Question

An electric dipole is placed in a uniform electric field of Intensity E as shown in the figure.

1. What are the forces on +q and -q?

2. What is the net force on the system?

3. Copy the diagram, mark the forces and derive an expression for torque.

4. If the dipole is placed in a non uniform electric field what nature of motion does it show?

Answer 1

1. Force on ⁺q, F₊ = ⁺qE (along the direction of electric field)

Force on -q, F = -qE (Opposite to the direction of electronic field)

2. Net force on the system is zero.

3.

Consider an electric dipole of dipole moment P = 2aq kept in a uniform external electric field, inclined at an angle θ to the field direction. torque = any one force × perpendicular

distance τ = qE × 2 a sin θ

Since P = 2aq

τ = p E sin θ

Vectorialy

\(\overrightarrow \tau = \overrightarrow P \times \overrightarrow E\)

4. In nonuniform electric field, total force is not zero. Hence the dipole undergoes for both translational and rotational motion.