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Homework Help: Is heat absorbed, when volume is increased?

  1. Oct 26, 2013 #1
    1. The problem statement, all variables and given/known data
    Hi! Greetings! This is not really a problem set; I just made the question up to better understand PV diagrams of Carnot cycle.

    Suppose you have a gas in a container with a movable piston, but is thermally insulated. If the volume of the gas increases, is heat absorbed by the system (the gas particles) or released by the system into the environment?

    2. Relevant equations

    [itex]dQ = dU + dW[/itex]
    [itex]dQ = dU + PdV[/itex]

    3. The attempt at a solution
    Hmm.. What I am thinking is that if the volume expands, the gas particles are less compressed and have more place to move around, so I guess the system should give off heat to the surroundings?
    Last edited: Oct 26, 2013
  2. jcsd
  3. Oct 26, 2013 #2
    Oh, I got it. It's thermally insulated, so no heat flow. It's the temperature that should change, and if it is an ideal gas, the volume is proportional with temperature and thus, the temperature should increase. Sorry for asking a silly question.. Haha..
  4. Oct 26, 2013 #3
    If the container is thermally insulated ,there is no heat exchange between the container and the surroundings
  5. Oct 26, 2013 #4

    Andrew Mason

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    Science Advisor
    Homework Helper

    How can it increase?

    Apply the first law. If PdV>0 and Q = 0 what must happen to U?

  6. Nov 23, 2013 #5
    I guess the internal energy increases if PdV.

    But in Carnot cycle there are two types of processes right? Adiabatic and isothermal. The process described above is adiabatic, so I thought there should be some temperature change.
  7. Nov 23, 2013 #6
    Ah wait.. When the volume increases, the particles are doing work against the system, so the internal energy will decrease. And temperature also would decrease. Am I right?
  8. Nov 23, 2013 #7

    dU = -pdV

    Now dV > 0 ,so dU <0 i.e internal energy decreases.
  9. Nov 24, 2013 #8
    Oh I see.. Thank you very much! :)
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