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BCECE Engineering BCECE Engineering Solved Paper-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 \[\frac{d\theta }{dt}=\frac{1}{12}\times {{10}^{-2}}{{\,}^{\text{o}}}\text{C/min,}\] specific heat\[s=420J/kg{{\,}^{\text{0}}}\text{C)}\]

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


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