# Heat Transfer MCQ’s – Kirchoff’s Law

1 - Question

If radiant energy E B emitted by the black surface strikes the non-black surface. If non-black surface has absorptivity α, it will absorb how much radiations?
a) α E B
b) 2 α E B
c) 3 α E B
d) 4 α E B
Explanation: The remainder (1 – α) will be reflected back for full absorption at the black surface.

2 - Question

If two surfaces are at the same temperature, then the conditions correspond to mobile thermal equilibrium for which the resultant interchange of heat is zero are
a) 3 E – α E B = 0
b) 2 E – α E B = 0
c) E – α E B = 0
d) ½ E – α E B = 0
Explanation: The remainder (1 – α) will be reflected back for full absorption at the black surface.

3 - Question

The absorptivity of black body equals to
a) 2
b) 1
c) 3
d) 4
Explanation: The absorptivity of black body equals to unity.

4 - Question

A diathermanous body
a) Shines as a result of incident radiation
b) Gets heated up a result of absorption of incident radiation
c) Allows all the incident radiation to pass through it
d) Partly absorbs and partly reflects the incident radiation
Explanation: It behaves like a body that allows all the incident radiation to pass through it.

5 - Question

Choose the false statement
a) Snow is nearly black to thermal radiation
b) Absorption of radiation occurs in a very thin layer of material near the surface
c) Transmissivity varies with the wavelength of incident radiation
d) Most of the engineering materials have rough surfaces, and these rough surfaces give regular (specular) reflections
Explanation: Rough surfaces give diffused reflections. Reflections from highly polished and smooth surfaces have regular characteristics.

6 - Question

The emissivity and the absorptivity of a real surface are equal for radiation with identical temperature and wavelength. This law is referred to as
a) Kirchhoff’s law
b) Lambert’s law
c) Planck’s law
d) Wein’s displacement law
Explanation: Emissivity and absorptivity are related by Kirchhoff’s law.

7 - Question

With an increase in wavelength, the monochromatic emissive power of a black body
a) Increases
b) Decreases
c) Decreases, reaches a minimum and then increases
d) Increases, reaches a maximum and then decreases
Explanation: It firstly increases to its maximum value and then decreases to zero.

8 - Question

The temperature of a solid surface changes from 27 K to 627 K. The emissive power changes would then confirm to the ratio
a) 6:1
b) 9:1
c) 81:1
d) 27:1
Explanation: E2/E1 = (T2/T1)4 = 81.

9 - Question

If the temperature of a hot body is increased by 50%, the amount of radiations emitted by it would increase by nearly
a) 200%
b) 500%
c) 50%
d) 100%
Explanation: E2/E1 = (T2/T1)4 = 5.06.

10 - Question

Consider two surfaces, one absolutely black and the other non-black. These surfaces are arranged parallel to each other and so close that the radiation of one falls totally on the other. Choose the correct option

a) 1 denotes the radiant energy E emitted by the non-black surface impinges on the black surface
b) 1 denotes the radiant energy E emitted by the black surface impinges on the non-black surface
c) 2 and 3 denotes the quantity α E b
d) 4 denotes the quantity (1 – α) E b