Physics midterm problem: light bulb power dissipation in circuit

Physics 205B Midterm 2, spring semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.65

An ideal 12 V emf source is connected to ideal light bulbs and an ideal resistor, as shown at right. Find the amounts of power used by each of the two light bulbs. Show your work and explain your reasoning.

Solution and grading rubric:

• p:
  Correct. Finds equivalent resistance of circuit, and uses Ohm's law to find the current flowing through the emf and top light bulb. Can use P = (I²)·R to find power used by top light bulb, and then Kirchhoff's loop rule to find the voltage remaining after the top light bulb, then P = (∆V²)/R to find the power used by the lower light bulb.
  
• r:
  Nearly correct, but includes minor math errors. Has at least one light bulb power correct, but has minor misapplications of Kirchhoff's loop rules or junction rules in finding the other light bulb power.
  
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least has systematic approach to finding equivalent resistance and current of circuit, and finding powers of each light bulb.
  
• 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: midterm02H3nR
p: 3 students
r: 3 students
t: 2 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7503):