The emissive power of sphere of area $0.04 \mathrm{~m}^2$ is $0.7 \mathrm{kcal} \mathrm{s}^{-1}…

The emissive power of sphere of area $0.04 \mathrm{~m}^2$ is $0.7 \mathrm{kcal} \mathrm{s}^{-1} \mathrm{~m}^{-2}$. The amount of heat radiated in 20 second is
  1. $2.8 \mathrm{kcal} \mathrm{s}^{-1} \mathrm{~m}^{-2}$
  2. $0.28 \mathrm{kcal} \mathrm{s}^{-1} \mathrm{~m}^{-2}$
  3. $5.6 \mathrm{kcal} \mathrm{s}^{-1} \mathrm{~m}^{-2}$
  4. $0.56 \mathrm{kcal} \mathrm{s}^{-1} \mathrm{~m}^{-2}$

Solution

Emissive power $E=\frac{Q}{A t}$ $\therefore Q=E A t=0.7 \times 0.04 \times 20=0.56 \mathrm{kcal} \mathrm{s}^{-1} \mathrm{~m}^{-2}$

Asked in: MHT CET 2021 (22 Sep Shift 2)

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