Question

An electric dipole is placed in an electric field generated by a point charge. Choose the correct option:

• 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 is not equal to zero

Answer 1

Answer: (D)

the torque on the dipole due to the field is not equal to zero

The electric field generated by the point charge is non-uniform which varies as $\propto \frac{1}{r^2}$. For the diplole the electric field acting on the two charges never be equal. Hence the net electric field on the dipole cannot be zero. Thus electric force acting on the the two charges will never cancel each other.

For the torque acting on the dipole, first remember that in uniform electric field the torque becomes zero when the dipole becomes aligned to the electric field direction because the forces acting on the two charges is same but in case of non uniform electric field by point charge as the force acting on the charges are not equal and hence the torque acting would never be zero even when the dipole is aligned to electric field.