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How well do I understand pressure?

  1. Mar 29, 2006 #1
    I have thought about negative and positive pressure for a while, and I decided that someone should test me whether I have some understanding right.

    This is what I typed:

     
    Last edited: Mar 29, 2006
  2. jcsd
  3. Mar 30, 2006 #2

    vanesch

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    I don't really understand what you are trying to say. Negative as well as positive pressure is possible in solid materials under stress, when you consider the classical stress tensor:
    http://en.wikipedia.org/wiki/Stress_(physics)

    It is sufficient to pull on a solid to instore a positive normal stress (= negative pressure)...
     
  4. Mar 30, 2006 #3

    FredGarvin

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    I really hate seeing the term "negative pressure." Negative stress, ok, but not with pressure.
     
  5. Mar 30, 2006 #4

    pervect

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    Negative pressure comes up mostly in cosmology. As the wikipedia says, dark energy has a negative pressure.

    http://en.wikipedia.org/wiki/Dark_energy

    Probably the most useful comment from the above link is the following. Note that the cosmological constant is one specific form dark energy could take.

     
  6. Mar 30, 2006 #5

    vanesch

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    Isn't pressure defined as -1/3 of the trace of the stress tensor ?
    (in other words, minus the average of the principal stresses)

    Of course, you cannot have that number to be negative in liquids or gas. But in a solid, there's no problem, no ?
     
  7. Mar 31, 2006 #6

    FredGarvin

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    I am assuming you are referring to stresses. In that case it's analagous to acceleration in that the minus sign simply indicates a direction or trend. And in that you are correct. I guess it's in how I was taught. I didn't realize that cosmology (as Pervect pointed out) utilized the negative pressure idea.
     
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