• # question_answer Two rods of equal length and area of cross-section are kept parallel and logged between temperature ${{20}^{o}}C$ and ${{80}^{o}}C$ The ratio of the effective thermal conductivity to that of the first rod is [The ratio ${{K}_{1}}/{{K}_{2}}=3:4$] A)  $7:4$                         B)  $7:6$C)  $4:7$                         D)  $7:8$

For parallel combination of two rods of equal length and equal area of cross-section. $K=\frac{{{K}_{1}}+{{K}_{2}}}{2}=\frac{{{K}_{1}}+\frac{4{{K}_{1}}}{3}}{2}$ $=\frac{7{{K}_{1}}}{6}$ (given,  ${{K}_{1}}/{{K}_{2}}=3/4$) Hence, $\frac{K}{{{K}_{1}}}=\frac{7}{6}$