Physics 205A Quiz 7, Spring Semester 2009
Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 14.8
[3.0 points.] A window conducts power P from a house to the cold outdoors. What power is conducted through another window in this house, made of the same material, that has half the thickness, but twice the area?
(A) P/4.
(B) P/2.
(C) P.
(D) 2P.
(E) 4P.
(F) (Not enough information is given.)
Correct answer: (E)
The rate of heat conducted per time (power) through an object of conductivity kappa, thickness d, and area A, with a temperature difference delta(T) on either side is given by:
P = kappa*A*delta(T)/d.
The new window will have the same kappa as the old window, but both halving the thickness and doubling the area with respect to the old window will each increase the power by a factor of 2, resulting in four times as much heat per time being conducted through the new window.
Student responses
Sections 30880, 30881
(A) : 3 students
(B) : 3 students
(C) : 18 students
(D) : 2 students
(E) : 13 students
(F) : 0 students
"Difficulty level": 33%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53
I came across your blog by chance. I think more students will be attracted to it if you could use symbols as such (and not the word 'delta' for instance). Nice work.
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