Question

A bar magnet (2.5 J/T) is aligned with a 0.32 T field. (a) Find work done to align it (i) normal, (ii) opposite to the field. (b) Find torque in both cases.

Hint:

Potential energy and torque of a magnetic dipole.

Answer 1

(a) W = U_final - U_initial = -mBcosθ₂ + mBcosθ₁. (i) θ₁ = 0°, θ₂ = 90°: W = 0.8J. (ii) θ₁ = 0°, θ₂ = 180°: W = 1.6J. (b) τ = mBsinθ. (i) θ=90°: τ = 0.8 Nm. (ii) θ = 180°: τ = 0 Nm.