Question

An electric dipole is placed in an electric field generated by a point charge.

• A. The net electric force on the dipole may be zero.
• B. The net electric force on the dipole must be zero.
• C. The torque on the dipole due to the field must be zero.
• D. The torque on the dipole due to field may be zero.

Answer 1

Answer: (D)

Generally the electric field due to the point charge is $E=kq/r^2$ and dipole moment is $p=qr$
Force on the dipole is $\overrightarrow{F}=(\overrightarrow{p}.\overrightarrow{\nabla })\overrightarrow{E}\ne 0$ always in this case.
Torque on the dipole $\overrightarrow{\tau }=\overrightarrow{p}\times \overrightarrow{E}$
if the dipole moment and the electric field are parallel to each other, then the torque may be zero because $pE\sin 0=0$