Question

A copper ball $2\,cm$ in radius is heated in a furnace to $327\,{}^{\circ}$C. If its emissivity is $0.3$, at what rate does it radiate energy?

• A. 1.72 W
• B. 2.73 W
• C. 11.0 W
• D. 2.15 W

Answer 1

Answer: (C)

The surface area of the ball
$A=4 \pi r^{2}=(4 \pi)(0.02 m )^{2}=0.005\, m ^{2}$
and its absolute temperature is
$T =327+273=600\, K . $ Hence
$P =e \sigma AT ^{4} $
$=\left(0.3 \times 5.67 \times 10^{-8} \times 0.005 \times(600)^{4}\right.$
$=11.0 \,W$