Absolute zeros other than temperature (e.g. pressure)

[greypilgrim]

Hi.

A version of the third law of thermodynamics states that no system can be cooled down to absolute zero temperature in finitely many steps.

But what about other quantities, for example pressure: Is it possible (in principle) to evacuate a system up to the last gas particle, or would this violate thermodynamics? Would we need a Maxwellian demon to get the last particles out?

If such no-go statements exist for other quantities, are they less fundamental than the one about temperature and can be derived from the usual laws of thermodynamics?

 

[russ_watters]

But what about other quantities, for example pressure: Is it possible (in principle) to evacuate a system up to the last gas particle, or would this violate thermodynamics? Would we need a Maxwellian demon to get the last particles out?

If such no-go statements exist for other quantities, are they less fundamental than the one about temperature and can be derived from the usual laws of thermodynamics?

For pressure I would think there is no violation in evacuating to the last molecule, and while it may seem like a Maxwell's Demon type situation, it wouldn't be a violation because the vacuum pump uses/inputs energy into the system. Maxwell's Demon is only a violation because he's assumed to not use any energy.

 

[greypilgrim]

Doesn't a vacuum pump work by creating a pressure gradient that causes a net flow of molecules out of the system? How is that supposed to work if we aim for zero pressure inside the system?

 

[Nugatory]

Doesn't a vacuum pump work by creating a pressure gradient that causes a net flow of molecules out of the system? How is that supposed to work if we aim for zero pressure inside the system?

In principle withdrawing a perfectly machined piston from a perfectly machined cylinder with a perfect seal around the piston would produce a perfect vacuum. In practice that will not be possible, but these are mechanical limitations, not any violation of thermodynamical law.

 

[f95toli]

Doesn't a vacuum pump work by creating a pressure gradient that causes a net flow of molecules out of the system? How is that supposed to work if we aim for zero pressure inside the system?

This only works for relatively high pressures (Google "Knudsen limit"). For high- and ultra-high vacuum system we use pumps that do not rely on a pressure gradient such as turbomolecular pumps or, for ultrahigh vacuum, ion pumps

 

[greypilgrim]

And how are you getting this piston inside the cylinder beforehand? If there is gas left inside the cylinder, pushing it in will make the pressure diverge and hence require infinite force, if there is no gas left, well then the vacuum existed before.

 

[greypilgrim]

Never mind. Obviously we could have a cylinder with a valve, open it, push the piston in and then close the valve.

What if we add radiation pressure to the picture? The walls cannot have zero temperature, hence they radiate.

 

[greypilgrim]

Actually:

In practice that will not be possible, but these are mechanical limitations, not any violation of thermodynamical law.

How can you be sure that there will always be mechanical limitations? Maybe because they all come down to the second and third law of thermodynamics?

 

[russ_watters]

What if we add radiation pressure to the picture? The walls cannot have zero temperature, hence they radiate.

That doesn't have anything to do with a gas/vacuum.

How can you be sure that there will always be mechanical limitations? Maybe because they all come down to the second and third law of thermodynamics?

The logic @Nugatory described is pretty straightforward and in keeping with physics laws. Can you think of a reason for/cause of a violation of such laws? "How can you be sure" isn't a law of physics.

 

[Mister T]

Doesn't a vacuum pump work by creating a pressure gradient that causes a net flow of molecules out of the system? How is that supposed to work if we aim for zero pressure inside the system?

Researchers refer to such a pump a fore-pump. It's just the first stage in achieving an ultra high vacuum (UHV). After that other devices, such as an oil diffusion pump, are used to remove more air.

Is it possible (in principle) to evacuate a system up to the last gas particle, or would this violate thermodynamics?

Note that even if gas particles remained in a vessel, you could find a region of the vessel that contains no gas particles.

Your main point is interesting. We have other quantities that have an absolute zero. For example, size. Is it possible for an object to have zero size? Some particles, for example electrons, appear to have a zero size but there's no way to know for sure if that's true. The current limit of technology puts them at a size smaller than about $10^{-18}$ meters.

 

[A.T.]

What if we add radiation pressure to the picture? The walls cannot have zero temperature, hence they radiate.

Right, so you cannot reach zero absolute pressure in a container, in practice.

 

[jbriggs444]

Right, so you cannot reach zero absolute pressure in a container, in practice.

On the other hand, tension is a thing. Somewhere in the spectrum between positive pressure and negative pressure, there should be [approximately] zero pressure.