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Determine how pressure,temperature and volume change

  1. Sep 12, 2008 #1
    well,in closed system ,for example piston-cylinder system i cant determine how pressure,temperature and volume change to relating each other(in liquids and gases)?

    iam confused abt that , i have some question abt that

    when heat is transferred to the (liquid for example) so there is higher velocity of molecules(higher momentum) so it is suppoed that volume increases while pressure is constant my question here
    (1)why volume increases? is that because when molecules have higher momentum so they can overcome intermolecular forces so distances between molecules increase so it starts to change its phase from liquid to vapor

    if that correct so doesnt that affect preesure ?is there arelation between intermolecular forces and pressure??
     
  2. jcsd
  3. Sep 12, 2008 #2

    stewartcs

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    Re: thermodynamics

    I believe the assumption you've overlooked is that in this type of piston cylinder system the piston is free to move. Thus as heat is added, the pressure increases just long enough to exert a force on the piston causing it to move. Since the piston has moved, the volume is increase proportionally and the pressure essentially remains constant.

    Does that help?

    CS
     
  4. Sep 12, 2008 #3
    Re: thermodynamics

    ok thx for ur reply but i already know that can happen in gases but iam talking particularly abt liquid in piston-cylinder system ,ok let me ask u question if there is amount of compressed or subcooled water in the cylinder and we fix the volume if there is heat transfer to the water what will happen?
    does its pressure will rise?
    and if we still add heat is there a possibility that this water starts to evaporize?
    or not beacuse vapour means larger volume and volume here is fixed?
     
  5. Sep 12, 2008 #4

    stewartcs

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    Re: thermodynamics

    Yes the pressure will rise as heat is added, as well as the temperature of the compressed liquid.

    The compressed liquid will transition to a saturated mixture as heat is added and then to a superheated vapor as more heat is added. The transition points would depend on the mass of the compressed liquid and the volume of the rigid container.

    For example, if you have 1 ft^3\lbm of water initially at 40 deg F and 0.12173 psia in a rigid container (i.e. constant volume) and add heat to it, the temperature and pressure will rise. Once the temperature of the water hits 459.50 deg F the pressure will be 464.46 psia and the water will be in the superheated vapor region.

    CS
     
  6. Sep 12, 2008 #5
    Re: thermodynamics

    well,iam afraid u need to review t-v diagram ,u say compressed water will transit to saturated mixture???how that?if it transits so sure its volume would increase but it's supposed constant volume??
     
  7. Sep 13, 2008 #6

    stewartcs

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    Re: thermodynamics

    Actually I didn’t check the quality at state 1 in the example I gave (it was really low 0.0004). I assumed it was 0 instead, so it was incorrect for me to say it would transition from a compressed liquid to a saturated mixture since it was already a saturated mixture.

    If you have a compressed liquid (0 quality) and hold the specific volume constant and heat it, you’ll just end up with pressurized hot water. If it is a saturated mixture (quality is greater than 0 and less than 1), depending on the initial specific volume, it may transition to a compress liquid or a superheated vapor when heat is added (see attachment).

    I’ve attached a file output from REFPROP (a program by NIST) that shows the two different phase changes depending on initial specific volume. It shows essentially the same thing that a T-v diagram shows but instead gives the actual values.

    Hope this helps.

    CS
     

    Attached Files:

  8. Sep 13, 2008 #7
    Re: thermodynamics

    ok iam really confused abt the relations between pressure,temperature and volume esprcially in liquid?for example for compressed water in piston cylinder system when pressure is decreased temperature is also decreased ??i cant get why temperature decreased?
     
  9. Sep 15, 2008 #8

    stewartcs

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    Re: thermodynamics

    A compressed fluid in a piston-cylinder won’t necessarily lose pressure. If the piston is allowed to move as a result of the pressure in the cylinder acting on it, then the temperature will drop due to boundary work being done by the system. This is due to the First Law of Thermodynamics:

    [tex]Q_{net} - W_{net} = \Delta{U}[/tex]

    (for a simple compressible system)

    It is best to describe the system in as much detail as possible so you can know what assumptions to make. For example, whether or not it is adiabatic, deciding to ignore potential and kinetic energy changes, etc.

    CS
     
  10. Mar 12, 2009 #9
    Re: thermodynamics

    Hi, sorry to bring this question up again, i know it has been a long time, but i kind of have the same problem with this, could you please help me again?

    my question is, when i have a rigid closed system of water lets say in amount of 500mL and then i heat it up until 600C which is way pass the saturation vapor stage, and i want to find the pressure, so how should i calculate it, since when i look at the phase diagram, the phase that exist in that temperature is superheated region, and when i check the superheated table, the value is vary with the pressure and temperature relationship, so there different value of temperature with 0.1MPa, 0.2MPa, 1MPa pressure. so how should i find the right pressure inside the vessel after i heat it up until 600C?

    Thank you
     
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