First slide

Radiation

Question

A body is heated to a temperature of $227^{o} C$ and allowed to coal in air at $27^{o} C$ . Initial rate of cooling of the body is $1^{o} C / \min$ what will be the rate of cooling of the body when its surface temperature is $127^{o} C$ ?

Moderate

A

$0 . 32^{0} C / m i n$
B

$0 . 42^{0} C / m i n$
C

$0.24 C / m i n$
D

$0 . 68^{0} C / m i n$

Solution

$R \propto (T^{4} - T_{0}^{4}) \Rightarrow \frac{R_{2}}{R_{1}} = (\frac{T_{2}^{4} - T_{0}^{4}}{T_{1}^{4} - T_{0}^{4}}) \Rightarrow \frac{R_{2}}{R_{1}} = \frac{400^{4} - 300^{4}}{500^{4} - 300^{4}}$
$\Rightarrow R_{2} = {0.32}^{0} C / m i n$

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