20150611

Physics final exam problem: switched-in voltmeter

Physics 205B Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 18.31, 18.72, 18.73

An ideal emf source is connected to two light bulbs (each with the same resistance R), a voltmeter, and an open switch. All of these components are ideal. Determine whether the voltmeter (magnitude) reading will increase, decrease, or remain constant when the switch is closed. 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. Understands that:
    1. when the switch is open, the voltmeter reading will be zero, as there is no potential difference between its terminals, and/or it is "not wired properly" and so will have a null/nonsense reading;
    2. when the switch is closed, no current will flow through the voltmeter (as it has infinite resistance), but it is wired in parallel to the left light bulb, and will read the light bulb's voltage drop ΔV = IR,
    thus closing the switch would result in a higher voltmeter reading.
  • 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.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. May confuse voltmeters with ammeters, and/or discusses how current will start to flow through voltmeter when switch is closed, etc.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: finalLd0c
p: 10 students
r: 7 students
t: 4 students
v: 9 students
x: 9 students
y: 2 students
z: 2 students

A sample "p" response (from student 1288):

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