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A Intensive Properties

  1. Mar 14, 2016 #1
    Can anyone tell me about how temperature is an intensive property.Intensive properties are roughly those which do not depend on the size of the system or amount of matter in the system,but temperature does depend on amount of matter because it has a relation with pressure also,and pressure depends on the amount.I'm talking about the internal pressure here
     
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  3. Mar 14, 2016 #2

    phinds

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    If you have a pan of water at, say 200 degrees F, and your remove a ladle-full, does the rest of the water change temperature?
     
  4. Mar 14, 2016 #3
    The way I think of it is to ask whether the quantity scales with particle number, holding all else constant. So, as per phinds, take a system at some temperature, hold pressure and volume constant, and change the particle number. Does the temperature change? No. Compare this to, for instance volume, which is an extensive property. Take a system which occupies some volume, hold pressure and temperature constant, and add or remove particles. Does the volume change? Necessarily yes.
     
  5. Mar 14, 2016 #4
    Sturk, you made an error there which I hope was just a typo. The first example you have is wrong; you can't change particle number while holding all those things constant.

    Esfand, you have to be careful about what you mean by "depends on" in thermodynamics. The equation of state relates several quantities together, and so they all depend on each other. You can talk about the dependence of any two quantities while holding the other quantities fixed. For example, for an ideal gas, temperature depends on number of moles if I hold volume and pressure fixed. But it is ambiguous to say temperature depends on number of moles without saying what I do with volume and pressure. If I let volume increase proportionally as I increase the temperature, then temperature doesn't depend on number of moles. It's all about the context.

    An intrinsic property can be thought of as an average value at any point in the system, whereas an extrinsic property isn't and is usually some kind of total of the entire system.
     
  6. Mar 14, 2016 #5
    The pressure is an intensive property too. It does not depend on the size of the sample.
    If you divide a chamber with uniform pressure into two (with a divider), the pressure in each half will be unchanged. So your reasoning relies on a wrong assumption.
     
  7. Mar 14, 2016 #6
    As a matter of fact, that was not a typo, but a brain-o. I resign.
     
  8. Mar 27, 2016 #7
    okay,now consider this. I've say 50 molecules of any gas in a chamber ,and because of their collisions ,they'll have a particular pressure and thus temprature in the system.Now , if remove 30 molecules from it ,the pressure will decrease ,but what will happen to the temprature?
     
  9. Mar 27, 2016 #8

    jtbell

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    What do you think, and why? If you're assuming an ideal gas, consider PV = NkT. Is P the only quantity that decreases?
     
  10. Mar 28, 2016 #9
    You have to specify how you remove the 50 molecules
     
  11. Mar 29, 2016 #10
    Can I say that because of the removal of 30 molecules,the pressure will decrease but so as the volume,thus temperature will remain constant?
     
  12. Mar 29, 2016 #11

    phinds

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    Why would the volume decrease?
     
  13. Apr 3, 2016 #12
    Yes,the volume will ot decrease in the first place.Then how is it explained
     
  14. Apr 3, 2016 #13

    ehild

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    Intensive and extensive properties are related when you make a system of more identical ones.
    Assume you have two identical vessels of water, with volume of 1 L and the same temperature, 20 °C. You put the vessels together and remove the wall between them. The new system has twice the volume of the original ones and the same temperature as the original ones. Volume is extensive property as it is multiplied when making a new system from more identical ones. The temperature does not change: it is an intensive property.

    upload_2016-4-3_9-14-44.png
     
  15. Apr 3, 2016 #14
    Beautiful answer. Thanks @ehild :smile:
     
  16. Apr 3, 2016 #15

    ehild

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    The same works for gases.
    You have a cylinder divided by a thin wall at the middle which separates it into two identical chambers of 1 m3 volumes. Both chambers are filled with 1 mol O2 gas at identical temperature of 300K. You remove the wall, so you have 2m3 volume of 2 mol of O2 gas, at the same temperature. Temperature is intensive property, volume and mol number are extensive properties.
    upload_2016-4-3_9-51-39.png
    What about the pressure? It depends on the volume and number of moles, according to PV=nRT.
    The pressure in both the 1 m3 chambers is P=nRT/V=(1mol)*R*(300K)/(1m3)=300R.
    The pressure in the united system is P=(2mol)*R*(300K)/(2m3 )= 300R.
    Pressure is also an extensive intensive property, it does not change if you unite identical systems.
     
    Last edited: Apr 4, 2016
  17. Apr 4, 2016 #16
    You mean pressure is an intensive property
     
  18. Apr 4, 2016 #17
    Yes. Pressure and temperature are intensive property.
     
  19. Apr 4, 2016 #18

    ehild

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    Yes, I corrected it now.
     
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