Question

The emissive power of a blackbody at $T=300K$ is $100W/m^2$. Consider a body B of area $A=10m^2$ coefficient of reflectivity $r=0.3$ and coefficient of transmission $t=0.5$. Its temperature is $300K$. Then which of the following is correct?

• A. The emissive power of B is $20$ $W/m^2$
• B. The emissive power of B is $200$ $W/m^2$
• C. The power emitted by B is $100$ Watts
• D. The emissivity of B is $0.3$

Answer 1

The correct option is A The emissive power of B is $20$ $W/m^2$

Emissivity of body B, $e=0.2$ (Since $e=a$ and $a=(1-r-t)=0.2$)

Emissive power of body B, $E=\left(100\right)\left(0.2\right)=20W/m^2$

Power emitted $=e.A=20\times 10=200Watts$