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

  • question_answer
    Three perfect gases at absolute temperatures \[{{T}_{1}},\,{{T}_{2}}\] and \[{{T}_{3}}\] are mixed. The masses of molecules are \[{{m}_{1}},\,{{m}_{2}}\] and \[{{m}_{3}}\] and the number of molecules are \[{{n}_{1}},\,{{n}_{2}}\] and \[{{n}_{3}},\] respectively. Assuming no loss of energy, the final temperature of the mixture is

    A) \[\frac{({{T}_{1}}+{{T}_{2}}+{{T}_{3}})}{3}\]

    B) \[\frac{{{n}_{1}}{{T}_{1}}+{{n}_{2}}{{T}_{2}}+{{n}_{3}}{{T}_{3}}}{{{n}_{1}}+{{n}_{2}}+{{n}_{3}}}\]

    C) \[\frac{{{n}_{1}}{{T}_{1}}+{{n}_{2}}T_{2}^{2}+{{n}_{3}}T_{3}^{2}}{{{n}_{1}}{{T}_{1}}+{{n}_{2}}{{T}_{2}}+{{n}_{3}}{{T}_{3}}}\]           

    D) \[\frac{n_{1}^{2}T_{1}^{2}+n_{3}^{2}T_{2}^{2}+n_{3}^{2}T_{3}^{2}}{{{n}_{1}}{{T}_{1}}+{{n}_{2}}{{T}_{2}}+{{n}_{3}}{{T}_{3}}}\]

    Correct Answer: B

    Solution :

    [b] Let \[{{T}_{3}}>{{T}_{2}}>{{T}_{1}}\] and final temperature is T such that \[{{T}_{3}}>T>{{T}_{2}}>{{T}_{1}}\]. Now heat gained by first tow gases is equal to heat lost by third gas
    \[{{n}_{1}}C(T-{{T}_{1}})+{{n}_{2}}C(T-{{T}_{2}})={{n}_{3}}C({{T}_{3}}-T)\]
    \[\Rightarrow \]            \[T=\frac{{{n}_{1}}{{T}_{1}}+{{n}_{2}}{{T}_{2}}+{{n}_{3}}{{T}_{3}}}{{{n}_{1}}+{{n}_{2}}+{{n}_{3}}}\]


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