Chapter 27, Problem 77GP

In a simple device for measuring the magnitude B of a magnetic field, a conducting rod (length d = 1.0 m. mass m = 150 g) hangs from a friction-free pivot and is oriented so that its axis of rotation is aligned with the direction of the magnetic field to be measured. Thin flexible wires (which exert negligible force on the rod) carry a current I = 12 A, which causes the rod to deflect an angle θ with respect to the vertical, where it remains at rest (Fig. 27–58). (a) Is the current flowing upward (toward the pivot) or downward in Fig. 27–58? (b) If θ = 13º, determine B. (c) What is the largest magnetic field magnitude that can be measured using this device?