Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 18.22, 18.23, Problems 18.40, 18.41
Two identical light bulbs (R1 = R2 = 2.0 Ω), a third light bulb (r = 0.50 Ω), and an ideal 9.0 V emf source are connected as shown in the diagram at right. If the R1 light bulb is completely removed from the circuit (and not replaced with anything, leaving that part of the circuit open), determine whether the brightness of the r light bulb would decrease, increase, or remain the same. Show your work and explain your reasoning. Solution and grading rubric:
- p:
Correct. Uses either a quantitative or qualitative argument to show that (1) the equivalent resistance is greater when the R1 light bulb is disconnected from the circuit, resulting in (2) less current flowing through the r light bulb, (3) making it dissipate less power, and thus is dimmer. - r:
Nearly correct, but includes minor math errors. - t:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Only one step in (p) correct: typically finding the resistances r and R. - v:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Problematic application of equivalent resistance, Ohm's law (and/or Kirchhoff's circuit rules), and power. - x:
Implementation of ideas, but credit given for effort rather than merit. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Sections 30882
Exam code: finalpL3x
p: 3 students
r: 3 students
t: 8 students
v: 14 students
x: 0 students
y: 2 students
z: 0 students
A sample "p" response (from student 1224):
A sample "p" response (from student 1337):
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