Chapter 22, Problem 24P

A current loop with magnetic dipole moment $\stackrel{\to }{\mu }$ is placed in a uniform magnetic field $\stackrel{\to }{B}$, with its moment making angle θ with the field. With the arbitrary choice of U = 0 for θ = 90°, prove that the potential energy of the dipole-field system is $U=-\stackrel{\to }{\mu }\cdot \stackrel{\to }{B}$.