Physics quiz question: changing emissivity of a brick

Physics 205A Quiz 7, fall semester 2019
Cuesta College, San Luis Obispo, CA

"Red bricks"
Ramesh NG

A red brick has an emissivity of 0.93[*], and has a surface temperature of 270 K. Ignore conduction and convection heat transfers to/from the environment. If the brick were coated with aluminum paint such that its emissivity was lowered to 0.45, the rate of heat per time the brick _________ would decrease.
(A) radiates to the environment.
(B) absorbs from the environment.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

[*] thermoworks.com/emissivity-table.

Correct answer (highlight to unhide): (C)

The net power (rate of heat per time) radiated is given by:

Power = –e·σ·A·((Tobj)4 + (Tenv)4) = –e·σ·A·(Tobj)4e·σ·A·(Tenv)4,

where the first (negative) term corresponds to the rate of heat being removed (radiated) from the object, while the second (positive) term corresponds to the rate of heat being put into (absorbed) by the object (in order to be consistent with the ±Q convention for removing heat from (–) or putting heat into (+) a thermodynamic system).

As it is a factor common to both the radiation and absorption terms, lowering the emissivity value from 0.93 to 0.45 would then decrease both the rate of heat per time radiated to the environment and the rate of heat per time absorbed from the environment.

Sections 70854, 70855
Exam code: quiz07VlnC
(A) : 10 students
(B) : 5 students
(C) : 35 students
(D) : 1 student

Success level: 69%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52

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