BCECE Engineering BCECE Engineering Solved Paper-2008

  • question_answer
    During an adiabatic process, the pressure of a gas is found to be proportional to the cube of its absolute temperature. The ratio \[{{C}_{p}}/{{C}_{v}}\]for the gas is

    A)  4/3                       

    B)         2

    C)  5/3                       

    D)         3/2

    Correct Answer: D

    Solution :

    Key Idea For adidbatic process \[p{{V}^{\gamma }}=\text{constant}\text{.}\] Give that, \[p\propto {{T}^{3}}\]                 or            \[p{{T}^{-3}}=\text{constant}\]                 ?(i) Also for adiabatic process, \[p{{V}^{\gamma }}=\text{constant}\]                 but         \[pV=RT\]           (ideal gas law)                 or            \[V=\frac{RT}{p}\]                 \[\therefore \]  \[p{{\left( \frac{RT}{p} \right)}^{\gamma }}=\text{constant}\]                                 \[{{p}^{1-\gamma }}{{T}^{\gamma }}=\text{constant}\]                 or            \[p{{T}^{\gamma /(1-\gamma )}}=\text{constant}\]        ?(ii) Equating Eqs. (i) and (ii), we get \[\frac{\gamma }{1-\gamma }=-3\]                 or            \[\gamma =-3+3\gamma \]                 or            \[2\gamma =3\]                 or            \[\gamma =3/2\]                 or            \[\frac{{{C}_{p}}}{{{C}_{v}}}=3/2\]


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