Heat Transfer Questions and Answers – Reflectivity

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

1. Reflectivity is defined as
a) Fraction of total energy transmitted by the body
b) Fraction of energy reflected by the body
c) Fraction of total energy absorbed by the body
d) Fraction of total energy absorbed and radiated by the body
View Answer

Answer: b
Explanation: It is total energy reflected by the body. Its value depends upon the nature of the surface of the body, its temperature and wavelength of incident radiations.

2. Of the radiant energy 350W/m2 incident upon a surface 250W/m2 is absorbed, 60W/m2 is reflected and the remainder is transmitted through the surface. Workout the value for reflectivity for the surface material
a) 0.181
b) 0.171
c) 0.161
d) 0.151
View Answer

Answer: b
Explanation: Reflectivity = fraction of total energy reflected by the body = 60/350 = 0.171.

3. A thin metal plate of 4 cm diameter is suspended in atmospheric air whose temperature is 290 K. This plate attains a temperature of 295 K when one of its face receives radiant energy from a heat source at the rate of 2 W. If heat transfer coefficient on both surfaces of the plate is stated to be 87.5 W/m 2 K, workout the energy lost by reflection
a) 0.8 W
b) 0.3 W
c) 0.9 W
d) 0.10 W
View Answer

Answer: c
Explanation: Heat loss by convection from both sides of the plates = 2 h A d t = 1.1 W. Energy lost by reflection = 2.0 – 1.1 = 0.9 W.
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4. The value of reflectivity depends upon
(i) Body temperature
(ii) Wavelength of reflected radiation
(iii) Nature of surface of body
Identify the correct statement
a) i and ii
b) ii and iii
c) ii only
d) i and iii
View Answer

Answer: d
Explanation: It should be wavelength of incident radiation. It is the fraction of total energy reflected by the body.

5. When a surface absorbs a certain fixed percentage of striking radiation the surface is called
a) Grey body
b) Black body
c) White body
d) Opaque body
View Answer

Answer: a
Explanation: It is absorbing a certain fixed amount of radiations.
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6. Which one is true regarding gases?
a) Reflectivity is unity
b) Absorptivity is zero
c) Reflectivity is zero
d) Transmissivity is zero
View Answer

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

7. Thin glass plate is an example of
a) Transparent body
b) Opaque body
c) Black body
d) White body
View Answer

Answer: a
Explanation: For a transparent body, transmissivity is zero.
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8. Radiant energy with an intensity of 800 W/m2 strikes a flat plate normally. The absorptivity is thrice the reflectivity and twice the transmissivity. Determine the rate of reflection
a) 518.20 W/m2
b) 418.20 W/m2
c) 318.20 W/m2
d) 218.20 W/m2
View Answer

Answer: d
Explanation: Rate of reflection = p Q 0 = 218.20 W/m2.

9. The total radiant energy A impinging upon a body would be partially or totally absorbed, reflected from its surface or transmitted through it. Identify the correct statement
B is the energy that is reflected by the body in given figure
a) B is the energy that is absorbed by the body
b) B is the energy that is reflected by the body
c) C is the energy that is absorbed by the body
d) D is the energy that is transmitted by the body
View Answer

Answer: b
Explanation: A = B + C + D.
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10. Which one is true for an opaque body?
a) Transmissivity is zero
b) Reflectivity is zero
c) Absorptivity is zero
d) Reflectivity is unity
View Answer

Answer: a
Explanation: For the opaque body, sum of absorptivity and reflectivity is unity.

Sanfoundry Global Education & Learning Series – Heat Transfer.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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