In a simple device for measuring the magnitude B of a magnetic field, a conducti

Question

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 theta with respect to the vertical, where it remains at rest (see figure), (a) Is the current flowing upward (toward the pivot) or downward in the figure? (b) If theta = 13 determine B. (c) What is the largest magnetic field magnitude that can be measured using this device?

Explanation / Answer

a. the current is flowing down, ie away from pivot.

b. force = B i l Sin balanced by, mg cos

B = mgcos 13 / IL

= 0.119 T

c. largest magnetic field = when angle = 90

mg / Il

= 0.1225 T

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