Punjab Medical Punjab - MET Solved Paper-2008

  • question_answer
    What fraction of the volume of a glass flask must be filled with mercury so that the volume of the empty space may be the same at all temperatures? \[({{\alpha }_{glass}}=9\times {{10}^{-6}}/{}^\circ C,\,\,{{\gamma }_{Hg}}=18.9\times {{10}^{-5}}/{}^\circ C)\]

    A) \[\frac{1}{2}\]                                   

    B) \[\frac{1}{7}\]

    C) \[\frac{1}{4}\]                                   

    D) \[\frac{1}{5}\]

    Correct Answer: B

    Solution :

    If \[V\] is the volume of glass flask, \[{{V}_{L}}\] of mercury and \[{{V}_{A}}\] of air in it                 \[V={{V}_{L}}+{{V}_{A}}\] Now as with change in temperature volume of air remains constant, the expansion of mercury will be equal to that of the glass flask, ie,                 \[\Delta V=\Delta {{V}_{L}}\] or            \[{{V}_{{{\gamma }_{G}}}}\Delta \theta ={{V}_{Hg}}{{\gamma }_{Hg}}\Delta \theta \]    \[(as\Delta V={{V}_{\gamma }}\Delta \theta )\] or            \[\frac{{{V}_{Hg}}}{V}=\frac{{{\gamma }_{G}}}{{{\gamma }_{Hg}}}=\frac{3\times 9\times {{10}^{-6}}}{18.9\times {{10}^{-5}}}=\frac{1}{7}\]


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