Physics midterm question: parallel circuit with switch

Physics 205B Midterm 2, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 18.21, 18.23

[10 points.] An ideal emf source is connected to an ideal resistor and an ideal light bulb of different resistances, with the switch in the open position. If the switch is closed, determine whether the brightness of the light bulb will be more than, less than, or the same as when the switch is open. Explain your reasoning using the properties of currents and potential differences, and Kirchhoff's rules and Ohm's law.

Solution and grading rubric:
  • p = 10/10:
    Correct. Applies Kirchhoff's rules and Ohm's law to determine that the voltage drop and current flowing through the light bulb is not affected by the switch, such that its brightness is unchanged.
  • r = 8/10:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Only explicitly discusses how voltage drop and current flowing through the remains is not affected by the switch.
  • t = 6/10:
    Nearly correct, but argument has conceptual errors, or is incomplete. Attempt at applying Kirchhoff's law and Ohm's law, but claims either more current will pass through light bulb (because equivalent resistance is less), or less current will pass through light bulb (because current will be divided).
  • v = 4/10:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  • x = 2/10:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y = 1/10:
    Irrelevant discussion/effectively blank.
  • z = 0/10:

Grading distribution:
Section 30882
Exam code: midterm02B1rD
p: 3 students
r: 1 student
t: 23 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 2372):
Another sample "p" response (from student 0524), explicitly comparing the unswitched and switched circuits:

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