Can pressure be negative?

[Ziggurat]

Pressure is never negative. It's always positive, but can be written as negative when it's less than a greater positive pressure to make math simpler.

That's simply not true. Water, for example, can support negative pressures of magnitudes significantly larger than 1 atmosphere. And that is absolute negative pressures, not negative relative to atmosphere. This is possible because water molecules attract each other, so they can pull on each other, not just push. It's basically the bulk equivalent of surface tension. There's a limit to this negative pressure, because beyond a certain tension the water will simply pull apart, but water definitely can go to significant negative absolute pressure.

 

[Ziggurat]

I am saying that there is a "absolute zero pressure" analogously to the absolute zero temperture. It might be defined as the pressure of a gas at absolute zero.

That's a really bad way to define pressure. Especially since much simpler, more rigorous, and more general definitions (like the one in the OP) are available.

A bit of interesting physics: There can be systems with temperatures below absolute zero. How to create temperatures below absolute zero

Negative temperatures are neat, but they aren't really analogous to negative pressures. You can't reach negative temperatures by lowering the temperature past zero, but only by heating past infinite temperature. The temperature scale is thus discontinuous (but inverse temperature is not). Pressures, however, vary continuously across zero.

 

[mike3]

That's simply not true. Water, for example, can support negative pressures of magnitudes significantly larger than 1 atmosphere. And that is absolute negative pressures, not negative relative to atmosphere. This is possible because water molecules attract each other, so they can pull on each other, not just push. It's basically the bulk equivalent of surface tension. There's a limit to this negative pressure, because beyond a certain tension the water will simply pull apart, but water definitely can go to significant negative absolute pressure.

You've piqued my interest with this one. What do you mean by absolute negative pressure? You mean like grabbing a chunk of real cold ice out in space and pulling on it really hard?

 

[mike3]

Negative temperatures are neat, but they aren't really analogous to negative pressures. You can't reach negative temperatures by lowering the temperature past zero, but only by heating past infinite temperature. The temperature scale is thus discontinuous (but inverse temperature is not). Pressures, however, vary continuously across zero.

Huh? You can't "heat to infinite temperature" to get past it since that would require an infinite amount of energy (same as cooling to absolute 0). I thought that to make negative temperature you needed to do something funny with the stuff that wasn't like normal heating or cooling.

 

[Reality Check]

That's a really bad way to define pressure. Especially since much simpler, more rigorous, and more general definitions (like the one in the OP) are available.
I was not trying to define pressure. I was trying to define a pressure scale.
As you say, the OP contains pressure definitions.

 

[Ziggurat]

I was not trying to define pressure. I was trying to define a pressure scale.
As you say, the OP contains pressure definitions.

But why even bother with a scale that's separate from the absolute pressure? All you need are units for the absolute pressure, and the zero point obviously doesn't define the units, your units of energy and length (or force and length, if you like) do.

 

[Ziggurat]

Huh? You can't "heat to infinite temperature" to get past it since that would0require an infinite amount of energy (same as cooling to absolute 0).

Systems which have constant heat capacities (such as ideal gasses) indeed cannot be heated to infinite temperature, and cannot support negative temperatures either. The only systems which can support negative temperatures are systems with an upper bound to their energy (such as magnetic moments in a magnetic field). As you heat them, their heat capacity drops (and goes to zero at infinite temperature), so it does not actually take an infinite amount of energy to heat them.

I thought that to make negative temperature you needed to do something funny with the stuff that wasn't like normal heating or cooling.

We frequently think of heating a system in terms of putting it in thermal contact with a heat bath at higher temperature, and letting heat flow from the higher temperature to the lower temperature. So it's true that I can't use such a technique to heat a system to infinite temperature if the heat baths I have available are all finite positive temperature. But if I add energy to the system directly, I'd still call that heating the system, though perhaps one might object that this isn't technically "heat" that I'm transferring. So all I really need to do is find a way to add energy to the system. And that's not necessarily very hard or exotic to do. In the case of magnetic moments in a field, if the moments are aligned with the field (positive temperature) I can just quickly flip the direction of the field, and that puts all the moments in a high energy state, and the system at a negative temperature.

 

[Halfcentaur]

I've always wanted to learn more about implosions.

 

[Ziggurat]

You've piqued my interest with this one. What do you mean by absolute negative pressure? You mean like grabbing a chunk of real cold ice out in space and pulling on it really hard?

One need not do anything so exotic.

Consider a simple piston filled with water, with cross-sectional area A. If you press on the piston with force F, you increase the pressure inside the piston by F/A. If you pull on the piston with force F, you decrease the pressure by -F/A. One might think that pulling on the piston merely cancels some of the ambient atmospheric pressure, but in fact one can pull on the piston with considerably more force than the ambient atmosphere applies. For very pure water, one can pull on the piston hard enough to produce something like 20 atmospheres of negative pressure before the water "rips" (ie, forms a void which will then expand as you pull).

 

[Michael Mozina]

I'm think I'm going to kick back and watch for awhile. This is fun!

Keep in mind that Guth claimed that a VACUUM (not a liquid) had negative pressure. No vacuum ever achieves even a "zero" pressure, and the limit of pressure of/in a vacuum is ZERO, not a negative number.

 

[Michael Mozina]

No-but other than gasses, other forms of "pressure"-hydraulic and stress, for instance are due to something other than molecular energy--gravity, or other applied load. In solids, "pressure' can be either tensile or compressive, for which we engineers use a sign convention to differentiate the two, because it is important to know which is which. Compressive stress is considered to be "negative", while tensile stress is "positive"- a matter of convention, and convenience. They are both positive values, however.
In a liquid, hydrostatic pressure is matter of applied load, and is always normal to whatever surface you are dealing with.
for a gas, P*V=n*R*T, where n is the molar amount, R is a gas constant, and T is measured from absolute zero.
Lab tricks and semantics aside, if heat flows from the hotter to the colder, the "negative temperature" system described in the above link is still at a temperature above 0 absolute.
Perhaps you have found a new state of matter?

Oh no, you used the same formula for pressure in a vacuum that I used.

FYI, RC's question relates back to the pressure of a VACUUM in the total absence of anything *OTHER THAN* the vacuum itself. The formula PV=nRT demonstrates that the limit of pressure in a vacuum is zero, but Guth said claimed his vacuum contained a "negative" pressure, not a RELATIVELY LOWER pressure. You're right and RC is wrong, but RC will not ever admit it IMO.

 

[Ziggurat]

Keep in mind that Guth claimed that a VACUUM (not a liquid) had negative pressure. No vacuum ever achieves even a "zero" pressure, and the limit of pressure of/in a vacuum is ZERO, not a negative number.

Can a vacuum have an energy?

 

[Michael Mozina]

Peer pressure.

...

(*Pause, hears crickets*)


...

I for one appreciated your humor.

 

[Michael Mozina]

Can a vacuum have an energy?

Sure, *POSITIVE* kinetic energy. You couldn't for instance remove every neutrino from a vacuum even if you could figure out a way to remove every single last atom.

 

[Ziggurat]

The formula PV=nRT demonstrates that the limit of pressure in a vacuum is zero

No it doesn't. It predicts zero pressure for a vacuum, but the ideal gas law is known to be wrong in multiple ways, so it demonstrates nothing of the sort.

 

[Ziggurat]

Sure, *POSITIVE* kinetic energy.

I'm talking about a vacuum. Which means, by definition, no mass. Whatever energy it has, labeling that energy as kinetic is bloody STUPID.

And yes, I'm talking about positive energy, so I don't know why you're stating that as if it somehow contradicts anything I said or am planning on saying.

 

[Michael Mozina]

I'm talking about a vacuum. Which means, by definition, no mass. Whatever energy it has, labeling that energy as kinetic is bloody STUPID.

Why would you say that? Even light has POSITIVE kinetic energy. Does your vacuum contain any photons?

And yes, I'm talking about positive energy, so I don't know why you're stating that as if it somehow contradicts anything I said or am planning on saying.

I don't profess to be clairvoyant about what you may or may not say in the future, I was simply responding to your question. The only energy that the vacuum might contain is KINETIC energy assuming any photons still exist in the chamber. If not, the PRESSURE drops to zero. You can't put negative kinetic energy inside the vacuum, so the absolute limit of pressure is zero, not a negative number.

 

[Michael Mozina]

No it doesn't. It predicts zero pressure for a vacuum, but the ideal gas law is known to be wrong in multiple ways, so it demonstrates nothing of the sort.

Ya it does, you just don't like the implication. That formula sets the lower limit of pressure in a vacuum to zero, not a negative number, so you're dancing.

 

[Ziggurat]

Ya it does, you just don't like the implication. That formula sets the lower limit of pressure in a vacuum to zero, not a negative number, so you're dancing.

Except that the formula is wrong. We know it's wrong. It's wrong for multiple reasons. So how can a wrong formula demonstrate anything?

And you're dancing. Can a vacuum have energy? Yes or no. Easy question.

 

[Michael Mozina]

Working in the hospital environment, we have "negative pressure isolation rooms" where we put patients with TB, for example.

I'm assuming it's negative relative to the pressure outside the room (and that's the only reason it's "negative"), and outside the room we have a digital reading which lets us know whether or not the room is operating correctly. There is also a simple test (quick and dirty so to speak) of dropping a kleenex slightly beyond the threshold and watching it "suck" into the room.

I know the question the OP is asking is mathematical in nature, but I thought I would throw this anecdote in there just for it's novelty value perhaps ...

Keep in mind that these are "relative" pressures, not ABSOLUTE pressures. One room simply has to have 'less" pressure than another and neither of them need to achieve an actual ZERO pressure (which would kill the patient) let alone a NEGATIVE pressure in absolute terms.