## 20171214

### Physics quiz question: thermal conductivity of bubble wrap

Physics 205A Quiz 7, fall semester 2017
Cuesta College, San Luis Obispo, CA

"Bubble Wrap"
Chrysti Hydeck
flic.kr/p/4wMG9t

A sheet of plastic bubble wrap (area of 0.56 m2, and thickness of 0.0065 m) has a thermal resistance of 0.43 K/watt[*]. The thermal conductivity of bubble wrap is:
(A) 0.0016 watts/m·K.
(B) 0.0050 watts/m·K.
(C) 0.027 watts/m·K.
(D) 37 watts/m·K.

[*] me.umn.edu/courses/me4054/archives/2013_Spring/S13_Design_Reports/S13%20Cushion%20V2.pdf.

Correct answer (highlight to unhide): (C)

The thermal resistance R of an object can be related to its thermal conductivity κ by:

R = d/(κ·A),

where d is the thickness of the object that heat must conduct through, and A is the cross-sectional area, such that the conductivity is given by:

κ = d/(R·A) = (0.0065 m)/((0.43 K/watt)·(0.56 m2)) = 0.02699335548 watts/m·K,

or to two significant figures, the thermal conductivity of the wall is 0.027 watts/m·K.

(Response (A) is d·R·A; response (B) is R/(A·d); response (D) is R·A/d).)

Sections 70854, 70855
Exam code: quiz07Whu7
(A) : 4 students
(B) : 3 students
(C) : 30 students
(D) : 5 students

Success level: 71%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53