Halliday, Resnick & Walker — Magnetic Fields: Problem 24

24 An electron moves in a circle of radius \( r = 5.29 \times 10^{-11} \text{ m} \) with speed \( 4.12 \times 10^6 \text{ m/s} \). Treat the circular path as a current loop with a constant current equal to the ratio of the electron’s charge magnitude to the period of the motion. If the circle lies in a uniform magnetic field of magnitude \( B = 7.10 \text{ mT} \), what is the maximum possible magnitude of the torque produced on the loop by the field?