20170601

Physics final exam problem: ammeter and voltmeter readings

Physics 205B Final Exam, spring semester 2017
Cuesta College, San Luis Obispo, CA


A lithium battery with an emf of 3.6 V and an internal resistance of r = 0.45 Ω is connected to two light bulbs (each with different resistances), an ammeter, and a voltmeter. Determine (a) the ammeter reading (in amps) and (b) the voltmeter reading (in volts). 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. Determines the ammeter and voltmeter readings by:
    1. finding the equivalent resistance of the circuit, and then uses Ohm's law to determine the current passing through the 0.45 Ω resistor; and
    2. knowing the current and the resistance, uses Ohm's law to determine the drop in voltage across the 0.45 Ω resistor; and
    3. knowing the voltage rise of the emf and the voltage drop across the 0.45 Ω resistor, uses Kirchhoff's loop rule and Ohm's law to determine the current passing through the 1.2 Ω resistor, which is the ammeter reading; then
    4. knowing the voltage rise of the emf and the voltage drop across the 0.45 Ω resistor, uses Kirchoff's loop rule to determine the voltage drop across the 2.2 Ω resistor, which is the voltmeter reading.
  • r:
    Nearly correct, but includes minor math errors. Has determined at least one of (1)-(2), but only one of (3)-(4) is complete/correct.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least only one of (1)-(2) is complete/correct.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Garbled attempt at applying Kirchhoff's rules, Ohm's law, and properties of ammeters.
  • 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 ammeters.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: finalmR3x
p: 2 students
r: 2 students
t: 4 students
v: 2 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student):

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