Physics final exam question: induced current in loop

Physics 205B Final Exam, fall semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 29.4.

The current in a long wire is increasing over time. A square metal loop (of resistance R) is located below the wire. What is the direction (clockwise/counterclockwise) of the current induced in the square loop? Explain your answers using the appropriate right-hand rules, Faraday's law and Lenz's law.

Solution and grading rubric:
  • p:
    Correct. Applies RHR2 to determine direction of (increasing) B field through the loop (out of the page), which means magnetic flux out of the page increases over time, then uses Faraday's law and Lenz's law to determine that the loop's induced magnetic field must point into the page, and uses RHR3 to determine the direction (clockwise) of the 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. May have direction/sign wrong in RHR2/flux/F's law/L's law/RHR3 process.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Few missing pieces, but still demonstrates RHR2/flux/F's law/L's law/RHR3 process.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:

Grading distribution:
Section 70856
p: 4 students
r: 3 students
t: 3 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1234), although "force" in the second sentence should refer to "flux" instead:

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