JEE Main & Advanced Physics Thermodynamical Processes JEE PYQ-Thermodynamical Processes

  • question_answer
    A vertical closed cylinder is separated into two parts by a frictionless piston of mass m and negligible thickness. The piston is free to move along the length of the cylinder. The length of the cylinder above the piston is\[{{l}_{1}}\]and that below the piston is\[{{l}_{2}},\]such that \[{{l}_{1}}>{{l}_{2}}.\]Each part of the cylinder contains n moles of an ideal gas at equal temperature T. If the piston is stationary, its mass m, will be given by (R is universal gas constant and g is the acceleration due to gravity) [JEE Main 12-Jan-2019 Evening]

    A) \[\frac{RT}{ng}\left[ \frac{{{l}_{1}}-3{{l}_{2}}}{{{l}_{1}}{{l}_{2}}} \right]\]   

    B)      \[\frac{nRT}{g}\left[ \frac{1}{{{l}_{2}}}+\frac{1}{{{l}_{1}}} \right]\]

    C) \[\frac{RT}{g}\left[ \frac{2{{l}_{1}}+{{l}_{2}}}{{{l}_{1}}{{l}_{2}}} \right]\]   

    D)      \[\frac{nRT}{g}\left[ \frac{{{l}_{1}}-{{l}_{2}}}{{{l}_{1}}{{l}_{2}}} \right]\]

    Correct Answer: D

    Solution :

    [d] Let A be area of cross section of the cylinder.
    As the piston is at rest, so the pressure from both the sides should be equal
    \[\frac{mg}{A}+\frac{nRT}{{{V}_{1}}}=\frac{nRT}{{{V}_{2}}}\]
    \[\Rightarrow \]\[\frac{mg}{A}+\frac{nRT}{A{{l}_{1}}}=\frac{nRT}{A{{l}_{2}}}\Rightarrow m=\frac{nRT}{g}\left( \frac{{{l}_{1}}-{{l}_{2}}}{{{l}_{1}}{{l}_{2}}} \right)\]


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