Physics final exam problem: brightness levels of light bulbs in circuit

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 18.21

An ideal 9.0 V emf battery is connected to a set of light bulbs which all have the same resistance. Verify whether or not there are three distinct levels of light bulb brightnesses in this circuit. Show your work and explain your reasoning using the properties of currents and potential differences, and Kirchhoff's rules and Ohm's law.

Solution and grading rubric:
  • p:
    Correct. Applies equivalent resistance rules, Ohm's law, and Kirchhoff loop and junction rules to determine that there are only two levels of brightnesses.
  • r:
    Nearly correct, but includes minor math errors.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Problematic application of equivalent resistance rules and Kirchhoff's rules.
  • x:
    Implementation of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:

Grading distribution:
Section 70856
p: 2 students
r: 2 students
t: 3 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7673):

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