20110520

Physics midterm problem: charge moving near two magnets

Physics 205B Midterm 2, spring semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.65

A positive charge moves near two identical bar magnets such that the magnetic force on it is to the right (in the +x direction). Determine (a) the direction of the magnetic field at the location of the charge, and (b) the direction of the velocity of the charge. Show your work and explain your reasoning.

Solution and grading rubric:
  • p:
    Correct. Determines (a) the magnetic field at the location of the positive charge knowing how magnetic field lines behave at the respective poles of the magnets, then (b) uses RHR1 to determine the direction of the charge's velocity, given the directions of the magnetic field and the force exerted on the charge.
  • r:
    Nearly correct, but includes minor math errors. Direction of one of the vectors in (a)-(b) is reversed.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Problems with reversing directions of both vectors in (a)-(b), but understands direction of magnetic field lines and RHR1.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. May involve currents and/or RHR2, RHR3, etc.
  • x:
    Implementation of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.

Grading distribution:
Section 30882
Exam code: midterm02H3nR
p: 6 students
r: 1 student
t: 0 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 3773):

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