Question

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

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

Answer 1

Answer: (A)

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