# Heat Transfer MCQ’s – Gray Body and Selective Emitters

1 - Question

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
d) None of them will radiates
Explanation: The radiation spectrum for a non-black body may be similar or radically different from that of a black body.

2 - Question

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
Explanation: The constant is different for different bodies.

3 - Question

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
Explanation: The radiation spectrum for a grey body, though reduced in vertical scale, is continuous and identical to the corresponding curve for a perfectly black surface.

4 - Question

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
Explanation: It is the ratio of radiating coefficient to that of black body radiating coefficient.

5 - Question

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
Explanation: E = (T b/T G) 4 = 0.4096.

6 - Question

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
Explanation: Emissivity = E/σ A T4 = 0.144.

7 - Question

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
Explanation: E = (Emissivity) σ A T4. So, A = 0.98 * 10 -4 m2.

8 - Question

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
Explanation: Total emissivity = 0.4 (-0.3181 – 0.0000) + 0.8 (0.8563 – 0.3118) = 0.5578.

9 - Question

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
Explanation: It should be measured in Steradian.

10 - Question

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