How well do I understand pressure?

[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:

Negative pressure, an excitation force, is a normal force in a given volume or area.
Expansion/Inflation:

• An aspect of an acceleration* and expansion towards a volume of lower density.

Wherein:

• A medium of dark energy, the repulsive electromagnetic force, or the weak nuclear force establishes precedent over...
• ...a medium of gravity, the attractive electromagnetic force, or the strong nuclear force.
• It includes accelerating galaxies, accelerating chemicals, and decaying fundamental particles

Positive pressure, an inhibition force, is a force countering the normal force in a given volume or area.
Collapse/Deflation:

• An aspect of an acceleration* and compression towards a volume of higher density.

Wherein:

• A medium of gravity, the attractive electromagnetic force, or the strong nuclear force, establishes precedent over...
• ...a medium of dark energy, a medium of the repulsive electromagnetic force, or the weak nuclear force.
• It includes collapsing nebulas, constraining of electrons in an atom, constraining of quarks in a nucleus.

* note: it's not a deceleration, it's an acceleration. Thus the air does not supply negative pressure whatsoever.

 

[vanesch]

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)...

 

[FredGarvin]

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

 

[pervect]

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.

The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate (see equation of state (cosmology)). The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics. The work done by a change in volume dV is equal to −p dV, where p is the pressure. But the amount of energy in a box of vacuum energy actually increases when the volume increases (dV is positive), because the energy is equal to ρV, where ρ is the energy density of the cosmological constant. Therefore, p is negative and, in fact, p = −ρ.

 

[vanesch]

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

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 ?

 

[FredGarvin]

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

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.