The emissive power of a black body at T=300K is 100W/m2. Consider a body B of area A=10m2, coefficient of reflectivity r=0.3 and coefficient of transmission t=0.5. Its temperature is 300K. Then choose the correct statement.
A
The emissive power of B is 20W/m2.
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B
The emissive power of B is 200W/m2.
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C
The power emitted by B is 100watts.
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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 20W/m2. We know that, a+r+t=1 ⇒a=1−r−t ⇒a=[1−0.3−0.5]=0.2
Applying Kirchhoff's law, emissivity(e)=absorptive power(a) ⇒e=0.2 Emissive power of body B, E=(100)(0.2)=20W/m2 Power emitted=Emissive Power×Area =20×10=200W Thus, only option (a) is correct.