Chapter 21, Problem 21.55E

Torque on a Dipole. An electric dipole with dipole moment $\stackrel{\to }{\text{p}}$ is in a uniform external electric field $\stackrel{\to }{\text{E}}$. (a) Find the orientations of the dipole for which the torque on the dipole is zero, (b) Which of the orientations in part (a) is stable, and which is unstable? (Hint: Consider a small rotation away from the equilibrium position and see what happens.) (c) Show that for the stable orientation in part (b), the dipole’s own electric field tends to oppose the external field.