If an electric dipole is placed parallel to the electric field, then the torque on the dipole will be:

CONCEPT:

Electric dipole in a uniform external field:

• We know that when a charge q is placed in electric field E, it experiences a force F, the force is given as,

⇒ F = qE     -----(1)

• So when an electric dipole is placed in the electric field according to the diagram,

The force on the +q and the -q charge due to the electric field is given as,

⇒ F = qE

• The net force on the electric dipole will be zero.
• The torque on the electric dipole is given as,

⇒ τ = pE.sinθ

Where θ = angle between the dipole and the electric field

• This torque will tend to align the dipole with the electric field.

EXPLANATION:

• When an electric dipole is placed parallel to the electric field, the angle between the electric field and the dipole will be zero.

⇒ θ = 0

So the torque is given as,

⇒ τ = pEsinθ

⇒ τ = pE sin0°

⇒ τ = 0

• Hence, option 3 is correct.