20140511

Physics midterm question: torque on magnetic dipole loop

Physics 205B Midterm 2, spring semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 19.13

A circular loop of current-carrying wire is placed in a uniform magnetic field. The loop is allowed to rotate from its initially stationary position. Discuss why the loop will begin to rotate counterclockwise. Explain your reasoning using the properties of magnetic fields and forces.

Solution and grading rubric:
  • p:
    Uses right-hand rule 1 (thumb: current, pointer finger: magnetic field, middle finger: force) to show how the force on each end of the loop will exert a net counterclockwise torque on the loop. May instead minimize potential energy by having magnetic dipole of loop align with direction of magnetic field.
  • 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 using right-hand rules, magnetic fields, forces, currents, and torques.
  • x:
    Implementation of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: midterm02iF47
p: 23 students
r: 0 students
t: 5 students
v: 10 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 0618) using right-hand rules to determine the directions of forces on the sections of current perpendicular to the page:

Another sample "p" response (from student 1993) arguing from a minimization of magnetic potential energy:

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