A tungsten body of diameter $2.3\, cm$ is at $2000\,^oC$. It radiates $30\%$ of the energy radiated by a black body of same radius and temperature. Find radius of black body which will radiate energy at same rate at the same temperature......... $cm$
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rate $\frac{\mathrm{E}}{\mathrm{t}}=\mathrm{e} \sigma \mathrm{AT}^{4} \quad(\mathrm{e}=1)$
$\sigma A_{1} T^{4}=30 \% \sigma A T^{4}$
$\sigma 4 \pi R_{1}^{2} T^{4}=0.3 \sigma 4 \pi R^{2} T^{4}$
$\mathrm{R}_{1}=\sqrt{0.3} \times\left(\frac{2.3}{2}\right) \mathrm{cm}$
$\mathrm{R}_{1}=0.629 \mathrm{cm}$
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