A copper rod $2\,m$ long has a circular cross-section of radius $1\, cm$. One end is kept at $100^o\,C$ and the other at $0^o\,C$ and the surface is covered by nonconducting material to check the heat losses through the surface. The thermal resistance of the bar in degree kelvin per watt is (Take thermal conductivity $K = 401\, W/m-K$ of copper):-
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Thermal resistance,
$\mathrm{R}=\frac{l}{\mathrm{KA}}=\frac{l}{\mathrm{K}\left(\pi \mathrm{r}^{2}\right)}$
$=\frac{2}{401 \times 3.14 \times\left(10^{-4}\right)}=15.9$ kelvin $/$ watt
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