Physics quiz question: circuit breaker

Physics 205B Quiz 5, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.97

The circuit breaker for a power outlet strip that supplies 120 V is rated for 7.0 A. Suppose a laptop computer draws 24 W, a lamp draws 300 W, and a popcorn popper draws 600 W. Which will draw the most current without tripping the breaker?
(A) Laptop and lamp.
(B) Popcorn popper.
(C) Laptop and popcorn popper.
(D) Popcorn popper and lamp.
(E) Laptop, lamp, and popcorn popper.

Correct answer: (C)

The maximum power output for the strip outlet, without tripping the breaker, is:

Power = I*delta(V) = (7.0 amps)*(120 volts) = 840 watts.

Running these appliances, respectively, uses these amounts of power:
(A) 24 watts + 300 watts = 324 watts.
(B) 600 watts.
(C) 24 watts + 600 watts = 624 watts.
(D) 600 watts + 300 watts = 900 watts.
(E) 24 watts + 300 watts + 600 watts = 924 watts.

Thus running the laptop and popcorn popper will draw the most current (I = Power/delta(V) = (624 watts)/(120 volts) = 5.2 amps) without tripping the breaker. Running the popcorn popper and lamp, the next higher power combination of appliances, would result in a current I = Power/delta(V) = (900 watts)/(120 volts) = 7.5 amps, which would trip the circuit breaker. Running all three appliances would result in a current I = Power/delta(V) = (924 watts)/(120 volts) = 7.7 amps, which would also trip the circuit breaker.

Student responses
Section 70856
(A) : 0 students
(B) : 2 students
(C) : 4 students
(D) : 1 student
(E) : 4 students

Success level: 36%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.83