed parallel to the electric field. (b) The two equal and opposite forces - \[-qE\] and \[+qE\] constitute a couple and hence a torque acts on the dipole, given by \[\tau =pE\sin {{90}^{\circ }}=q.2aE\]\[[\because p=q.2a]\] This torque rotates the dipole about an axis perpendicular to the electric field and passing through the midpoint of the dipole.