8.2 Exercise 12

Show that $U = − \overset{\rightarrow}{\mu} \cdot \overset{\rightarrow}{B}$.

(Hint: An infinitesimal amount of work is done to align the magnetic moment with the external field. This work rotates the magnetic moment vector through an angle $- d \theta$ (toward the positive z-direction), where $d \theta$ is a positive angle change.)