Chapter 27, Problem 27.44E

Both circular coils A and B (Fig. E27.44) have area A and N turns. They are free to rotate about a diameter that coincides with the x-axis. Current I circulates in each coil in the direction shown. There is a uniform magnetic field $\stackrel{\to }{\text{B}}$ in the +z-direction. (a) What is the direction of the magnetic moment $\stackrel{\to }{\mu }$ for each coil? (b) Explain why the torque on both coils due to the magnetic field is zero, so the coil is in rotational equilibrium. (c) Use Eq. (27.27) to calculate the potential energy for each coil. (d) For each coil, is the equilibrium stable or unstable? Explain.

Figure E27.44