JEE Main & Advanced Physics Thermodynamical Processes Question Bank Critical Thinking

  • question_answer
    Work done by a system under isothermal change from a volume \[{{V}_{1}}\] to \[{{V}_{2}}\] for a gas which obeys Vander Waal's equation \[(V-\beta n)\,\left( P+\frac{\alpha {{n}^{2}}}{V} \right)=nRT\]

    A)            \[nRT{{\log }_{e}}\left( \frac{{{V}_{2}}-n\beta }{{{V}_{1}}-n\beta } \right)+\alpha \,{{n}^{2}}\,\left( \frac{{{V}_{1}}-{{V}_{2}}}{{{V}_{1}}{{V}_{2}}} \right)\]

    B)            \[nRT{{\log }_{10}}\left( \frac{{{V}_{2}}-\alpha \beta }{{{V}_{1}}-\alpha \beta } \right)+\alpha \,{{n}^{2}}\,\left( \frac{{{V}_{1}}-{{V}_{2}}}{{{V}_{1}}{{V}_{2}}} \right)\]

    C)            \[nRT{{\log }_{e}}\left( \frac{{{V}_{2}}-n\alpha }{{{V}_{1}}-n\alpha } \right)+\beta \,{{n}^{2}}\,\left( \frac{{{V}_{1}}-{{V}_{2}}}{{{V}_{1}}{{V}_{2}}} \right)\]

    D)            \[nRT{{\log }_{e}}\left( \frac{{{V}_{1}}-n\beta }{{{V}_{2}}-n\beta } \right)+\alpha \,{{n}^{2}}\,\left( \frac{{{V}_{1}}{{V}_{2}}}{{{V}_{1}}-{{V}_{2}}} \right)\]

    Correct Answer: A

    Solution :

                       According to given Vander Waal?s equation                    \[P=\frac{nRT}{V-n\beta }-\frac{\alpha {{n}^{2}}}{{{V}^{2}}}\]                    Work done, \[W=\int_{{{V}_{1}}}^{{{V}_{2}}}{PdV}=nRT\int_{{{V}_{1}}}^{{{V}_{2}}}{\frac{dV}{V-n\beta }}-\alpha {{n}^{2}}\int_{{{V}_{1}}}^{{{V}_{2}}}{\frac{dV}{{{V}^{2}}}}\]                        \[=nRT\,\left[ {{\log }_{e}}(V-n\beta ) \right]\,_{{{V}_{1}}}^{{{V}_{2}}}+\alpha {{n}^{2}}\left[ \frac{1}{V} \right]_{{{V}_{1}}}^{{{V}_{2}}}\]                        \[=nRT{{\log }_{e}}\frac{{{V}_{2}}-n\beta }{{{V}_{1}}-n\beta }+\alpha {{n}^{2}}\left( \frac{{{V}_{1}}-{{V}_{2}}}{{{V}_{1}}{{V}_{2}}} \right)\]


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