When StefanBoltzmann law is applied to a black body, it takes the form
The constant is different for different bodies.
Consider two bodies, one absolutely back and the other nonblack and let these be at same temperature. Which one of the following statement is correct?
The radiation spectrum for a nonblack body may be similar or radically different from that of a black body.
When the emissivity of nonblack surface is constant at all temperatures and throughout the entire range of wavelength, the surface is called
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.
The emissivity of the gray surface may be expressed as
It is the ratio of radiating coefficient to that of black body radiating coefficient.
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?
E = (T _{b}/T _{G})^{ 4 }= 0.4096.
The radiant heat transfer from a plate of 2.5 cm^{2} area at 1250 K to a very cold enclosure is 5.0 W. Determine the emissivity of the plate at this temperature
Emissivity = E/σ A T^{4} = 0.144.
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
E = (Emissivity) σ A T^{4}. So, A = 0.98 * 10 ^{4} m^{2}.
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
Total emissivity = 0.4 (0.3181 – 0.0000) + 0.8 (0.8563 – 0.3118) = 0.5578.
For a hemisphere solid angle is measured as
It should be measured in Steradian.
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
T _{b}^{4 }= E T _{g}^{4}.
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