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BCECE Medical BCECE Medical Solved Papers-2011

  • question_answer
    If the emissive power of black surface at same temperature is 400 \[W/{{m}^{2}}\], the emissive and absorptive powers of the surface assuming it was initially ordinary surface, are (Given, Mass of the body m = 4.2 kg, Area of body \[=5\times {{10}^{-2}}{{m}^{2}}\], Rate of cooling \[\frac{d\theta }{dt}=\frac{1}{12}\times {{10}^{-2}}{{\,}^{o}}C/\min \] Specific heat \[s=420J/kg{{\,}^{o}}C\])

    A)  e = a = 0.0735   

    B)  e = a = 0.0435

    C)  e = a = 0.0535   

    D)  e = a = 0.0235

    Correct Answer: A

    Solution :

    Using the relation                 \[E=\frac{Q}{At}\]                 \[=\frac{ms\,\theta }{At}=\frac{4.2\times 420\times 1\times {{10}^{-2}}}{5\times {{10}^{-2}}\times 12}=29.4W/{{m}^{2}}\] Emissive and absorptive powers are given by                 \[e=\frac{{{E}_{a}}}{{{E}_{b}}}=\frac{29.4}{400}\] 0(Here,\[{{E}_{b}}=400\,W/{{m}^{2}}\]) = 0.0735


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