# Heat Transfer MCQ’s – Planck’s Law

1 - Question

The energy emitted by a black surface should not vary in accordance with
a) Wavelength
b) Temperature
c) Surface characteristics
d) Time
Explanation: It is time independent. For a prescribed wavelength, the body radiates much more energy at elevated temperatures.

2 - Question

In the given diagram let r be the length of the line of propagation between the radiating and the incident surfaces. What is the value of solid angle W?

a) A sin α
b) A cos α
c) 2A cos α
d) 2A cos α
Explanation: The solid angle is defined by a region by the rays of a sphere, and is measured as A n/r2.

3 - Question

Likewise the amount of emitted radiation is strongly influenced by the wavelength even if temperature of the body is
a) Constant
b) Increasing
c) Decreasing
d) It is not related with temperature
Explanation: Temperature must remain constant in order to emit radiation.

4 - Question

A small body has a total emissive power of 4.5 kW/m2. Determine the wavelength of emission maximum
a) 8.46 micron m
b) 7.46 micron m
c) 6.46 micron m
d) 5.46 micron m
Explanation: (Wavelength) max t = 2.8908 * 10 -3.

5 - Question

The sun emits maximum radiation of 0.52 micron meter. Assuming the sun to be a black body, Calculate the emissive ability of the sun’s surface at that temperature
a) 3.47 * 10 7 W/m2
b) 4.47 * 10 7 W/m2
c) 5.47 * 10 7 W/m2
d) 6.47 * 10 7 W/m2
Explanation: E = σ b t 4 = 5.47 * 10 7 W/m2.

6 - Question

The law governing the distribution of radiant energy over wavelength for a black body at fixed temperature is referred to as
a) Kirchhoff’s law
b) Planck’s law
c) Wein’s formula
d) Lambert’s law
Explanation: This law gives a relation between energy over wavelength.

7 - Question

The Planck’s constant h has the dimensions equal to
a) M L 2 T -1
b) M L T -1
c) M L T -2
d) M L T
Explanation: It has unit equal to J s and its value is 6.626 * 10 -34.

8 - Question

Planck’s law is given by
a) (E) b = 2 π c 2 h (Wavelength) -5/[c h/k (Wavelength) T] – 2
b) (E) b = π c 2 h [exponential [c h/k (Wavelength) T] – 3].
c) (E) b = 2 π c 2 h (Wavelength) -5/exponential [c h/k (Wavelength) T] – 1
d) (E) b = 2 c 2 h (Wavelength) -5/exponential [c h/k (Wavelength) T] – 6
Explanation: Planck suggested the following law for the spectral distribution of emissive power.

9 - Question

A furnace emits radiation at 2000 K. Treating it as a black body radiation, calculate the monochromatic radiant flux density at 1 micron m wavelength
a) 5.81 * 10 7 W/m2
b) 4.81 * 10 7 W/m2
c) 3.81 * 10 7 W/m2
d) 2.81 * 10 7 W/m
Explanation: (E) b = C 1 (Wavelength) -5/exponential [C 2/ (Wavelength) T] – 1.

10 - Question

A metal sphere of surface area 0.0225 m2 is in an evacuated enclosure whose walls are held at a very low temperature. Electric current is passed through resistors embedded in the sphere causing electrical energy to be dissipated at the rate of 75 W. If the sphere surfaces temperature is measured to be 560 K, while in steady state, calculate emissivity of the sphere surface
a) 0.498
b) 0.598
c) 0.698
d) 0.798