Physics quiz question: loop pulled across magnetic field threshold

Physics 205B Quiz 6, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Example 20.1

A square metal loop (of resistance R) moves at constant velocity. An external magnetic field has uniform magnitude and direction in a certain region, and is zero everywhere outside of this region. The direction of current induced in the square loop is:
(A) clockwise.
(B) counterclockwise.
(C) (No direction, as there is no induced current.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

Since the magnetic flux Φ_B through the square metal loop does not change over time, there will be no induced emf and no induced current in the loop. (Alternatively, positive charges in both the left and the right sides of the loop experience upwards forces, but these induced clockwise and counterclockwise currents will effectively cancel out, leaving no net induced current in the loop.)

Section 30882
Exam code: quiz06LR1r
(A) : 12 students
(B) : 10 students
(C) : 5 students
(D) : 0 students

Success level: 19%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38