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AFMC AFMC Solved Paper-2004

  • question_answer
    Ice starts forming in a lake with water at 0°C, when atmospheric temperature is -10°C. If the time taken for 5 mm thickness of ice to be formed is 2 h, then the time taken for the thickness of ice to change from 5 mm to 20 mm, is:

    A) 30 h                                       

    B) 18 h

    C) 6 h                                         

    D) 32 h

    Correct Answer: A

    Solution :

    For growth of ice, we have \[t=\frac{\rho L}{2K\theta }(x_{2}^{2}-x_{1}^{2})\]where, \[\rho \]is density, L latent heat, \[{{x}_{2}},{{x}_{1}}\]thickness and\[\theta \]temperature. Similarly, for second case \[t'=\frac{\rho L}{2K\theta }(x_{4}^{2}-x_{3}^{2})\] \[\therefore \]              \[\frac{t'}{t}=\frac{(x_{4}^{2}-x_{3}^{2})}{(x_{2}^{2}-x_{1}^{2})}\] \[=\frac{({{20}^{2}}-{{5}^{2}})}{({{5}^{2}}-0)}\] \[=\frac{375}{25}=15\] \[\therefore \]     \[t'=15t=15\times 2=30\,h\]


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