A magnetic needle is placed in a uniform magnetic field and is aligned with the field. The needle is now rotated by an angle of 60∘ and the work is W. The torque on the magnetic needle at this position is:
θ1=0∘,θ2=60∘W=PE1−PE2=MBcos0∘−MBcos60∘=MB−MB2W=MB2.......(1)τ(θ−60∘)=MBsin60∘=√32MB
using equation (1) in this we get
τ=√3W