20110606

Physics final exam question: slide-wire generator

Physics 205B Final Exam, spring semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.5

A conducting rod in a uniform external magnetic field slides on top of metal rails, which are connected to a light bulb to form a complete circuit. Determine the direction of the current (clockwise or counterclockwise) through the circuit if the rod is moved to the right at constant speed. Explain your reasoning using the properties of circuits, right-hand rules, Faraday's law and/or Lenz's law.

Solution and grading rubric:
  • p:
    Correct. Applies RHR2 to determine direction of force on positive charges in moving rod, which results in a counterclockwise current in the loop. Or uses Faraday's law and Lenz's law to determine that the increasing flux through the loop into the page must be counteracted, and uses RHR3 to determine the direction (counterclockwise) 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:
    Blank.

Grading distribution:
Section 30882
Exam code: finalv0L7
p: 5 students
r: 0 students
t: 1 student
v: 1 student
x: 0 students
y: 1 student
z: 0 students

A sample "p" response (from student 9904):

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