# Heat Transfer Questions and Answers – Transmissivity

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

1. The value of transmissivity may vary from
a) 0-1
b) 1-2
c) 3-4
d) 4-5

Explanation: This is a very small quantity. The response of the body to incident radiations is, however, completely independent of and unaffected by the simultaneous emission from the body.

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 absorptivity for the surface material
a) 0.113
b) 0.114
c) 0.115
d) 0.116

Explanation: Transmissivity = fraction of total energy transmitted by the body = 350 – (250 + 60)/350 = 0.115.

3. Transmissivity can also be defined as a ratio of transmitted radiation to that of incident energy flow.
a) True
b) False

Explanation: It is the ratio of incident energy flow to that of transmitted radiations.
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4. A cavity with a small hole will always behave as a
a) White body
b) Transparent body
c) Black body
d) Opaque body

Explanation: No radiation passing in and out will have any effect upon the equilibrium of radiation inside the cavity.

5. What is the wavelength band of gamma rays?
a) 1 * 10 -7 to 4 * 10 -4 micron meter
b) 2 * 10 -7 to 2.4 * 10 -4 micron meter
c) 3 * 10 -7 to 3.4 * 10 -4 micron meter
d) 4 * 10 -7 to 1.4 * 10 -4 micron meter

Explanation: This is the maximum and minimum wavelength for gamma rays.

6. Transmissivity is defined as
a) Fraction of total energy transmitted by the body
b) Fraction of total 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

Explanation: It is a fraction of total energy transmitted by the body.

7. A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 11.35 W/m2 degree. Calculate the overall coefficient of heat transfer by the combined mode of convection and radiation
a) 11.04 W/m2 degree
b) 12.04 W/m2 degree
c) 13.04 W/m2 degree
d) 14.04 W/m2 degree

Explanation: The total heat exchange can be expressed as, Q = U A d t where U is the overall coefficient of heat transfer.

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 transmission
a) 155.47 W/m2
b) 145.47 W/m2
c) 135.47 W/m2
d) 125.47 W/m2

Explanation: Rate of transmission = T Q 0 = 145.47 W/m2.

9. With respect to incident radiation, transmissivity varies with
a) Time
b) Temperature
c) Surface area
d) Wavelength

Explanation: For transparent body, transmissivity is equal to unity.

10. A body through which all the incident radiations passes, is called
a) Opaque body
b) Black body
c) Transparent body
d) White body

Explanation: For transparent body, transmissivity is unity.

Sanfoundry Global Education & Learning Series – Heat Transfer.

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