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A rod of length $l$ with thermally insulated lateral surface is made of a material whose thermal conductivity $K$ varies as $K = C/T$ , where $C$ is a constant. The ends are at temperatures $T_1$ and $T_2$ . The heat current density is
  • A$C\,\ln \,\frac{{{T_2}}}{{{T_1}}}$
  • B$\frac{C}{l}\,\ln \,\left( {\frac{{{T_2}}}{{{T_1}}}} \right)$
  • C$\frac{C}{l}\,\ln \,\left( {{T_1}{T_2}} \right)$
  • D$Cl\,\ln \,\left( {\frac{{{T_2}}}{{{T_1}}}} \right)$
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