# Heat Transfer Questions and Answers – Absorptivity

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

1. Energy strikes a vertical hemispherical plate with an intensity of 640 W/m2. The absorptivity is thrice the transmissivity and twice the reflectivity. Determine the rate of transmission
a) 236.74 W/m2
b) 116.37 W/m2
c) 187.87 W/m2
d) 456.09 W/m2

Explanation: Rate os transmission = (0.5455) (640)/3 = 116.37 W/m2.

2. The absorptivity of a surface depends upon
(ii) Temperature of the surface
(iii) Composition
Identify the correct statements
a) i, ii and iii
b) i and iii
c) ii only
d) ii and iii

Explanation: It depend upon the direction of incident radiation. Absorptivity is the fraction of total energy absorbed by the body.

3. What is the absorptivity of the gray body?
a) Below unity
b) Unity
c) 2
d) 2.5

Explanation: Gray body absorbs a certain fixed percentage of impinging radiations.

4. A body that reflects all the incident thermal radiations is called a
a) Opaque body
b) Gases
c) Black body
d) Specular body

Explanation: In this case reflection is regular. For such bodies reflectivity is unity and transmissivity is equal to zero.

5. Energy strikes a vertical hemispherical plate with an intensity of 640 W/m2. The absorptivity is thrice the transmissivity and twice the reflectivity. Determine the rate of absorption
a) 449.12 W/m2
b) 349.12 W/m2
c) 249.12 W/m2
d) 149.12 W/m2

Explanation: Rate of absorption = (0.5455) (640) = 349.12 W/m2.
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6. Let 220 W/m2 of radiant energy is absorbed by a convex surface, 90 W/m2 is reflected and 40 W/m2 is transmitted through it. What is the value of absorptivity?
a) 0.72
b) 0.62
c) 0.52
d) 0.42

Explanation: It is the ratio of incident energy flow to that of absorbed radiations. Absorptivity = 220/(220 + 90 + 40) = 0.62.

7. For a grey surface
a) Reflectivity equals emissivity
b) Emissivity equals transmissivity
c) Absorptivity equals reflectivity
d) Emissivity is constant

Explanation: Emissivity is constant only for grey surfaces because grey surface radiates much more than any of the other surface.

8. Which quantity can be neglected for gases?
a) Transmissivity
b) Reflectivity
c) Absorptivity
d) None can be neglected

Explanation: Gases are known to reflect very little of the radiation incident on their surface.

9. Absorptivity of a body is equal to its emissivity
a) Under thermal equilibrium conditions
b) For a polished surface
c) At one particular temperature
d) At shorter wavelengths

Explanation: In most case absorptivity of a body is equal to its emissivity under thermal equilibrium conditions.

10. Electromagnetic waves strikes a hot body maintained at 900 degree Celsius which has a reflectivity of 0.66 and a transmissivity of 0.022. Let the absorbed flux be 60 W/m2. Determine the rate of incident flux
a) 168.67 W/m2
b) 178.67 W/m2
c) 188.67 W/m2
d) 198.67 W/m2

Explanation: Absorptivity + Reflectivity + Transmissivity = 1. So, incident flux = 60/0.318 = 188.67 W/m2.

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