• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Can pressure be negative?

Reality Check said:
It is still going on because Michael Mozina cannot understand the concept that forces other than those caused by particles bumping against surfaces exert pressure.
We have given him the example of the Casimir effect which in theory and experiment gives negative pressure.
We have given him the example of the cosmological constant in general relativity causing a negative pressure.

He is now trying to ignore the next example (the force between 2 oppositely charged plates causes a negative pressure).

All defintions of pressure allow negative pressure to exist.
Click to expand...
Now you're back into semantics again.
2 opposite poled magnets would then exert a "negative pressure" if held apart, if your only definition of pressure is "surface area dependent force"
Hell, even frictional forces could be considered negative pressures under that definition...
 
rwguinn said:
Now you're back into semantics again.
2 opposite poled magnets would then exert a "negative pressure" if held apart, if your only definition of pressure is "surface area dependent force"
Click to expand...

Only if the plates were magnetized perpendicular to the plate. Otherwise the force won't scale with area. But I don't see what's wrong with this as an example of a pressure, and you haven't said why it's a problem.

Hell, even frictional forces could be considered negative pressures under that definition...
Click to expand...

Not if you only look at the force normal to the surface. Which is pretty normal to do (pun intended).
 
Ziggurat said:
You say that like pressure and force are unrelated. But they aren't. Pressure = Force/area, or force = pressure x area. The force between the plates scales as the area of the plates. That's a pressure, Michael.
Click to expand...

Sure, and if Guth was trying to get "negative pressure" from another charged plate, or an external gravity field, you and I wouldn't be having this conversation. He claimed the 'vacuum' produced "negative pressure", not a *SECOND OBJECT*.

That's where this whole analogy goes to hell in a handbasket. Guth never had an *EXTERNAL* thing to work with, just a VACUUM. You keep trying to use a "two physical object" analogy when Guth only had *ONE* physical thing and a vacuum.
 
Last edited:
Michael Mozina said:
As long as you treat it as a "positive" kinetic energy/pressure state, or even a ZERO kinetic energy/pressure state, you're welcome to do so.
Click to expand...

And if the universal conditions had sufficient force to remover the ideal piston from the ideal sealed chamber?;)
 
rwguinn said:
Now you're back into semantics again.
2 opposite poled magnets would then exert a "negative pressure" if held apart, if your only definition of pressure is "surface area dependent force"
Hell, even frictional forces could be considered negative pressures under that definition...
Click to expand...
My defintion is not "surface area dependent force" whatever you think that is.

The definition I have used is the standard definition of pressure.
The pressure is the scalar proportionality constant that relates the two normal vectors:
d2a035095f1ff78a27e468cb4abd225e.png

The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outwards
Click to expand...
That definition means that a force toward a surface has positive pressure and a force away from a surface has a negative pressure.

Thus the standard definition of pressure means that the force on the surface of an magnetic exerted by an opposite pole magnetic is attractive and so the pressure will be negative. Ditto for friction.

There is a more general definition that is also used: P = -dE/dV
For a negative pressure all you need is an energy that remains constant as volume increases.
 
Last edited:
Assume your sealed chamber in your lab to be 100m in length.

What would the pressure in the ideal sealed chamber be as the piston progresses from the sealed end?

Would it always remain positive?

Substitute "universal condition" for piston pulling force of your choice.:D
 
Michael Mozina said:
Sure, and if Guth
Click to expand...

Once again, I'm not talking about Guth. Forget Guth. Let's talk about the scenario YOU proposed. This isn't an analogy, it's a physical scenario in its own right. Does it or does it not have a negative pressure? Answer the question. It's quite simple, it's just a yes/no question, and the answer is staring you in the face.
 
Ziggurat said:
Only if the plates were magnetized perpendicular to the plate. Otherwise the force won't scale with area. But I don't see what's wrong with this as an example of a pressure, and you haven't said why it's a problem.
Click to expand...
Not a problem, but the actual force is due to something else. Call it a pressure if you want. It's a matter of convention--semantics
Not if you only look at the force normal to the surface. Which is pretty normal to do (pun intended).
Click to expand...
Ho-ho--I was thinking mode along the lines of go stick your foot in a swamp. Wait a few minutes, then try to pull it out. The friction generated by the fluid, trying to hang on to your foot (oops-lost a shoe!) would be negative pressure.
of course, we knuckle-dragging engineers would not describe it that way, but the result is tensile stress (AKA Pressure) in the leg...
 
Ziggurat said:
"by definition". What's the definition? Because according to standard definitions of pressure, it can indeed be negative. What definition of pressure precludes negative values? I've never seen such a definition.



There is a very standard definition which works for both classical mechanics and quantum mechanics. So I'm not sure why you think the answer "depends on your definition", when a very standard definition is available for both classical and quantum mechanics.



I have yet to see a case where the definition P = -dE/dV does not work.
Click to expand...
To (likely) 99.9% of folk it's still a theoretical masturbation exercise. Most of us don't work to those lveles of precison. Top the rest of us you measure absolute gas pressure which CANNOT be less than zero at these levels of measurement. Hence my comments about definitions. Guage pressure is a different kettle of fish, obviously you can have a negative pressure differential (yet another definition...)

IOf you want to bring in Casimir effects, then you have to define your levels of measurement and tghe environment, yet another definition. I wasn't attacking the idea, simply that for the majority of us it's a pointless annd tedious exercise. If you are defining and carrying out experiments to measure the Casimir effect then such arguements have merit, as they will try to minize experimental error.

Meanwhile the rest of us will not give a damn, as it doesn't effect us.

Given that, I will retire from this conversation (for the reasons given above). If you want to talk about Casimair effects, raise a thread about that, rather than raising a thread about negative pressure, when 'you' are actually trying to talk about something completely different!
 
rwguinn said:
Not a problem, but the actual force is due to something else. Call it a pressure if you want. It's a matter of convention--semantics
Click to expand...

Sure, it's partly just semantics, but why NOT call it a pressure? It fits the standard definition P = -dE/dV perfectly.

Ho-ho--I was thinking mode along the lines of go stick your foot in a swamp. Wait a few minutes, then try to pull it out. The friction generated by the fluid, trying to hang on to your foot (oops-lost a shoe!) would be negative pressure.
Click to expand...

The function of friction in such a scenario is not really to exert force directly on your foot, but to prevent the rapid equalization of pressure within the fluid mud. Which means that as you pull your foot up, the pressure at the bottom of your foot drops, whereas the ambient atmospheric pressure still exerts its full force downward from above, leading to your difficulty in extracting your foot.
 
Mikemcc said:
To (likely) 99.9% of folk it's still a theoretical masturbation exercise.
Click to expand...

To the extent that basically all physics can be described this way for most "folk", sure. But if that describes you, what the hell are you even doing in this thread? And if it doesn't, then what's the problem?

Most of us don't work to those lveles of precison. Top the rest of us you measure absolute gas pressure which CANNOT be less than zero at these levels of measurement.
Click to expand...

But why limit yourself to gasses? The definition of pressure is much more generic, and it SHOULD be. You can have pressure in water, can't you? Of course you can. And if it's just a matter of the level of precision, well, you may not be aware of this, but water can support negative absolute pressures (not just gauge pressures) far greater than 1 atmosphere. You don't need extraordinary levels of precision to figure that out, or for it to matter.

Given that, I will retire from this conversation (for the reasons given above).
Click to expand...

I still don't know why you decided to participate in the first place.

If you want to talk about Casimair effects, raise a thread about that, rather than raising a thread about negative pressure, when 'you' are actually trying to talk about something completely different!
Click to expand...

Except it's not different. It's an example case using the same general definition of pressure that one uses for ordinary gasses or liquids.
 
Ziggurat said:
Once again, I'm not talking about Guth. Forget Guth. Let's talk about the scenario YOU proposed. This isn't an analogy, it's a physical scenario in its own right.
Click to expand...

Bait and switch much Zig? My "complaint" was specifically directed at Guth's claim about getting "negative pressure" out of a "vacuum". You can't move the goalposts like that! Guth didn't have an EXTERNAL anything to work with. Had he attributed the "negative pressure" effect to something 'external' to his mass thingy, I probably wouldn't have complained. If he claimed "external mass did it" for instance, I'd probably be 'ok' with the term "negative pressure" from the other mass. Guth however specifically tried to get a *VACUUM* (not another object) to convey "negative pressure" to his mass/energy thingy. There's nothing in a vacuum at best case, and "positive pressure" and positive kinetic energy in all real life scenarios. His 'vacuum' cannot possible hold "negative pressure" of any sort that might interact with that mass/energy thingy.
 
Last edited:
You must log in or register to reply here.

Back
Top Bottom