# According to Stefan’s law which must be modified by Quantum Physics the total radiant intensity of a block body over all wave length is given as If the radiant intensity is measured as    R(λ)=8π/λ4 kTc/4 What are the units of the Stefan-Boltzmann constant S in MKS notation Note: Boltzmann’s constant k is measured as J/K

1.  J/K4

2.  W/m4 K4

3.  W/m2

4.  W/m2 K4

4

W/m2 K4

Explanation :
No Explanation available for this question

# A source of radiation has an intensity of 840 watts/m2. Find the number of photons per second per square meter represented by this intensity, if the wavelength is 500 nm. (Use speed of light = 3 * 104 m/s, and Plank’s constant h = 7 * 1034 J.s.)

1.  10.4 * 1021

2.  6.8 * 1021

3.  4.4 * 1021

4.  2.2 * 1021

4

2.2 * 1021

Explanation :
No Explanation available for this question

# When α is absorbability, ρ is reflectivity and τ is transitivity, then for diathermias body which of the following relation is valid

1.  α = 1, ρ = 0, τ = 0

2.  α = 0, ρ = 1, τ = 0

3.  α = 0, ρ = 0, τ = 1

4.  α + ρ = 1, τ = 0

4

α + ρ = 1, τ = 0

Explanation :
No Explanation available for this question

# Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75 % of this radiant heat transfer by inserting thin parallel shields of emissivity on both sides. The number of shields should be

1.  One

2.  Two

3.  Three

4.  Four

4

Three

Explanation :
No Explanation available for this question

# The burnout heat flux in the nucleate boiling regime is a function of

1.  Heat of evaporation

2.  Density of vapour and Density of liquid

3.  Vapour-liquid surface tension

4.  All of these

4

Vapour-liquid surface tension

Explanation :
No Explanation available for this question

# What is the next radiant interchange per square meter for two very large plates at temperatures 800 K and 500 K respectively (The emissivity of the hot and cold plates is 0.8 and 0.6 respectively. Stefan Boltz Mann constant is 5.67 * 10 W/m2 K4)

1.  1.206 kW/m2

2.  10.26 kW/m2

3.  102.6 kW/m2

4.  1026 kW/m2

4

10.26 kW/m2

Explanation :
No Explanation available for this question

# Consider two infinity long blackbody concentric cylinders with a diameter ratio D2/D1 = 3. The shape factor for the outer cylinder with itself will be

1.  0

2.  1/3

3.  2/3

4.  1

4

0

Explanation :
No Explanation available for this question

# A large spherical enclosure has a small opening. The rate of emission of radioactive flux through this opening is 7.35 kW/m2. The temperature at the inner surface of the sphere will be about (Assume Stefan Boltz Mann constant σ = 5.67 * 10-8 W/m2 K4)

1.  6000 C

2.  3300 C

3.  373 K

4.  1000 K

4

3300 C

Explanation :
No Explanation available for this question

# The spectral emissive power E1 for a diffusely emitting surface is E1 = 0 for λ < 3 μm E­1 = 150 W/m2 μm for 3 < λ < 12 μm E­1 = 300 W/m2 μm for 12 < λ < 25 μm E­1 = 0 for λ > 25 μm The total emissive power of the surface over the entire spectrum is

1.  1250 W/m2

2.  2500 W/m2

3.  4000 W/m2

4.  5250 W/m2

4

5250 W/m2

Explanation :
No Explanation available for this question

# On an summer day a scooter rider feels more comfortable while on the move than while at a stop light because

1.  An object in motion captures les solar radiation

2.  Air is transparent to radiation and hence it is cooler than the body

3.  More heat is lost by convection and radiation while in motion

4.  Air has a low specific heat and hence it is cooler

4