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BCECE Medical BCECE Medical Solved Papers-2007

  • question_answer
    A gas expands isothermally against a constant external pressure of 1 atm from a volume of \[10\,\,d{{m}^{3}}\]to a volume of \[20\,\,d{{m}^{3}}\]. It absorbs 300 J of thermal energy from its surroundings. The \[\Delta U\] is

    A)  -312J        

    B)  +123J

    C)  -213J        

    D)  + 231 J

    Correct Answer: C

    Solution :

    \[W=-\int_{{{V}_{1}}}^{{{V}_{2}}}{PdV=-P\,\,({{V}_{2}}-{{V}_{1}})}\] \[W=-1\,\,(20-10)=-10\,\,d{{m}^{3}}\,\,atm\]      \[=-10\,\,d{{m}^{3}}\times \frac{8.314\,J{{K}^{-1}}mo{{l}^{-1}}}{0.821\,\,d{{m}^{3}}\,{{K}^{-1}}mo{{l}^{-1}}}\]      = - 1013 J From 1st law of thermodynamic                 \[\Delta U=q+W\]          \[=800\,\,J+(-1013\,J)=-213\,J\]


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