Physics final exam question: magnetic force of wire on moving charge

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 19.65, 19.66

A positive charge q moves upwards in the presence of a current-carrying straight wire coming out of the plane of this page. Determine the direction of the wire's magnetic field at the location of the charge, and the resulting magnetic force on the charge. Explain your reasoning using the properties of magnetic fields and forces.

Solution and grading rubric:

• p:
  Correct. Applies RHR2 to the wire (thumb out of page, fingers curling counterclockwise in the plane of the page) to find that B field is to the right in the plane of the page at the location of the positive charge. Then applies RHR1 to the charge (thumb up along plane of the page, index finger to the right in the plane of the page, middle finger into the page) to find that the force on the charge is into the page.
  
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Has at least only one RHR completely correct.
  
• 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.
  
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distribution:
Section 31988
p: 3 students
r: 5 students
t: 4 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1987):