Physics quiz question: thermal resistance of cork coaster

Physics 205A Quiz 7, fall semester 2018
Cuesta College, San Luis Obispo, CA

A coaster is made from a square piece 
(12 cm × 12 cm) of cork (thermal conductivity 0.040 watts/m·K)[*]. Heat is conducted through its thickness of 0.50 cm. The thermal resistance of this cork coaster is:
(A) 2.9×10^–6 K/watt.
(B) 0.014 K/watt.
(C) 0.12 K/watt.
(D) 8.7 K/watt.

[*] corkstore24.co.uk/properties-of-cork-material/.

Correct answer (highlight to unhide): (D)

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 thermal resistance is:

R = (0.0050 m)/((0.040 watts/m·K)·(0.12 m)·(0.12 m)) = 8.680555555... K/watt,

or to two significant figures, the thermal resistance of the coaster is 8.7 K/watt.

(Response (A) is d·κ·A; response (B) is d·κ/A; response (C) is κ·A/d.)

Sections 70854, 70855
Exam code: quiz07PeA7
(A) : 3 students
(B) : 5 students
(C) : 5 students
(D) : 37 students

Success level: 74%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.73