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Thermodynamics

# A metal ball having surface area of $\;2 \times 10^{-2}\;m^2\;$ and at 900 K is kept in an enclosure at 300 K . If the surface emissivity of metal is 0.4 , find the rate of which heat is lost by the ball

$(a)\;293\;W\qquad(b)\;193\;W\qquad(c)\;173\;W\qquad(d)\;483\;W$

Answer : $\;293\;W$
Explanation :
Stefan Boltzman law
$\large\frac{dE}{dt} = eA \sigma\; [T_{0}^{4}-T_{S}^{4}]$
$T_{0}\;$ - Temperature of object
$T_{s}$ - Temperature of surroundings
$= 0.4 \times 2 \times 10^{-2} \times 5.67 \times 10^{-8} \;[(900)^4 - (300)^4]$
$= 293 W$