# Heat Transfer MCQ’s – Wein’s Displacement Law

1 - Question

The relationship (Wavelength) MAX T = constant, between the temperature of a black body and the wavelength at which maximum value of monochromatic emissive power occurs is known as
a) Planck’s law
b) Kirchhoff’s law
c) Lambert’s law
d) Wein’s law
Explanation: This is the Wien’s law. From the spectral distribution of black body emissive power, it is apparent that the wavelength associated with a maximum rate of emission depends upon the absolute temperature of the radiating surface.

2 - Question

A body at 500 K cools by radiating heat to ambient atmosphere maintained at 300 K. When the body has cooled to 400 K, the cooling rate as a percentage of original rate is about
a) 31.1
b) 41.5
c) 50.3
d) 80.4
Explanation: Q2/Q1 = (400)4 – (300)4/ (500)4 – (300)4 = 0.32.

3 - Question

Two spheres A and B of same material have radius 1 m and 4 m, and temperatures 4000 K and 2000 K respectively. Then the energy radiated by sphere A is
a) Greater than that of sphere B
b) Less than that of sphere B
c) Equal to that of sphere B
d) Two times that of sphere B
Explanation: E A/E B = (1)2 (4000)4/ (4)2 (2000)4 = 1.

4 - Question

A small body has a total emissive power of 4.5 kW/m2. Determine its surface temperature of maximum emission
a) 530.77 K
b) 345.65 K
c) 236.54 K
d) 367.8 K
Explanation: E = σ T4. So, T = 530.77 K.

5 - Question

A small black body has a total emissive power of 4.5 k W/m2. In which range of the spectrum does this wavelength fall?
a) Thermal region
b) Cosmic region
c) Visible region
d) Infrared region
Explanation: (Wavelength) T = 2.8908 * 10 -3. This must be the wavelength of infrared region.

6 - Question

The sun emits maximum radiation of 0.52 micron meter. Assuming the sun to be a black body, Calculate the surface temperature of the sun
a) 2345 K
b) 5573 K
c) 9847 K
d) 6492 K
Explanation: T = 2.8908 * 10 -3/0.52 * 10 -6 = 5573 K.

7 - Question

Consider the previous problem, determine the maximum monochromatic emissive power of the sun’s surface
a) 4.908 * 10 13 W/m2
b) 5.908 * 10 13 W/m2
c) 6.908 * 10 13 W/m2
d) 7.908 * 10 13 W/m2
Explanation: (E) MAX = 1.285 * 10 -5 T5 = 6.908 * 10 13 W/m2.

8 - Question

A furnace emits radiation at 2000 K. Treating it as a black body radiation, calculate the wavelength at which emission is maximum
a) 1.449 * 10 -6 m
b) 2.449 * 10 -6 m
c) 3.449 * 10 -6 m
d) 4.449 * 10 -6 m
Explanation: (Wavelength) T = 2.8908 * 10 -3. So, wavelength = 1.449 * 10 -6 m.

9 - Question

Four identical pieces of copper painted with different colors of paints were heated to the same temperature and then left in the environment to cool. Which of the following paint will give fast cooling?
a) White
b) Rough
c) Black
d) Shining
Explanation: The emissivity of black paint is highest i.e. unity. Consequently, the emitted radiant energy will be maximum when painted black.

10 - Question

A surface for which emissivity is constant at all temperatures and throughout the entire range of wavelength is called
a) Opaque
b) Grey
c) Specular
d) Diathermanous