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:
- p:
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. - r:
Nearly correct, but includes minor math errors. May have confused voltmeter reading as absolute voltage value, rather than voltage difference. - t:
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. - 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. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Grading distribution:
Section 30882
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:
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