JEE Main & Advanced Physics Transmission of Heat Question Bank Critical Thinking

  • question_answer
    Five rods of same dimensions are arranged as shown in the figure. They have thermal conductivities K1, K2, K3, K4 and K5. When points A and B are maintained at different temperatures, no heat flows through the central rod if [KCET 2002]

    A)            \[{{K}_{1}}={{K}_{4}}\,\text{and}\,\ {{K}_{2}}={{K}_{3}}\]

    B)            \[{{K}_{1}}{{K}_{4}}={{K}_{2}}{{K}_{3}}\]

    C)            \[{{K}_{1}}{{K}_{2}}={{K}_{3}}{{K}_{4}}\]

    D)            \[\frac{{{K}_{1}}}{{{K}_{4}}}=\frac{{{K}_{2}}}{{{K}_{3}}}\]

    Correct Answer: B

    Solution :

                       For no current flow between C and D                    \[{{\left( \frac{Q}{t} \right)}_{AC}}={{\left( \frac{Q}{t} \right)}_{CB}}\]Þ \[\frac{{{K}_{1}}A({{\theta }_{A}}-{{\theta }_{C}})}{l}=\frac{{{K}_{2}}A({{\theta }_{C}}-{{\theta }_{B}})}{l}\]                    Þ \[\frac{{{\theta }_{A}}-{{\theta }_{C}}}{{{\theta }_{C}}-{{\theta }_{B}}}=\frac{{{K}_{2}}}{{{K}_{1}}}\]                   ...(i)                    Also \[{{\left( \frac{Q}{t} \right)}_{AD}}={{\left( \frac{Q}{t} \right)}_{DB}}\]Þ\[\frac{{{K}_{3}}A({{\theta }_{A}}-{{\theta }_{D}})}{l}=\frac{{{K}_{4}}A({{\theta }_{D}}-{{\theta }_{B}})}{l}\]                    Þ\[\frac{{{\theta }_{A}}-{{\theta }_{D}}}{{{\theta }_{D}}-{{\theta }_{B}}}=\frac{{{K}_{4}}}{{{K}_{3}}}\]                    ...(ii) It is given that \[{{\theta }_{C}}={{\theta }_{D}},\] hence from equation (i) and (ii) we get  \[\frac{{{K}_{2}}}{{{K}_{1}}}=\frac{{{K}_{4}}}{{{K}_{3}}}\] Þ \[{{K}_{1}}{{K}_{4}}={{K}_{2}}{{K}_{3}}\]


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