How well do I understand pressure?

1. Mar 29, 2006

kmarinas86

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. Mar 30, 2006

vanesch

Staff Emeritus
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)...

3. Mar 30, 2006

FredGarvin

I really hate seeing the term "negative pressure." Negative stress, ok, but not with pressure.

4. Mar 30, 2006

pervect

Staff Emeritus
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.

5. Mar 30, 2006

vanesch

Staff Emeritus
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 ?

6. Mar 31, 2006

FredGarvin

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.