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Understanding gas pressure/temperature relation

  1. Jun 26, 2014 #1
    Let's say we have a sealed container that has an adjustable volume, so the volume of the container can change from big to very small. When the volume is big, the container is filled with a certain amount of an ideal gas. After this the amount of gas is not changed but remains constant. Also we don't do anything to change the temperature of the system. All we do is now to decrease the volume of the container. What will happen? I understand the pressure will increase, since less volume per certain amount of gas implies higher pressure.

    My question is, what (if anything) will happen to the temperature? Will it change or remain constant? I'm wondering because for an ideal gas there is the relation P*V=n*R*T. We already said we keep n constant and R is of course constant. If T is also constant, then follows another question:

    If the pressure is increased enough, the gas will eventually change to liquid form. But is the temperature still the same? So if we start at for example room temperature (20 degrees C) and decrease the volume until the gas liquifies, does that mean the boiling temperature of the gas will increase significantly (actually becoming equal to the starting temperature), since it liquifies without changing temperature?
     
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  3. Jun 26, 2014 #2

    UltrafastPED

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    The ideal gas law doesn't apply to liquids, and an ideal gas doesn't have any phase changes.

    For a real gas things are more complicated.
     
  4. Jun 26, 2014 #3

    CWatters

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    Your example could be better specified. What happens depends on the set up. Decide if the walls are ideal thermal insulators or ideal thermal conductors.

    Pehaps google "isothermal vs adiabatic"

    Experiment with a bicycle tyre pump.
     
  5. Jun 27, 2014 #4
    Ok, so an ideal gas doesn't liquify. What would happen if it's for example a gas like nitrogen? Would the temperature change then? Or would the gas liquify at the same temperature as the starting temperature?

    I was thinking thermal insulators.
     
    Last edited: Jun 27, 2014
  6. Jun 27, 2014 #5

    sophiecentaur

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    @Maxo
    Have you read the basic stuff about the derivation of the laws for an ideal gas? All the text books include this and Wiki is pretty good at explaining what happens in terms of Kinetic Theory. Of course, the Hyperphysics site has all the information - albeit a bit condensed. The way that the pressure is caused by collisions with the walls of a container and the effect of temperature and volume can be very neatly explained with some fairly simple maths. It would be worth while visiting those links.
     
  7. Jun 27, 2014 #6
    sophiecentaur: I guess you didn't read UltrafastPEDs post (although it's quite obvious, maybe you don't know that you are supposed to read previous posts before writing), where he said that the ideal gas law doesn't apply here, to which I already answered. So these questions still remain:

    Ok, so an ideal gas doesn't liquify. What would happen if it's for example a gas like nitrogen? Would the temperature change then? Or would the gas liquify at the same temperature as the starting temperature?
     
    Last edited: Jun 27, 2014
  8. Jun 27, 2014 #7

    sophiecentaur

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    Before moving on from the ideal gas laws, it's as well to establish that people actually know them first. Had that actually been established (in the light of some of the statements made)? Many of these threads leap in at the deep end without establishing the basics. Had you not noticed this? The departure from the gas laws, as Potential Energy starts to be relevant, is fairly straightforward to discuss, as long as you start from the right place.
     
  9. Jun 27, 2014 #8
    There are very many helpful people on these forums but you are definitely not one of them. Please do as all a favor and refrain from posting, at least in my threads, and let the better people post instead.

    So the questions (only directed towards good people) are still the following: What would happen if it's for example a gas like nitrogen? Would the temperature change then? Or would the gas liquify at the same temperature as the starting temperature?
     
  10. Jun 27, 2014 #9

    sophiecentaur

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    So it's not helpful to suggest going back to basics? If you understand how the ideal gas laws involve just KE and then introduce the consequences of PE then I don't think you can go far wrong. Are you saying that all of this is trivial and that you understand it fully. If so, why the original question?

    There really is no reason to get so rude so quickly. It is usually a sign of other problems which you should take elsewhere, rather than PF.
     
  11. Jun 27, 2014 #10
    Since you were rude already from the beginning, the quickest way possible, according to your own theory you are the one who must have very serious problems, so why don't you follow your own advice and take those elsewhere? I already told you you are not helpful at all, unlike very many other people on this forum, so there is really no reason for you to keep posting here. Stop interfering now and let other people on the forum - excluding yourself - get back on topic.
     
  12. Jun 27, 2014 #11

    sophiecentaur

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    I had no intention of being rude. What did I write that you interpreted as rudeness? If you interpreted my suggestion about going back to basics as being rude then I am sorry; the message was well intentioned and my suggestion still applies.
    I am still not sure of your level of understanding because you didn't make it clear.
     
  13. Jun 27, 2014 #12

    berkeman

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    Thread closed temporarily for Moderation...

    EDIT -- Thread will remain closed.
     
    Last edited: Jun 27, 2014
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