Physics quiz question: light bulb plugged into different voltage outlets

Physics 205B Quiz 5, spring semester 2013
Cuesta College, San Luis Obispo, CA

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

A 100 W light bulb plugs into a 120 V outlet. If this light bulb is (somehow) plugged into a 240 V outlet, it will draw __________ current and use __________ power compared to plugged into a 120 V outlet.
(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 (highlight to unhide): (H)

Assuming that the resistance R of the light bulb should remain (approximately) constant regardless of where it is plugged in, the current used by the light bulb in the 120 V outlet:

I₂₄₀ = (240 V)/R,

should be more than the current when plugged into the 120 V outlet:

I₁₂₀ = (120 V)/R.

The power used by the light bulb in the 240 V outlet:

P₂₄₀ = (240 V)²/R,

will also be more than the power used in 120 V outlet:

P₁₂₀ = (120 V)²/R.

Section 30882
Exam code: quiz05c4Rb
(A) : 2 students
(B) : 4 students
(C) : 4 students
(D) : 0 students
(E) : 2 students
(F) : 0 students
(G) : 6 students
(H) : 14 students

Success level: 44%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.27