Physics midterm problem: magnetic force on moving charge

Physics 205B Midterm 2, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Questions 19.1-19.4, Problem 19.19

A +6.0 nC charge moves at constant speed of 5.5×10⁴ m/s in the region between two magnets, and experiences a force of 8.5×10^–6 N. Calculate (a) the magnitude of the magnetic field 
at the location of the charge, and (b) determine 
the direction of the magnetic force on the +6.0 nC 
charge. Show your work and explain your 
reasoning.

Solution and grading rubric:

• p:
  Correct. Determines magnitude of magnetic field at the location of the charge, where the angle θ between velocity v and the magnetic field B is 45°; and the direction of the force on the moving charge from right-hand rule 1.
• r:
  Nearly correct, but includes minor math errors. Magnitude incorrect (typically from using θ = 90°); or direction of right-hand rule incorrect.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Magnitude incorrect (typically θ = 90°), and direction of right-hand rule incorrect.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
• x:
  Implementation of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882
Exam code: midterm02w1Ig
p: 10 students
r: 12 students
t: 7 students
v: 4 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7187):