Halliday, Resnick & Walker — Maxwell's Equations; Magnetism of Matter: Problem 18

18 A magnetic rod with length 6.00 cm, radius 3.00 mm, and (uniform) magnetization \(2.70 \times 10^3 \text{ A/m}\) can turn about its center like a compass needle. It is placed in a uniform magnetic field \(\vec{B}\) of magnitude 50.0 mT, such that the directions of its dipole moment \(\vec{\mu}\) and \(\vec{B}\) make an angle of \(68.0^\circ\). (a) What is the magnitude of the torque on the rod due to \(\vec{B}\)? (b) What is the change in the orientation energy of the rod if the angle changes to \(34.0^\circ\)?