Cuesta College, San Luis Obispo, CA
An ideal 9.0 V emf source is connected to a light bulb and two resistors, with a voltmeter connected to the light bulb, and another voltmeter connected to one of the resistors. Discuss why the two voltmeters have the same reading (in volts). Show your work and explain your reasoning using Kirchhoff's laws, Ohm's law, and properties of voltmeters. Solution and grading rubric:
- p:
Correct. Discusses/demonstrates how the two voltmeters have the same reading because:- due to Kirchhoff's junction rule, the current flowing through each of the 4.0 Ω resistors is one-half of the current flowing through the 2.0 Ω resistor; and
- since each voltmeter will read the voltage drop (I⋅R) of their respective resistors, the smaller current (factor of one-half) flowing through the 4.0 Ω resistor will be compensated by its larger resistance (factor of two).
- r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. - t:
Nearly correct, but argument has conceptual errors, or is incomplete. - v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at applying properties of Kirchhoff's rules, Ohm's law, and properties of ammeters. - x:
Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at systematically applying Kirchhoff's rules, Ohm's law, and properties of ammeters. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Sections 30882, 30883
Exam code: midterm02u7aH
p: 28 students
r: 4 students
t: 2 students
v: 8 students
x: 0 students
y: 0 students
z: 1 student
A sample "p" response (from student 1950):
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