Physics midterm question: total magnetic field of two perpendicular wires

Physics 205B Midterm 2, spring semester 2016
Cuesta College, San Luis Obispo, CA

Two straight wires carry equal amounts of current, crossing perpendicular to each other in the plane of the page. Discuss why the direction of the total magnetic field at the location shown points out of the page. Show your work and explain your reasoning using the properties of magnetic fields, and superposition.

Solution and grading rubric:

• p:
  Correct. Clearly discusses or draws diagrams showing how from RHR2 the magnetic field from each wire points out of the xy plane in the lower right quadrant, resulting in a total magnetic field that points in that direction.
• 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 applying properties of magnetic fields, and superposition.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of magnetic fields, and superposition.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: midterm02Mc4s
p: 26 students
r: 2 students
t: 1 student
v: 13 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1157):