Consider two insulating sheets with thermal resistances $R_1$ and $R_2$ as shown. The temperatures $\theta $ is
Medium
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Heat flow in $(1)=$ Heat flow in $(2)$
$\frac{\Delta \mathrm{T}_{1}}{\mathrm{R}_{1}}=\frac{\Delta \mathrm{T}_{2}}{\mathrm{R}_{2}}$
$\frac{\theta_{1}-\theta}{R_{1}}=\frac{\theta-\theta_{2}}{R_{2}} \Rightarrow \theta=\frac{\theta_{1} R_{2}+\theta_{2} R_{1}}{R_{1}+R_{2}}$
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