20110517

Physics midterm question: direction of current

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.37

A two ideal batteries and two resistors are connected in series as shown at right, where ε1 = ε2, but R1 > R2. Determine which battery has more current flowing through it, or if there is a tie. Explain your reasoning using the properties of Kirchhoff's rules and/or Ohm's law.

Solution and grading rubric:
  • p:
    Correct. Due to Kirchhoff's junction rule applied to this series circuit, both emfs must have the same current flowing through them. May also remark that from Ohm's law, there is actually no current flowing through this circuit.
  • 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.
  • x:
    Implementation/application 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: 7 students
r: 0 students
t: 0 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 9904):

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