How would you create a true vacuum engine?

[Darkside]

Okay, I had this idea randomly when I was at a friend's house. For various reasons, I was thinking:

1)vacuum is a lack of pressure

2)something inside a pressurized environment exposed to vacuum is "sucked" out

If you had a container that was expandable filled to the brim with a liquid in it's unexpanded form. When the container is expanded, a vacuum is created by the extra space.

Could you use this principle to somehow create a self-generating vacuum engine? It seems to me there has to be a way.:hmmm:

[Dayman]

You may want to check out the laws of thermodynamics first...

[Simetrical]

I'm not familiar with the engineering principles of vacuum creation in practice, but in principle you're certainly correct. Take a piston, say, push it all the way closed, then add an airtight seal around the edge of the piston and pull it back. It will take a considerable amount of force, easily hundreds of pounds depending on the size of the piston, and you have to make sure your container (including sealant, if any) will withstand the pressure difference. But you can do it. Approximate vacuums aren't very hard to create.

Of course, this won't be a perfect vacuum. Even if you close the piston all the way, there will likely still be a small amount of air trapped inside, that will remain when you open it up. Also your seal might leak, and for that matter you might have bits of your piston coming off or messy things like that.

Regarding your suggestion that one use "a container that was expandable filled to the brim with a liquid", though, expanding that won't create a vacuum. Liquid in a vacuum will immediately evaporate, and in a closed container that means you no longer have a vacuum. Specifically, the liquid will evaporate until either it's all evaporated, or the pressure in the (non-liquid-filled) part of the container equals the liquid's vapor pressure.

In practice, of course, probably more complicated machines are used to create vacuums, for engineering reasons (cost, reliability, quality of vacuum, etc.).

[Morkonan]

...

If you had a container that was expandable filled to the brim with a liquid in it's unexpanded form. When the container is expanded, a vacuum is created by the extra space...

No.

What you have done in the above example is compress a liquid. When it expands it is not creating a vacuum, it is simply returning to its "preferred" state. There is no "extra empty space" that would help define any vacuum here.

[Sphere]

Vacuums are fairly easy to create. The pressure of the atmosphere is only 14.7 psi, once you have a pump than can overcome that, and a container that can handle 14.7psi, you can create a vacuum.

Could you use this principle to somehow create a self-generating vacuum engine

Yeah, its called a toilet.

[Simetrical]

No.

What you have done in the above example is compress a liquid. When it expands it is not creating a vacuum, it is simply returning to its "preferred" state. There is no "extra empty space" that would help define any vacuum here.

Nothing in what he said suggested to me that he compressed the liquid. Compressing liquids is actually quite difficult, of course. I took what he said to mean he took a (very strong) balloon, say, filled it all the way up with water, sealed it, and then stretched it out.

[Sphere]

took what he said to mean he took a (very strong) balloon, say, filled it all the way up with water, sealed it, and then stretched it out.

You would create water vapor, not a vaccum, unless the temperature is below the triple point. In which case it would be ice, not liquid water. Even then, once you reach a low enough pressure, the ice would sublimate, so no true vaccum would form. Anyways, fundementally liquid water cannot exist unless there is a pressure on top of it, else it will evaporate away.

[Gaidin]

self-generating

What starts it?

[Gaidin]

No...nothing like that would happen with Simetrical's description. Volume doesn't change. The only thing that changes is the shape of the container, which doesn't affect the pressure as far as I'm aware.

OK...the best part is the second post to instructing to look at thermo has really nothing to do with pressure laws. Given the original post makes the assumption that the container can stand the pressure difference between the vacuum and the liquid. The container, with the given description, has to be assumed to be equally strong at all points. If we assume it is dropped in a pure vacuum with the liquid filling it in its unexpanded form, vector physics comes into play. Pressure differentials will be yanking at the liquid in all directions(unless your environment description is in error and only one side of the container is exposed to vacuum). The forces pulling any given liquid molecule cancel each other out and nothing in the world happens to the container. There is no expansion, and nothing will happen to the liquid.

How the hell do you think they survive in space suits in zero-g?

[Sphere]

You are incorrect, "vector physics" says there would be no net force or shear stresses on a material under uniform pressure, but there are normal forces. And these normal forces can contract/expand a material. This video of marshmellows in a vaccum illustrates this point well.

If you had a container that was expandable filled to the brim with a liquid in it's unexpanded form. When the container is expanded, a vacuum is created by the extra space.

Expanding the container means increasing its volume. That "extra space" would contain water vapor not a vaccum. At pressures lower than the triple point of water, the liquid form doesn't exist at all. So you cannot have a liquid/vaccum boundary in equalibrium, there will always be evaporation.

How the hell do you think they survive in space suits in zero-g?

Atmospheric absolute pressure is only 14.7 psi, which is equivalent to a 30 mph wind. Any design that would hold up to a 30mph wind can contain the pressure inside a space-vessal or space suit. The real difficulty is in manned deep sea vessels that have to resist hundreds of atmospheres of pressure.

[Gaidin]

You are incorrect, "vector physics" says there would be no net force or shear stresses on a material under uniform pressure, but there are normal forces. And these normal forces can contract/expand a material. This video of marshmellows in a vaccum illustrates this point well.

I've stated that with the given descriptions of the OP, one must assume that the container can withstand such things. If you don't wish to make that assumption that is your prerogative.

[Sphere]

The OP says

If you had a container that was expandable filled to the brim with a liquid in it's unexpanded form. When the container is expanded, a vacuum is created by the extra space.

My thoughts have nothing do with the container, just what is inside of it when it is expanded.

I agree, if you drop a infinitely ridgid container filled with liquid into a vaccum nothing changes. But that is not the expandable container like the OP talks about.

[Gaidin]

The OP says



My thoughts have nothing do with the container, just what is inside of it when it is expanded.

I agree, if you drop a infinitely ridgid container filled with liquid into a vaccum nothing changes. But that is not the expandable container like the OP talks about.

I was running on the thought that the container, while expandable, will not break. This means there is no way for there to be a pressure leak for the liquid to escape from, favoring any particular force pulling on it(in my thoughts). That's where my idea that on any given molecule the vacuum pressure forces were net zero. Given it was a vacuum, there would be no outer forces(things like wind, gravity) acting on the container or the liquid and they would be limited to the pressure forces from the differences and the pressures trying to equalize. At the least, when it came to forces like gravity, they would be negligible for the purposes of the experiment(unless we want to spend a lot of time on the math anyway).

I'll be the first to admit that when you start adding forces things start changing. But I was only attempting to account for the OP's pressurized environment and nothing else(including the container, as demonstrating by my assumption of its strength).

[Gaidin]

My thought isn't so much boundary, just that if all we're accounting for is the pressure difference between inside the container and outside, and if there's no gap for the pressure to equalize through, then the forces will zero out because the vacuum is acting on all sides of the container. We need more information about the container(material, etc) before we can account for anything else unless we wish to assume a common material.

Edit: yes...been years since I got to take physics...hell if I know if I'm wording things right.

[Sphere]

I found this cute little water phase diaghram to describe my point. The solid lines are the pressure-temperature conditions were two states of matter can exist in equalibrium. As you can see, when you drop the pressure in a liquid, it will turn into a gas. If you lower the temperature it will turn into a solid. But there is no point along the 0 pressure line (x-axis) were you can have liquid water. There may be substances that can do this, but I dont know of them.

My only point is that the OP's method of creating a vaccum would fail, no matter what containment system you used. The water inside would boil as you expand the container creating water vapor.

[ajm317]

There may be substances that can do this, but I dont know of them.

Because I'm bored, the phase diagram of Helium 3:

[Sphere]

Theoretically yeah helium can do it, but at temps under 1K? The technical term for that is mind-blowingly-fricken-crazy cold.

I had a peak at mercury because I thought it might work, but it seems to have a triple point, so no.

[ajm317]

Theoretically yeah helium can do it, but at temps under 1K? The technical term for that is mind-blowingly-fricken-crazy cold.

I had a peak at mercury because I thought it might work, but it seems to have a triple point, so no.

Well yeah, I made no promises of practicality.

[Simetrical]

You would create water vapor, not a vaccum, unless the temperature is below the triple point. In which case it would be ice, not liquid water. Even then, once you reach a low enough pressure, the ice would sublimate, so no true vaccum would form. Anyways, fundementally liquid water cannot exist unless there is a pressure on top of it, else it will evaporate away.

Yup, exactly as I said. Although if it were below the triple point's temperature, it's unlikely you would have liquid water, of course:

No...nothing like that would happen with Simetrical's description. Volume doesn't change. The only thing that changes is the shape of the container, which doesn't affect the pressure as far as I'm aware.

If only the shape changed, of course the pressure wouldn't change. However, the OP said "When the container is expanded". Expansion is nothing other than increase in volume. Since the liquid won't expand, but the container did, the liquid will no longer wholly fill the container. The otherwise empty space will fill with water vapor.

If we assume it is dropped in a pure vacuum with the liquid filling it in its unexpanded form

Nobody said that. He's talking about creating a vacuum inside the container, not placing in an exterior vacuum.

vector physics comes into play. Pressure differentials will be yanking at the liquid in all directions(unless your environment description is in error and only one side of the container is exposed to vacuum). The forces pulling any given liquid molecule cancel each other out and nothing in the world happens to the container. There is no expansion, and nothing will happen to the liquid.

Yes, true, if the container is rigid it will insulate its contents from any external pressure whatsoever. However, the problem a) didn't resemble what you said to begin with (see above), and b) didn't state or (I think) intend to imply that the container would expand by itself. It would have to be expanded by the deliberate application of some external force.

You're correctly answering your interpretation of the problem, but I'm pretty sure that interpretation isn't what the original poster intended at all. It doesn't have anything to do with creating a vacuum.

You are incorrect, "vector physics" says there would be no net force or shear stresses on a material under uniform pressure, but there are normal forces. And these normal forces can contract/expand a material.

But if, as in Gaidin's scenario, you put the liquid inside a rigid container, the pressure outside the container will exert no force on the liquid, normal or otherwise. The container itself will be subject to normal forces from pressure, but since it's rigid and the net force is zero, they won't be transmitted to the liquid.

This video of marshmellows in a vaccum illustrates this point well.

The marshmallows are not in a rigid container.

You've been hanging around Simetrical haven't you? He is a bad influence.

Pfft, only theoretical science is interesting.

[Gaidin]

But if, as in Gaidin's scenario, you put the liquid inside a rigid container, the pressure outside the container will exert no force on the liquid, normal or otherwise. The container itself will be subject to normal forces from pressure, but since it's rigid and the net force is zero, they won't be transmitted to the liquid.

I wasn't assuming a rigid container. I was assuming that with all other forces being negligible, the pressure forces would zero out, not giving the container a reason to expand or contract. If we don't want to assume other forces are negligible we need to know what material the container is, we need to know how well made it is, we need to know what nearby gravitational bodies are, and in general we need a lot of information the OP just flat out doesn't give us.