Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.75
An ideal 12.0 V emf source is connected to two ideal light bulbs, an ideal resistor, an ideal voltmeter and an ideal ammeter, as shown at right. The voltmeter has a reading of 4.2 V, and the ammeter has a reading of 0.50 A. The two light bulbs have the same resistance r. Determine the resistance R of the resistor. Show your work and explain your reasoning.
Solution and grading rubric:
Correct. Applies Kirchhoff's loop rule and Ohm's law to find the voltage drop across the resistor R = 12.0 V - 4.2 V = 7.8 V, such that its resistance R = ΔV/I = (7.8 V)/(0.50 A) = 16 Ω.
Nearly correct, but includes minor math errors. May have confused voltmeter reading as absolute voltage value, rather than voltage difference.
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. May have included all three resistor voltage drops in same loop rule equation, but at least demonstrates a methodical application of Kirchhoff's rules, Ohm's law, and equivalent resistance.
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
Implementation of ideas, but credit given for effort rather than merit.
Irrelevant discussion/effectively blank.
Exam code: midterm02B1rD
p: 9 students
r: 7 students
t: 10 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students
A sample "p" response (from student 4027), methodically applying Kirchhoff's junction and loop rules:
Another sample "p" response (from student 0514), explicitly applying Ohm's law to the resistor R, given the voltmeter and ammeter readings: