Physics quiz question: loop pulled parallel to wire

Physics 205B Quiz 6, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 19.4; Problem 20.15.

Consider a long straight wire with constant current. In the plane of the page, a square metal loop (of resistance R) moves parallel to the wire, opposite the direction of current in the wire. While the loop is moving, the direction of current induced in the loop is:
(A) clockwise.
(B) counterclockwise.
(C) (No direction, as there is no induced current.)
(D) (Not enough information is given.)

Correct answer: (C)

The magnetic field of the wire creates a magnetic flux that points out through the square loop. Since the square loop is pulled parallel to the wire, the amount of flux through the square loop does not change over time, such that from Faraday's law:

emf = -N*(delta(phi_B)/delta(t));

since the magnetic flux phi_B is is constant, then there is no induced emf in the square loop, and thus no current induced in the square loop.

Student responses
Section 70856
(A) : 4 students
(B) : 6 students
(C) : 1 student
(D) : 0 students

Success level: 9%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.17