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Confusion over intensive properties (Thermo)

  1. Mar 10, 2012 #1
    Disclosure: I am studying biochemistry so my thermodynamics education is lacking and I'm likely misinterpreting some of these concepts (that's why I'm here for help).

    I have always had trouble defining some properties as either intensive or extensive. I understand that extensive are supposed to depend on the amount of the substance present, but intensive properties do not. For example I can easily understand why boiling point is intensive and mass is extensive.

    However I get confused when I try to place some properties into either category. One that always gets me is pressure. I have heard the argument that if I have sealed a box with a pressure of 1atm inside and I split the box in two I have reduced the number gas molecules as well as the volume by half and the pressure doesn't change. So pressure is not dependent on amount.... Ok, that makes sense to me, but what if I doubled the amount of gas in the box (increases the amount of substance) Hasn't the pressure gone up?

    Im sure there is some fundamental flaw in my reasoning and I'm hoping that when this is cleared up I will have a much more sound understanding of intensive and extensive properties (or pressure for that matter).

    Thanks in advanced!
     
  2. jcsd
  3. Mar 10, 2012 #2

    Andrew Mason

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    Why does a property have to be either intensive or extensive?

    AM
     
  4. Mar 10, 2012 #3
    -AM

    I'm not sure what you are getting at. Are you saying that some properties can be extensive or intensive under different contexts? That doesn't seem to fit with my understanding, or are you saying that pressure specifically is neither intensive or extensive?

    I have read and heard in a recent lecture that pressure is intensive. I just don't see this as being the case specifically when looking at a gas.
     
  5. Mar 11, 2012 #4

    Rap

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    If you take two systems that are duplicates of each other, same thermodynamic parameters, and connect them, intensive properties will not change, extensive properties will double.

    Doubling the amount of gas in a box is not doing this. When you double the amount of gas in a box, you are basically connecting two like systems (pressure remains the same) then compressing to half the combined volume.
     
  6. Mar 11, 2012 #5
    Rap-

    Many thanks, your explanation of combining two like systems definitely clarifies what is meant by intensive and extensive properties. I think I can work through the other classifications that had me confused now!
     
  7. Mar 12, 2012 #6

    Andrew Mason

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    My point is that there is no particular reason why a property has to fit perfectly into one of these categories. I would say that pressure does not fit either category.

    AM
     
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