Physics quiz question: US vs. European electricity

Physics 205B Quiz 5, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Comprehensive Problem 18.113(d)

A 1000 W room heater from Europe that plugs into a 240 V outlet is brought home by a Physics 205B student. The heater is then (somehow) plugged into a 120 V outlet, where it will draw __________ current and use __________ power compared to Europe.
(A) less; less.
(B) less; the same.
(C) less; more.
(D) the same; less.
(E) the same; more.
(F) more; less.
(G) more; the same.
(H) more; more.

Correct answer: (A)

The resistance R of the heater should remain (approximately) constant regardless of where it is plugged in, such that the current used by the heater in the U.S.:

I_U.S. = (120 V)/R,

should be less than the current in Europe:

I_Europe = (240 V)/R.

The power used by the heater in the U.S.:

P_U.S. = (120 V)²/R,

will also be less than the power used in Europe:

P_Europe = (240 V)²/R.

Section 30882
Exam code: quiz05Vo7x
(A) : 10 students
(B) : 1 student
(C) : 3 students
(D) : 3 students
(E) : 0 students
(F) : 1 student
(G) : 7 students
(H) : 1 student

Success level: 39%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38