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Consider a long cylindrical tube of inner and outer radii, $r_i$ and $r_o$ , respectively, length, $L$ and thermal conductivity, $k$. Its inner and outer surfaces are maintained at $T_i$ and $T_o$ , respectively ( $T_i > T_o$ ). Assuming one-dimensional steady state heat conduction in the radial direction, the thermal resistance in the wall of the tube is

- $\dfrac{1}{2\pi kL}\ln \bigg(\dfrac{r_i}{r_o} \bigg ) \\$
- $\dfrac{L}{2\pi r_ik} \\$
- $\dfrac{1}{2\pi kL}\ln \bigg ( \dfrac{r_o}{r_i} \bigg ) \\$
- $\dfrac{1}{4\pi kL}\ln \bigg (\dfrac{r_o}{r_i} \bigg )$