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Homework Help: Temperature of ideal gas in a cylinder

  1. Nov 22, 2015 #1
    Qn : https://www.dropbox.com/s/dpcws9q4bjpzvtp/20151123_112037.jpg?dl=0

    Why is option A and C wrong or correct?

    My attempted answer :
    A :A is plausible as if the piston is suddenly moved inwards, the gas molecules bounce off the piston at higher speeds
    Since Temp proportional to rms speed, temp increases
    based on 1st law of thermodynamics
    The piston suddenly moved in.... So means Q=0 cos no time for heat transfer.
    Since compression, W is +ve so U is +ve. That implies T increases.

    C : as P increases, V decreases

    Please help to correct my understanding! !
    Thanks a million! !!
    Last edited: Nov 22, 2015
  2. jcsd
  3. Nov 22, 2015 #2
    "Suddenly moves inwards" in the question implies that there was little or no heat exchange during this compression process. With this information, apply the First Law and try and deduce for yourself whether A or C is the correct answer.
  4. Nov 22, 2015 #3
    What if the piston is slowly moved inwards? How will the answer change?
  5. Nov 22, 2015 #4
    If the piston is moved slowly enough, the temperature will not change. Can you rationalize why?
  6. Nov 22, 2015 #5
    Is it because there's time for heat distribution throughout the gas??
  7. Nov 22, 2015 #6
    There's sufficient time during the process for heat to enter or leave the system.
  8. Nov 22, 2015 #7
    If the piston is suddenly moved inwards and the temp increased, and i leave the system there for some time, will the system return to it's prior temperature (room temp) assuming this is carried out at room temp?
  9. Nov 22, 2015 #8
    Yes, heat will flow from higher temperature region to lower temperature region. It would be more precise to say that the system and its surroundings reach an equilibrium temperature, which may or may not be the original room temperature. (before the compression)
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