# Negative Pressure

1. Feb 26, 2014

### qitara

Hi guys

Is there any thing such as Negative Pressure, or is there only total vacuum 0 and nothing below that ?

2. Feb 26, 2014

### adjacent

3. Feb 26, 2014

### qitara

isn't a vacuum and negative pressure the same thing ?.

4. Feb 26, 2014

### Staff: Mentor

No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum, there are no negative pressures.

What's confusing you is that people often say "pressure" when they mean "difference from atmospheric pressure" - by that definition any "pressure" that is less than atmospheric is negative.

5. Feb 26, 2014

### qitara

So it's just a way to say that the pressure is below the reference pressure ?

6. Feb 26, 2014

### abitslow

In almost all areas of Engineering and Chemistry, yes its relative pressure and it can be negative.
In some areas of Physics, it has an entirely different meaning. This is the second post I've answered in the last 10 minutes where the poster didn't explain in what context the term was being used in. *sigh* Unless you are dealing with quantum field theory, you can NOT have negative absolute pressure and you CAN have negative relative pressure (also known as "gauge pressure").

7. Feb 26, 2014

### qitara

Term used in Engineering, but the picture is clear now thanks to you guys

8. Feb 26, 2014

### Varun Bhardwaj

There is no any perfect vaccum because if there is perfect vaccum than it will exert infinite force to fill it.

9. Feb 26, 2014

### Moidaki

Consider the atmospheric at sea level (1atm) as the reference pressure.

10. Feb 26, 2014

### Staff: Mentor

Why do you think that?

11. Feb 26, 2014

### A.T.

Sure there are:
http://en.wikipedia.org/wiki/Pressure#Negative_pressures

12. Feb 26, 2014

### Staff: Mentor

13. Feb 26, 2014

### A.T.

This here is absolute negative pressure:

14. Feb 26, 2014

### Staff: Mentor

If you also want to count solids, and, if you define the pressure in a solid under tension as minus the isotropic part of the stress tensor, then, yes, you can have a "negative pressure" in a solid. Tree sap, which is a polymer solution, exhibits viscoelastic rheological behavior, which is a combination of viscous-fluid and solid behavior. So tree sap can also exhibit, to some degree, what would be defined as negative pressure. But ordinary Newtonian viscous gases and liquids are not viscoelastic, and do not exhibit negative pressure.

15. Feb 26, 2014

### Staff: Mentor

That wikipedia article is stretching things a bit by characterizing capillary action (which is what the tree is using to lift its sap above 10 meters) as negative absolute pressure. It's more akin to a mechanical system with a pump at the bottom... except that the pump is distributed all along the surface of the xylem cells. It's would be more accurate to say that at all points in the column the pressure is positive and greater than the weight of the fluid above that point.

However, I think that is all a second-order sort of subtlety when OP is asking "isn't a vacuum and negative pressure the same thing?".

16. Feb 26, 2014

### A.T.

Pure water can have negative absolute pressure too:
http://www.nature.com/nature/journal/v455/n7210/full/nature07226.html

Last edited by a moderator: May 6, 2017
17. Feb 26, 2014

### A.T.

No, it's not (just) capillary action (attraction by the walls), but actually negative absolute pressure (attractive forces between the water molecules).

18. Feb 26, 2014

### Staff: Mentor

That article is amazing, if correct. I've never heard of this type of thing before. Thanks for calling it to my attention. Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?

Chet

Last edited by a moderator: May 6, 2017
19. Feb 26, 2014

### dauto

The article is correct. There is an excellent Veritasium video on youtube called "The Most Amazing Thing About Trees" about that topic. It's worth watching.

20. Feb 26, 2014

### A.T.

Here it is:

https://www.youtube.com/watch?v=BickMFHAZR0

According to the video the key is the lack of air bubbles see (3:33min). The article also discuses probability of cavitation.

21. Feb 26, 2014

### sophiecentaur

That is not correct. the maximum negative pressure (what you are calling force) that can be exerted by a 'vacuum' is Atmospheric Pressure (1Bar). The force on a piston to produce a pressure of 0.01Bar is very little different than the force on a piston producing a pressure of 0.001Bar. The forces needed will be (the Piston area times 0.99 Bar) compared with (Piston area times 0.999 Bar) -i.e. virtually no difference at all.
It's not the need for a massive force that limits how deep a vacuum you can produce; it's the practical matter of how you can extract the molecules of a gas (air) out of a space without more molecules getting back in via the pump. Very low vacuums are limited by contamination by odd molecules coming of the insides of the chamber and pipes.

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