Heat Transfer Questions and Answers – Gray Body and Selective Emitters

This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Gray Body And Selective Emitters”.

1. Consider two bodies, one absolutely back and the other non-black and let these be at same temperature. Which one of the following statement is correct?
a) Black body radiates less intensively than a non-black body
b) Non-black body radiates less intensively than a black body
c) Both will radiates equally
d) None of them will radiates
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2. When Stefan-Boltzmann law is applied to a black body, it takes the form
a) E = σ T
b) E = σ T ²
c) E = σ T ³
d) E = σ T ⁴
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3. When the emissivity of non-black surface is constant at all temperatures and throughout the entire range of wavelength, the surface is called
a) Gray body
b) Transparent body
c) Opaque bodies
d) Perfect black body
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4. The emissivity of the gray surface may be expressed as
a) σ /2 σ _b
b) σ / 3 σ _b
c) σ / σ _b
d) ½ σ / σ _b
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5. If a black body at 1000 K and a gray body at 1250 K emit the same amount of radiation, what should be the emissivity of the gray body?
a) 0.3096
b) 0.4096
c) 0.5096
d) 0.6096
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6. The radiant heat transfer from a plate of 2.5 cm² area at 1250 K to a very cold enclosure is 5.0 W. Determine the emissivity of the plate at this temperature
a) 0.444
b) 0.344
c) 0.244
d) 0.144
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7. A 100 W light bulb has a tungsten filament (emissivity = 0.30) which is required to operate at 2780 K. If the bulb is completely evacuated, calculate the minimum surface area of the tungsten filament
a) 0.98 * 10 ^-4 m²
b) 1.98 * 10 ^-4 m²
c) 2.98 * 10 ^-4 m²
d) 3.98 * 10 ^-4 m²
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8. The monochromatic emissivity € of a diffuse surface at 1600 K varies with wavelength in the following manner
€ = 0.4 for 0 < λ < 2
= 0.8 for 2 < λ < 5
Determine the total emissivity

a) 0.5558
b) 0.5568
c) 0.5578
d) 0.5588
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9. For a hemisphere solid angle is measured as
a) Radian and its maximum value is π
b) Degree and its maximum value is 180 degree
c) Steradian and its maximum value is 2π
d) Steradian and its maximum value is π
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10. A gray body (E = 0.8) emits the same amount of heat as a black body at 1075 K. Find out the required temperature of the gray body
a) 1146.72 K
b) 1136.72 K
c) 1126.72 K
d) 1116.72 K
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Sanfoundry Global Education & Learning Series – Heat Transfer.

To practice all areas of Heat Transfer, here is complete set of 1000+ Multiple Choice Questions and Answers.