he ratio of the coefficient of thermal conductivity of two different materials is $5 : 3$ . If the thermal resistance of the rod of same thickness resistance of the rods of same thickness of these materials is same, then the ratio of the length of these rods will be
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(b) Thermal resistances are same
==> $\frac{{{l_1}}}{{{K_1}{A_1}}} = \frac{{{l_2}}}{{{K_2}{A_2}}}$ ==> $\frac{{{l_1}}}{{{K_1}}} = \frac{{{l_2}}}{{{K_2}}}$ $(\because {A_1} = {A_2})$
==> $\frac{{{l_1}}}{{{l_2}}} = \frac{{{K_1}}}{{{K_2}}} = \frac{5}{3}$
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