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Increase pressure in closed container

  1. Feb 12, 2008 #1
    an experiment^^ in a closed container, means tat it has a constant volume. when i increase the air pressure inside(without heating), the result is the temperature will increase. why? can i hav a theoretical explanation?
    thx for reading
     
  2. jcsd
  3. Feb 12, 2008 #2

    Hootenanny

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    Welcome to PF,

    Consider the first law of thermodynamics together with the definition of kinetic temperature.
     
  4. Feb 12, 2008 #3
    wow, answering so fast....thank you guys
    let me try to think about it 1st
     
  5. Feb 12, 2008 #4

    Hootenanny

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    No problem, as an additional hint, think about how you would increase the pressure.
     
  6. Feb 12, 2008 #5
    It is because of the equation (p)(v)/(t) which is constant and if you increase the pressure the temperature will rise. According to the equation (law of thermodynamics)
     
  7. Feb 12, 2008 #6

    DaveC426913

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    Boyle's Law.
     
  8. Feb 12, 2008 #7
    thanks for the correction
     
  9. Feb 12, 2008 #8
    I think a good physical explanation (no math or equations) is that this "air" is always moving hitting each other and the inside of the container. If you deacrease the volume or add more air molecules, the container would be more crowded and this molecules would start hitting each other more constantly than when it was not. This "friction" and constant hitting between the molecules would heat them increasing temperature.
    kynetic energy into heat

    -link
     
  10. Feb 12, 2008 #9

    Hootenanny

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    Bad physics. Kinetic energy is not heat nor can a molecule or anything else for that matter "have heat". Furthermore, one cannot talk about temperature at the molecular level since temperature is only defined at the macroscopic level, i.e. for a large collection of molecules.
     
    Last edited: Feb 12, 2008
  11. Feb 12, 2008 #10
    Come on, Is just a really simplify explanation and I didn't wrote that kinetic energy is heat, kinetic energy into heat. First law of thermodynamics, as you said, would explain how this kinetic energy can change into heat (conservation of energy).
    I tried to make a simple explanation because not everyone has a degree on physics or engineering and some mathematical explanations doesn't help to visualize how things happens such as Ideal gas equation.

    Sorry is true that molecules or any matter can have heat, is internal energy that would increase.
    The real explanation is that W of compresion is equal to the change on internal energy
    W=U2-U1 and U2-U1=Cv(T2-T1)*m.
     
  12. Feb 12, 2008 #11

    Hootenanny

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    No, no, no! You've missed the point, a molecule, gas, liquid, solid, anything cannot have heat. See my post here.
    There's a difference between simplification and just plain wrong and misleading. I don't intend to cause offense, I just want to make sure that both you and the OP understand that your previous post was both incorrect and misleading.
    With respect to molecules heaving heat, see me previous comment. With respect to the internal energy of a molecule, could you please define internal energy for me and then explain to me how a molecule has internal energy?
     
    Last edited: Feb 12, 2008
  13. Feb 12, 2008 #12

    Hootenanny

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    With respect to the OP's question, from which we have deviated a little*, please accept my apologies glenn, we cannot apply Boyle's law here since Boyle's law assumes constant temperature. I restate my original question, how would you increase the pressure of the gas, given that it's volume must remain constant?

    As an aside Link's alternative argument, although incorrectly put, in essence is still correct. Consider how the kinetic energy (and hence temperature) of the gas molecules would change if you increased the number of molecules in the same volume.

    * Doc, Zz et al., feel free to split of this discussion into a different thread.
     
    Last edited: Feb 12, 2008
  14. Feb 12, 2008 #13
    Internal Energy would be the one that can't be classified as kinetic energy and potential energy such as sensible energy, latent energy, chemical energy and nuclear energy. Latent would be vibration, rotation and translation, that in a macroscopic point of view would be kinetic energy.

    And as I said before,
    Sorry if my explanation of how the increase in temperature is proportional to increase in pressure is not the best. Since I don't know the guy or girl who post the interrogant or what knowledge in thermodynamics has I tried to explain it as simple as possible. Is true that is incorrect and misleading.

    How would you explain this phenomena to a kid with a middle school science knowledge? No thermodynamics, no heat transfer and no equations.

    -link

    PD. Nice!!
    But Q-W=[tex]\Delta[/tex]U
     
  15. Feb 12, 2008 #14

    Doc Al

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    Whether the work gets a negative sign or not depends on your sign convention. More modern treatments use W to mean the work done on the system, thus giving Q+W=[tex]\Delta[/tex]U. (I prefer this form as it makes the connection with energy conservation even more apparent.)
     
  16. Feb 12, 2008 #15

    DaveC426913

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    :confused:

    The OP wants to know what will happen if he increases the pressure of the gas and does not heat the gas. What will happen is the the gas's temperature will increase - as per Boyle's Law.
     
  17. Feb 12, 2008 #16

    Doc Al

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    :confused: You posted a link to Boyle's law. Did you read it? :wink:

    (Boyle's law applies to an ideal gas kept at constant temperature.)
     
  18. Feb 13, 2008 #17

    Hootenanny

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    Well read from Wikipedia, but I think your a little confused here. For a general fluid, the internal energy can be described as the sum of the translation, rotational, vibration kinetic energies and the potential energies of the molecules within a system. This can be 'broken down' into the energy 'types' you mentioned, but it isn't necessary. So in fact, in a monatomic gas, the total translational kinetic energy of the molecules within a system is the internal energy. I also repeat my early question "Could you please now explain to me how a single molecule can have internal energy"?
    And as I said before, there is a difference between simplification and simply incorrect. As I said in my previous post, there is nothing wrong with the concept of your argument, just in how you put it. The idea behind your argument is the correct one, but the way your argued your point is incorrect and invalidates your argument.

    It is imperative, particularly in Thermodynamics and physics in general, the one uses correct terms. Physical terms have very specific meanings and one must chose which terms to use carefully.
    I was use the concept of kinetic theory, but correctly put.
    Thank you for the compliment. It looks like Doc's got your comment covered, however, you should notice that in that post I defined W as the work on on the system, it is simply a matter of sign convention. After reading my post, I hope you now understand the difference between heat, temperature and internal energy.

    To cronusmin
    Just to summarise so that you need to read anymore than necessary. Up to now, we have mentioned two ways in which you can explain the increase in temperature,

    (1) Consider the first law, how would you increase the pressure given that V is constant? What kind of device would you use? What does that device need to operate?

    (2) Kinetic Theory, use the concept that Link introduced, but use the proper terms correctly. Consider how the kinetic energy (and hence temperature) of the gas molecules would change if you increased the number of molecules in the same volume.
     
    Last edited: Feb 13, 2008
  19. Feb 13, 2008 #18
    seem like its a big argue here....
    1st at all, i do this experiment by using a closed container with air inside at the begining.then i use a machine to pump in the air. for sure tat the pressure inside will increase, i found tat the temperature also increase too. this is wat i've done b4.

    i have 1 question from Hootenanny, from the msg u said,'an additional hint, think about how you would increase the pressure.'can i know wat is this mean?
     
  20. Feb 13, 2008 #19

    Hootenanny

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    You have answered your own question,
     
  21. Feb 13, 2008 #20
    haha....i thought that your hints maybe something like slowly pump in or faster pump in. thx
     
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