Question

An electric dipole is placed in an electric field generated by a point charge.
(a) The net electric force on the dipole must be zero.
(b) The net electric force on the dipole may be zero.
(c) The torque on the dipole due to the field must be zero.
(d) The torque on the dipole due to the field may be zero.

Answer 1

(d) The torque on the dipole due to the field may be zero.
Torque acting on a dipole placed in an electric field,
$$\begin{gathered} \tau =r\times F \\ =rF\sin \theta , \end{gathered}$$
where $\theta$ is the angle between the force F and the arm of the couple.
So, for $\theta$=0 or pi ,the torque will be zero. Hence, the amount of torque acting on the dipole depends on the orientation of the dipole in the given electric field.