JCECE Engineering JCECE Engineering Solved Paper-2009

  • question_answer
    A given mass of a gas is compressed isothermally until its pressure is doubled. It is then allowed to expand adiabatically until its original volume is restored and its pressure is then found to be \[0.75\] of its initial pressure. The ratio of the specific heats of the gas is approximately

    A) \[1.2\]                                  

    B) \[1.41\]

    C) \[1.67\]                               

    D) \[1.83\]

    Correct Answer: B

    Solution :

    In isothermal process, temperature of the gas remains constant, so the gas obeys Boyle's law. That is,                 \[p\propto \frac{1}{V}\Rightarrow \frac{{{p}_{2}}}{{{p}_{1}}}=\frac{{{V}_{1}}}{{{V}_{2}}}\] \[\Rightarrow \]                               \[\frac{2p}{p}=\frac{{{V}_{1}}}{{{V}_{2}}}\] \[\therefore \]                  \[\frac{{{V}_{1}}}{{{V}_{2}}}=2\]                                               ? (i) Now, the gas is expanded adiabatically, so                 \[p{{V}^{\gamma }}=\]constant                 \[\frac{{{p}_{1}}}{{{p}_{2}}}={{\left( \frac{{{V}_{2}}}{{{V}_{1}}} \right)}^{\gamma }}\] \[\Rightarrow \]               \[\frac{2p}{0.75p}={{\left( \frac{2}{1} \right)}^{\gamma }}\] (since volume: is restored) \[\Rightarrow \]               \[\log \left( \frac{8}{3} \right)=\gamma \log 2\] \[\Rightarrow \]               \[\log 8-\log 3\gamma =\log 2\]


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