Physics clicker question: blackbody versus silverbody

Physics 5A, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 14.22

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the start of their learning cycle:

[0.6 participation points.] What should the outside of your jacket be like, in order to stay as warm as possible when hiking at night (in the dark)?
(A) Shiny, reflective.
(B) Black, matte.
(C) (It depends on how cold it is outside.)
(D) (It doesn't matter what type of jacket is worn outside in the dark.)
(E) (I'm lost, and don't know how to answer this.)

Sections 4987, 4988
(A) : 11 students
(B) : 5 students
(C) : 0 students
(D) : 15 students
(E) : 0 students

Correct answer: (A)

The rate of heat absorbed/emitted by an object depends on its emissivity: 1 for a perfect blackbody, and 0 for a perfect silverbody. While a blackbody will be an efficient absorber, it will also be a perfect emitter, which is desirable when cooling is an issue (e.g., car radiators, bottom tiles on the Space Shuttle), but at night, when heat loss by radiation needs to be minimized, a jacket approximating a silverbody would be better.