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

  • question_answer
    Two bodies \[A\]and \[B\] have thermal emissivities of 0.01 and 0.81 respectively. The outer surface areas of the two bodies are the same. The two bodies emit total radiant power at the same rate. The wavelength \[{{\lambda }_{B}}\] corresponding to maximum spectral radiancy in the radiation from \[B\] is shifted from the wavelength corresponding to maximum spectral radiancy in the radiation from \[A\], by \[1.00\mu m\]. If the temperature of \[A\] is \[5802\ K\]                                                                      [IIT 1994; DCE 1996]

    A)            The temperature of \[B\] is \[1934\ K\]

    B)            \[{{\lambda }_{B}}=1.5\mu m\]

    C)            The temperature of \[B\] is \[11604\ K\]

    D)            The temperature of \[B\] is \[2901\ K\]

    Correct Answer: A

    Solution :

     According to Stefan?s law                    \[E=eA\sigma {{T}^{4}}\Rightarrow {{E}_{1}}={{e}_{1}}A\sigma T_{1}^{4}\] and \[{{E}_{2}}={{e}_{2}}A\sigma T_{2}^{4}\]                    \[\because \]\[{{E}_{1}}={{E}_{2}}\] \[\therefore \]\[{{e}_{1}}T_{1}^{4}={{e}_{2}}T_{2}^{4}\]                    \[\Rightarrow \]\[{{T}_{2}}={{\left( \frac{{{e}_{1}}}{{{e}_{2}}}T_{1}^{4} \right)}^{\frac{1}{4}}}={{\left( \frac{1}{81}\times {{(5802)}^{4}} \right)}^{\frac{1}{4}}}\]\[\Rightarrow \]\[{{T}_{B}}=1934\ K\]                    And, from Wein?s law \[{{\lambda }_{A}}\times {{T}_{A}}={{\lambda }_{B}}\times {{T}_{B}}\]                    \[\Rightarrow \frac{{{\lambda }_{A}}}{{{\lambda }_{B}}}=\frac{{{T}_{B}}}{{{T}_{A}}}\]\[\Rightarrow \]\[\frac{{{\lambda }_{B}}-{{\lambda }_{A}}}{{{\lambda }_{B}}}=\frac{{{T}_{A}}-{{T}_{B}}}{{{T}_{A}}}\]            \[\Rightarrow \]\[\frac{1}{{{\lambda }_{B}}}=\frac{5802-1934}{5802}=\frac{3968}{5802}\Rightarrow {{\lambda }_{B}}=1.5\ \mu m\]


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