Question

A body at a temperature of $728^{\circ} C$ and has surface area $5\, cm ^{2}$, radiates $300\, J$ of energy each minute. The emissivity is (Given Boltzmann constant $=5.67 \times 10^{-8} Wm ^{2} K ^{4}$ )

• A. e =0.18
• B. e=0.02
• C. e=0.2
• D. e=0.15

Answer 1

Answer: (A)

e =0.18

Answer 2

$a=E A t=e \sigma\left(T^{4}-T_{0}^{4}\right) A t$ where, $t=$ time $T_{0}=$ temperature of surrounding when $T>T_{0}, Q=e \sigma T^{4} A t$ $300 =e \times\left(5.67 \times 10^{-8}\right)(1000)^{4}\left(5.00 \times 10^{-4}\right)$ $e=0.18$