Physics final exam problem: shunted voltmeter readings

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA


A 4.5 V emf source is connected to two light bulbs (each with different resistances), two voltmeters, and a switch. All of these components are ideal. Discuss why the voltmeters have equal readings while the switch is closed, and have unequal readings after the switch is opened. Show your work and explain your reasoning using the properties of voltmeters, Kirchhoff's rules and Ohm's law.

Solution and grading rubric:

• p:
  Correct. Explicitly explains/demonstrates that the voltmeters in the closed-switch circuit have equal readings, and the voltmeters in the open-switch circuit have unequal readings by:
  1. applying Kirchhoff's loop rule to both circuits; and/or
  2. explicitly calculating the voltage drops for each of the voltmeters in both circuits.
• r:
  Nearly correct, but includes minor math errors. For the closed-switch circuit, may instead compare the different amount of currents flowing through each light bulb, instead of through each ammeter.
• 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.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying Kirchhoff's rules, Ohm's law, and properties of voltmeters.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: finalm4u1
p: 11 students
r: 8 students
t: 7 students
v: 7 students
x: 4 students
y: 0 students
z: 2 students

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