20180505

Physics midterm problem: non-ohmic light bulb

Physics 205B Midterm 2, spring semester 2018
Cuesta College, San Luis Obispo, CA

A power supply with a built-in voltmeter is connected to a light bulb. When 4.5 V is applied to the light bulb, it uses 1.8 W of power. When the voltage applied is increased to 5.0 V, the light bulb then uses 2.0 W of power. Demonstrate how these measurements show that the light bulb is non-ohmic (that is, its resistance is not constant). Explain your reasoning using Kirchhoff's rules, Ohm's law, and properties of electrical power.

Solution and grading rubric:
  • p:
    Correct. Demonstrates that the resistance of the light bulb is not constant by calculating the different values it has for each different applied voltage value.
  • r:
    Nearly correct, but includes minor math errors.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some attempt at applying Kirchhoff's rules, Ohm's law, and properties of electrical power.
  • 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 electrical power.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 30882, 30883
Exam code: midterm02iFtW
p: 28 students
r: 1 student
t: 1 student
v: 4 students
x: 0 student
y: 0 students
z: 0 students

A sample "p" response (from student 3693):

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