First slide

Radiation

Question

A black body radiates 20 W at temperature $227^{0} C . If temperature of the black body is changed o 727^{0} C, then$ its radiating power will be:

Moderate

A

120 W
B

240 W
C

320 W
D

360 W

Solution

For a black body $\frac{Q}{t} = P = {AσT}^{4}$
$\Rightarrow \frac{P_{2}}{P_{1}} = {(\frac{T_{2}}{T_{1}})}^{4} \Rightarrow \frac{P_{2}}{20} = {(\frac{273 + 727}{273 + 227})}^{4}$
$\Rightarrow \frac{P_{2}}{20} = {(2)}^{4} \Rightarrow P_{2} = 320 W$

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