Physics midterm question: loop descending into uniform magnetic field

Physics 205B Midterm 2, spring semester 2016
Cuesta College, San Luis Obispo, CA

A square metal loop of resistance R (seen edge-on) is dragged down into a region with an external magnetic field that points into the plane of this page. Discuss why there will be no induced current in the loop while it enters into this region. Explain your reasoning using the properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.

Solution and grading rubric:

• p:
  Correct. Explains how there is no induced current in the loop using at least one of two similar arguments:
  1. plane of square metal loop is parallel to the magnetic field, such that there is zero magnetic flux through the loop, and since the magnetic flux is constantly zero, then there is no induced emf, and thus no induced current in the loop; or
  2. from using RHR1, the force on the fictitious positive charges in the square metal loop is to the right (in the +x direction), such that for the section of the loop closest to the viewer, this would induce a counterclockwise current (as viewed down into the magnetic field, along the −y direction), but in the section of the loop farthest from the view, a clockwise current is induced, such that there will be no (net) induced current in the loop.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, forces, motional emf, Faraday's law and Lenz's law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02Mc4s
p: 17 students
r: 7 student
t: 5 students
v: 10 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 1614), discussing the Lorentz force exerted on fictitious positive charges in the top and bottom segments of the wire loop:
A sample "p" response (from student 3214), using both the Lorentz force, and also applying Lenz's law to the changing flux through the wire loop: