Halliday, Resnick & Walker — Magnetic Fields: Problem 16

16 Figure 28-28 gives the orientation energy $U$ of a magnetic dipole in an external magnetic field $\vec{B}$, as a function of angle $\phi$ between the directions of $\vec{B}$ and the dipole moment. The vertical axis scale is set by $U_s = 2.0 \times 10^{-4} \text{ J}$. The dipole can be rotated about an axle with negligible friction in order to change $\phi$. Counterclockwise rotation from $\phi = 0$ yields positive values of $\phi$, and clockwise rotations yield negative values. The dipole is to be released at angle $\phi = 0$ with a rotational kinetic energy of $1.0 \times 10^{-4} \text{ J}$, so that it rotates counterclockwise. To what maximum value of $\phi$ will it rotate? (In the language of Module 8-3, what value $\phi$ is the turning point in the potential well of Fig. 28-28?)