Physics midterm question: possible directions of magnetic force

Physics 205B Midterm 2, spring semester 2017
Cuesta College, San Luis Obispo, CA

An unspecified amount of charge q moves to the right in a uniform magnetic field that is present everywhere, and experiences a force up along this page. Discuss how there can be two possible directions for the magnetic field, and how those directions depend on the sign of the moving charge. Explain your reasoning using the properties of magnetic fields and forces.

Solution and grading rubric:

• p:
  Correct. Clearly discusses or draws diagrams showing how from RHR1 the magnetic field must point into the page for a positive charge, and the magnetic field must point out of the page for a negative charge.
• 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. May have magnetic field directions switched for positive and negative charges, or may conflate field and forces in discussion.
• 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 and forces.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields and forces.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02GruT
p: 21 students
r: 0 students
t: 4 student
v: 4 students
x: 0 students
y: 0 students
z: 0 student

A sample "p" response (from student 0428):
Another sample "p" response (from student 2525):