Halliday, Resnick & Walker — Maxwell's Equations; Magnetism of Matter: Problem 28

28 The magnitude of the dipole moment associated with an atom of iron in an iron bar is \(2.1 \times 10^{-23} \text{ J/T}\). Assume that all the atoms in the bar, which is \(8.0 \text{ cm}\) long and has a cross-sectional area of \(1.0 \text{ cm}^2\), have their dipole moments aligned. (a) What is the dipole moment of the bar? (b) What torque must be exerted to hold this magnet perpendicular to an external field of magnitude \(1.5 \text{ T}\)? (The density of iron is \(7.9 \text{ g/cm}^3\).)