JEE Main & Advanced JEE Main Paper Phase-I Held on 07-1-2020 Morning

  • question_answer
    A litre of dry air at STP expands adiabatically to a volume of 3 litres. If \[\gamma =1.40\], the work done by air is \[\left( {{3}^{1.4}}=4.6555 \right)\] [Take air to be an ideal gas]                         [JEE MAIN Held on 07-01-2020 Morning]

    A) 90.5 J

    B) 60.7 J

    C) 48 J                 

    D) 100.8 J

    Correct Answer: A

    Solution :

    [a] \[{{\text{V}}_{\text{1}}}\text{=1}\,\,\text{litre,}\,\,{{\text{P}}_{\text{1}}}\text{=1}\,\,\text{atm}\] \[{{\text{V}}_{2}}\text{=3}\,\,\text{litre,}\,\,\gamma \text{=1}\text{.40}\], \[{{P}_{2}}V_{2}^{\gamma }={{P}_{1}}V_{1}^{\gamma }\] \[\Rightarrow {{P}_{2}}={{P}_{1}}\times {{\left( \frac{1}{3} \right)}^{1.4}}=\frac{1}{4.6555}atm\] \[\therefore w=\frac{{{P}_{1}}{{V}_{1}}-{{P}_{2}}{{V}_{2}}}{\gamma -1}\] \[=\frac{\left( 1\times 1-\frac{1}{4.6555}\times 3 \right)1.01325\times {{10}^{5}}\times {{10}^{-3}}}{0.4}\] \[=90.1\,\,J\] Closest value of \[W=90.5\,\,J\]   


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