AFMC AFMC Solved Paper-2011

  • question_answer
    An ideal gas (\[\gamma \]= 1.5) is expanded adiabatically. How many times has the gas to be expanded to reduce the root mean square velocity of the molecules 2.0 times?

    A) 8 times                                

    B) 16 times

    C) \[\frac{1}{8}\] times                                      

    D) 4 times

    Correct Answer: B

    Solution :

    \[{{v}_{rms}}=\sqrt{\frac{3RT}{M}}\] ie, \[{{v}_{rms}}\propto \sqrt{T}\] To reduce rms velocity 2 times, temperature must be reduced 4 times, ie, \[{{T}_{2}}=\frac{1}{4}{{T}_{1}}\] In an adiabatic process, \[{{T}_{2}}V_{2}^{\gamma -1}={{T}_{1}}V_{1}^{\gamma -1}\] \[{{\left( \frac{{{V}_{2}}}{{{V}_{1}}} \right)}^{\gamma -1}}=\frac{{{T}_{1}}}{{{T}_{2}}}=\frac{4}{1}\] \[\frac{{{V}_{2}}}{{{V}_{1}}}={{(4)}^{1/\gamma -1}}={{(4)}^{\frac{1}{1.5-1}}}={{(4)}^{2}}=16\] \[{{V}_{2}}=16{{V}_{1}}\]


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