Question

Assertion The rate of loss of heat of a body at $300 K$ is $R$. At $900 K$, the rate of loss becomes $81\, R$.
ReasonThis is as per Newton's law of cooling.

• A. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
• B. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.
• C. Assertion is correct but Reason is incorrect.
• D. Assertion is incorrect but Reason is correct.

Answer 1

Answer: (C)

Stefan's law is applied here not the Newton's law of cooling.
According to Stefan's law,
$\frac{E_{2}}{E_{1}}=\left(\frac{T_{2}}{T_{1}}\right)^{4}=\left(\frac{900}{300}\right)^{4}=81 $
$\Longrightarrow \frac{F_{2}}{R}=81$
Therefore, $ E_{2}=81 \,R$
Therefore, Assertion is correct but Reason is incorrect.