How well do I understand pressure?

  • Thread starter kmarinas86
  • Start date
  • Tags
    Pressure
In summary, the conversation discusses the concepts of negative and positive pressure, as well as their relationship to forces such as gravity and dark energy. Negative pressure is often seen in cosmology and is defined as the work done by a change in volume, while positive pressure is a force countering the normal force. The conversation also touches on the use of the term "negative pressure" and its relation to stresses in solid materials.
  • #1
kmarinas86
979
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:

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.
 
Last edited:
Physics news on Phys.org
  • #2
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
I really hate seeing the term "negative pressure." Negative stress, ok, but not with pressure.
 
  • #4
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.
 
  • #5
FredGarvin said:
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 ?
 
  • #6
vanesch said:
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.
 

1. What is pressure?

Pressure is defined as the force exerted by a substance per unit area. It is a measure of how much force is applied over a given area and is typically measured in units of Pascals (Pa) or Newtons per square meter (N/m2).

2. How does pressure affect different substances?

The effect of pressure on a substance depends on its physical properties. For example, an increase in pressure may cause a solid to compress, a liquid to change state, or a gas to become more dense. In some cases, pressure can also cause chemical reactions to occur.

3. How do I measure pressure?

Pressure can be measured using various devices such as barometers, manometers, and pressure gauges. These instruments work by converting the force exerted by a substance into a readable value. The choice of instrument depends on the specific application and the range of pressures being measured.

4. What factors affect pressure?

The factors that affect pressure include the amount of force applied, the area over which the force is applied, and the physical properties of the substance. Additionally, pressure can also be influenced by external factors such as temperature, altitude, and the presence of other substances.

5. How is pressure used in scientific research?

Pressure plays a crucial role in various fields of scientific research, including chemistry, biology, and physics. It is used to study the behavior of substances under different conditions, to measure the strength and stability of materials, and to create new materials with unique properties. Pressure is also used in many industrial processes, such as in the production of electronics and pharmaceuticals.

Similar threads

Replies
22
Views
2K
Replies
5
Views
1K
Replies
9
Views
2K
Replies
4
Views
2K
  • Mechanics
2
Replies
49
Views
6K
Replies
10
Views
1K
  • Advanced Physics Homework Help
Replies
1
Views
751
  • High Energy, Nuclear, Particle Physics
Replies
14
Views
2K
Back
Top