Question

Derive the magnitude of torque acting on an electric dipole in a uniform electric field. Which of the following correctly represents this torque?

Options

• τ = pE cos θ

• τ = 2pE sin θ

• τ = pE sin θ

• τ = pE tan θ

Answer 1

The torque on a single charge about the dipole center is qE·l·sin θ. Since both the positive and negative charges contribute equally, the total torque is τ = qE·l·sin θ + qE·l·sin θ = 2qE·l·sin θ. Since the dipole moment p = q·2l, we get τ = pE sin θ, where θ is the angle between the dipole moment and the electric field.