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 eiectric 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

The correct option is D The torque on the dipole due to the field may be zero.

A and B cannot be true in any case. Because for the net force on a dipole to be zero, the field lines should be parallel to each other and the dipole must be placed perpendicular to the field. However, in case of point charge you cannot have parallel field lines.
C: Not true again. Because in any orientation (except radial) the force on the positive and negative charge will form a couple and you will have torque.
D: True. If you place the dipole radial from the point object, then there will be no net torque. Since torque $=F\times R\sin \theta$. If it is radial, $\sin \theta =0$, so torque $=0$.