JEE Main & Advanced Physics Kinetic Theory of Gases Question Bank Topic Test - Kinetic Theory of Gases

  • question_answer
    A box contains N molecules of a perfect gas at temperature \[{{T}_{1}}\] and pressure \[{{P}_{1}}\]. The number of molecules in the box is doubled keeping the total kinetic energy of the gas same as before. If the new pressure is \[{{P}_{2}}\] and temperature \[{{T}_{2}},\] then

    A) \[{{P}_{2}}=P,\,\,{{T}_{2}}={{T}_{1}}\]

    B) \[{{P}_{2}}={{P}_{1}},\,{{T}_{2}}=\frac{{{T}_{1}}}{2}\]

    C) \[{{P}_{2}}=2{{P}_{1}},\,{{T}_{2}}={{T}_{1}}\]

    D)  \[{{P}_{2}}=2{{P}_{1}},\,{{T}_{2}}=\frac{{{T}_{1}}}{2}\]

    Correct Answer: B

    Solution :

    [b] Kinetic energy of N molecule of gas, \[E=\frac{3}{2}NkT\]
    Initially, \[{{E}_{1}}=\frac{3}{2}{{N}_{1}}k{{T}_{1}}\] and finally, \[{{E}_{2}}=\frac{3}{2}{{N}_{2}}k{{T}_{2}}\]
    But according to \[{{E}_{1}}={{R}_{2}}\] and \[{{N}_{2}}=2{{N}_{1}},\]
    \[\frac{3}{2}{{N}_{1}}k{{T}_{1}}=\frac{3}{2}\,(2{{N}_{1}})k{{T}_{2}}\Rightarrow \,{{T}_{2}}=\frac{{{T}_{1}}}{2}\]
    Since the kinetic energy constant,
    \[\frac{3}{2}{{N}_{1}}k{{T}_{1}}=\frac{3}{2}{{N}_{2}}k{{T}_{2}}\]
    \[\Rightarrow \]            \[{{N}_{1}}{{T}_{1}}={{N}_{2}}{{T}_{2}}\]
    \[\therefore \]    \[NT=\]Constant
    From ideal gas equation of N molecules,
    \[PV=NkT\]
    \[\Rightarrow \]            \[{{P}_{1}}{{V}_{1}}={{P}_{2}}{{V}_{2}}\Rightarrow \,{{P}_{1}}={{P}_{2}}\]
    [as \[{{V}_{1}}={{V}_{2}}\] and \[NT=\] constant]


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