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Question

The emissive power of a black body at T=300 K is 100 W/m2. Consider a body B of area A=10 m2, coefficient of reflectivity r=0.3 and coefficient of transmission t=0.5. Its temperature is 300 K. Then choose the correct statement.

A
The emissive power of B is 20 W/m2.
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B
The emissive power of B is 200 W/m2.
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C
The power emitted by B is 100 watts.
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D
The emissivity of B is 0.3.
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Solution

The correct option is A The emissive power of B is 20 W/m2.
We know that,
a+r+t=1
a=1rt
a=[10.30.5]=0.2

Applying Kirchhoff's law,
emissivity(e)=absorptive power(a)
e=0.2
Emissive power of body B,
E=(100)(0.2)=20 W/m2
Power emitted=Emissive Power×Area
=20×10=200 W
Thus, only option (a) is correct.

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