# Mass Transfer Questions and Answers – Emissivity

This set of Mass Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Emissivity”.

1. What is the value of emissivity for polished aluminum?
a) 0.01
b) 0.02
c) 0.03
d) 0.04

Explanation: The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation.

2. Which one is having highest emissivity?
a) Tarnished copper
b) Polished aluminum
c) Aluminum foil
d) Polished copper

Explanation: Emissivity of tarnished copper is 0.75 while that of polished aluminum, aluminum foil and polished copper are 0.03, 0.05 and 0.03 respectively.

3. What is the value of emissivity for polished copper?
a) 0.02
b) 0.03
c) 0.04
d) 0.05

Explanation: This is the total hemispherical emissivity from the surfaces. It describes thermal radiation emitted near specific wavelength and at specific angles to surface.

4. What is the value of emissivity for tarnished copper?
a) 0.78
b) 0.77
c) 0.76
d) 0.75

Explanation: This is optically thick material. The absorptivity at the wavelengths doesn’t depend on the thickness of the material.

5. Which one is having the lowest emissivity?
a) Polished stainless steel
b) White marble
c) Snow
d) Asphalt

Explanation: Emissivity of polished stainless steel is 0.60 while that of white marble, snow and asphalt are 0.95, 0.97 and 0.90 respectively.
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6. What is the value of emissivity for dull stainless steel?
a) 0.24
b) 0.23
c) 0.22
d) 0.21

Explanation: It is transparent with low emissivity coatings. It emits less thermal radiation than an ordinary window.

7. What is the value of emissivity for white marble?
a) 0.96
b) 0.95
c) 0.94
d) 0.93

Explanation: It is based on evacuated glass tube collectors. Sunlight is absorbed by a selective surface.

8. Which one is having highest emissivity?
a) Red brick
b) Concrete
c) Black paint
d) Dull stainless steel

Explanation: Emissivity of black paint is 0.97 while that of red brick, concrete and dull stainless steel are 0.93, 0.88 and 0.21 respectively.

9. What is the value of emissivity for asphalt?
a) 0.90
b) 0.80
c) 0.70
d) 0.60

Explanation: It is an ordinary black surface that absorbs sunlight efficiently but emits thermal radiation copiously.

10. Which one is having the lowest emissivity?
a) Snow
b) White marble
c) Concrete
d) Polished stainless steel

Explanation: Emissivity of polished stainless steel is 0.60 while that of snow, white marble and concrete are 0.97, 0.95 and 0.88 respectively.

Sanfoundry Global Education & Learning Series – Mass Transfer.

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