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Thermodynamics Pressure

  1. Apr 16, 2016 #1
    1. The problem statement, all variables and given/known data
    I'm doing a class on thermodynamics and energy and well theres this chapter about pure substances and subcooled liquids, saturated liquids, superheated, etc. stuff like that.
    The chapter begins by showing a piston-cylinder in an initial state containing liquid water and a mass is kept on top of it to keep the pressure constant. Thats what I don't understand, how can a mass on top of the piston keep the pressure constant?, I mean first of all as the water is heated should the volume not increase(as gas) by the PV=mRT, and since volume is proportional to pressure it should increase too?

    Can someone explain how the fixed mass on top of the piston-cylinder device keeps pressure constant as the water is heating through several phases.



    2. Relevant equations


    3. The attempt at a solution
     
    Last edited: Apr 16, 2016
  2. jcsd
  3. Apr 16, 2016 #2

    Andrew Mason

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    Neglect friction between the piston and cylinder and assume that any changes to the gas will occur slowly. What are the external forces on the gas in the cylinder? Do those forces change? Does the area of the piston change?

    If you add heat flow to the gas (increasing T), PV will increase. But in this case, since the piston is free to move V increases. Why would P increase?

    AM
     
  4. Apr 16, 2016 #3
    For an ideal gas at constant m and T, volume is inversely proportional to pressure. But, that is not what is happening in your experiment. Moreover, liquid water does not even obey the ideal gas low.

    In your experiment, both P and T are constant as liquid evaporates to form vapor. If V is increasing, that must mean that the number of moles of gas m is increasing. And, the number of moles of liquid must be decreasing by the same amount (since mass is conserved). But the molar volume of the liquid is much less than the molar volume of the vapor that is produced. So the total volume of liquid and gas increases.
     
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