20110608

Physics final exam problem: voltmeter reading

Physics 205B Final Exam, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.65

[20 points.] An ideal 9.0 V emf battery is connected to an ideal voltmeter, a 4.0 Ω resistor and two 2.2 Ω light bulbs, as shown at right. What is the reading of the voltmeter? Show your work and explain your reasoning.

Solution and grading rubric:
  • p = 20/20:
    Correct. Finds equivalent resistance of series circuit to find current, then applies some part of Kirchhoff's voltage rule to determine difference in voltage, either using the left part of the circuit (delta(V) = |-(2.2 ohms)*I + (9.0 V)|), or the right part (delta(V) = |–I*(2.2 ohms + 4.0 ohms)|).
  • r = 16/20:
    Nearly correct, but includes minor math errors.
  • t = 12/20:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
  • v = 8/20:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Problematic application of equivalent resistance rules and Kirchhoff's rules.
  • x = 4/20:
    Implementation of ideas, but credit given for effort rather than merit.
  • y = 2/20:
    Irrelevant discussion/effectively blank.
  • z = 0/20:
    Blank.

Grading distribution:
Section 30882
Exam code: finalv0L7
p: 1 student
r: 2 students
t: 3 students
v: 1 student
x: 0 students
y: 1 student
z: 0 students

A sample "p" response (from student 7503):

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