# speed of vacuum

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#### spacehugo

##### Guest
1000 ml of vacuum will weight in on the as ,lets say 100000000000000 ml, they fill volume but when released they will dissappear. they have no mass (ok there are a lot of energy jumping in and out of existence), it is nothing. what about a tube 300.000 km long with vacuum inside, if we release in one end, how long before we see pressurefall in the other end of tube. I think maybe no time elapsed because vacuum has no direction and it is nothing so pressurefall will be all over at the same time. in air or gasses the pressure will have to move in a similar as sound does and thatb is rather slow.

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#### Mee_n_Mac

##### Guest
spacehugo":14myq30w said:
1000 ml of vacuum will weight in on the as ,lets say 100000000000000 ml, they fill volume but when released they will dissappear. they have no mass (ok there are a lot of energy jumping in and out of existence), it is nothing. what about a tube 300.000 km long with vacuum inside, if we release in one end, how long before we see pressurefall in the other end of tube. I think maybe no time elapsed because vacuum has no direction and it is nothing so pressurefall will be all over at the same time. in air or gasses the pressure will have to move in a similar as sound does and thatb is rather slow.

Given English isn't your primary language it's hard to understand your question. I have to ask what's at each end of the tube ? I understand the tube is evacuated. Is the other end to be opened into normal atmopheric pressure ? Is your question then how long is it before I see the pressure RISE (not fall) at the other end ?

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#### origin

##### Guest
spacehugo":3h19d6f9 said:
1000 ml of vacuum will weight in on the as ,lets say 100000000000000 ml, they fill volume but when released they will dissappear. they have no mass (ok there are a lot of energy jumping in and out of existence), it is nothing. what about a tube 300.000 km long with vacuum inside, if we release in one end, how long before we see pressurefall in the other end of tube. I think maybe no time elapsed because vacuum has no direction and it is nothing so pressurefall will be all over at the same time. in air or gasses the pressure will have to move in a similar as sound does and thatb is rather slow.

This is essentially a fluid dynamics problem. The differntial pressure is 14.7psi and the fluid is air. The velocity of the air flow will depend on the diameter of the tube and the roughness of the wall. The turbulent flow at the entrance of the tube would probably limit the velocity to subsonic speeds.

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#### bobw

##### Guest
origin":3ej4v1l8 said:
This is essentially a fluid dynamics problem. The differntial pressure is 14.7psi and the fluid is air. The velocity of the air flow will depend on the diameter of the tube and the roughness of the wall. The turbulent flow at the entrance of the tube would probably limit the velocity to subsonic speeds.

Yes but the pipe length makes it almost a trick question so there are a couple other things: Friction with the pipe causees a drop in pressure so pretty soon along a 300000km pipe the pressure will drop to zero and you will be limited to the rate of diffusion. (For example, a velocity of 25 fps in black iron pipe represents about 0.25 psi loss per 100 ft. of run.) Also if you stand your pipe on end and let it hang out of the atmosphere you will never see pressure at the other end, air will quit going into the pipe at the edge of space.

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#### origin

##### Guest
bobw":11vfbv23 said:
origin":11vfbv23 said:
This is essentially a fluid dynamics problem. The differntial pressure is 14.7psi and the fluid is air. The velocity of the air flow will depend on the diameter of the tube and the roughness of the wall. The turbulent flow at the entrance of the tube would probably limit the velocity to subsonic speeds.

Yes but the pipe length makes it almost a trick question so there are a couple other things: Friction with the pipe causees a drop in pressure so pretty soon along a 300000km pipe the pressure will drop to zero and you will be limited to the rate of diffusion. (For example, a velocity of 25 fps in black iron pipe represents about 0.25 psi loss per 100 ft. of run.) Also if you stand your pipe on end and let it hang out of the atmosphere you will never see pressure at the other end, air will quit going into the pipe at the edge of space.

I see your point. I think you are right that the air flow would have a very low velocity. 14.7 psi over even just 30 km ain't much with the pipe losses figured in. I assumed he was talking about a pipe in space, you are of course correct that the gas in a vertical pipe (assuming the gravitation field earth) would only fill the pipe until the mass of the gas in the pipe exerted a pressure of 14.7 psi at the pipe entrance.

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#### spacehugo

##### Guest
the tube is up in space and closed in both end and totaly empty, in one end there is a device registering any difference in pressure and in the opposite end there is a valve to let pressure in. we also have a enclosed container which contains atmosperic pressure (760 mm mercury). when that pressure is released from that container into the tube thru the valve, there is nothing inside and there is no distance because pressure will distribute all over at the same amount. also keep in mind that no matter how much "nothing" you have, even the smallest particle will make it "something". if you have something, whatever you will put in, it must blend, before distributed all over in the same amount . this is the essential point in this question

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#### origin

##### Guest
spacehugo":1qmj5s51 said:
the tube is up in space and closed in both end and totaly empty, in one end there is a device registering any difference in pressure and in the opposite end there is a valve to let pressure in. we also have a enclosed container which contains atmosperic pressure (760 mm mercury). when that pressure is released from that container into the tube thru the valve, there is nothing inside and there is no distance because pressure will distribute all over at the same amount. also keep in mind that no matter how much "nothing" you have, even the smallest particle will make it "something". if you have something, whatever you will put in, it must blend, before distributed all over in the same amount . this is the essential point in this question

Huh? What do you mean there is no distance? Do you want to know what would happen if a fixed volume of air is released into a container under vacuum? So are you asking how fast molecules of air would diffuse into the vacuum?

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#### spacehugo

##### Guest
if you fill a ballon with air , it will have a certain volume and a tube filled with air will also have volume in addition to length. open this under water and you will see bubbles rising towards the surface, also in this ballon or tube with air, sound can travel at 360 m/s. the same procedure with the vacuum tube will not make any bubbles rising towards the surface and further more, sound can not travel thru vacuum and sound travel in a similar way (or the same way) as pressure do. so if you have 360 m of air between you and the source of sound, the sound use 1 s. to reach you, but if there is 1mm or 10 km of vacuum between you and the source of sound, the sound will never reach you.

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#### DrRocket

##### Guest
spacehugo":1mjlxa8f said:
if you fill a ballon with air , it will have a certain volume and a tube filled with air will also have volume in addition to length. open this under water and you will see bubbles rising towards the surface, also in this ballon or tube with air, sound can travel at 360 m/s. the same procedure with the vacuum tube will not make any bubbles rising towards the surface and further more, sound can not travel thru vacuum and sound travel in a similar way (or the same way) as pressure do. so if you have 360 m of air between you and the source of sound, the sound use 1 s. to reach you, but if there is 1mm or 10 km of vacuum between you and the source of sound, the sound will never reach you.

You have managed to start with a poorly posed question and then scramble it into compeete incoherence.

Basically this is, or ought to be, a standard compartment venting problem. Here you start with one volume, which happens to be evacuated, separated from another volume that contains some volume (unspecified) of some gas (unspecified) at some pressure (apparently unspecified though mention was made later of 14.67 psi). Then at time 0 a diaphram separting the two compartments if broken (or a valve is opened) and gas from the first compartment flows into the second compartment.

The problem is that you asked WHEN the first pressure would be seen at the far end of the tube. That is fairly difficult to answer since you have not specified criteria for what you mean by "first pressure". If you mean when might the first molecule hit the far end, then that could be pretty quick, since the question one of what the HIGHEST speed might be of the molecules that make up the gas at the specified initial temperatue. Even then you will have to be more precise, since the distribution permits extremely high, but low probability speeds.

On the other hand you might be considering a classical continuum mechanics model of a compressible fluid, which would provide a solution, somewhat complex, in terms of the Navier-Stokes equation. That should provide flow vectors, pressure, temperatureand density throughout the volumes as a function of time. This will be complicated by the expansion of the gas into the vacuum which changes the temperature, which affects the speed of sound. There is additinal complication because when you vent into a vacuum you will at times find that the flow is supersonic.

You might also be asking what speed gas can reach when it is expanded isentropically from a given pressure to zero pressure. This is precisely the quantity that is used as a measure of the efficiency of rocket propellants -- vacuum specific impulse. It is dependent in detail on the thermodynamics properties of the gas but basically varies like the square root of temperature divided by molecular weight.

Finally, this has nothing whatever to do with the "speed of vacuum" which as a term makes no sense at all. The vacuum is not moving.

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#### spacehugo

##### Guest
"Finally, this has nothing whatever to do with the "speed of vacuum" which as a term makes no sense at all. The vacuum is not moving". Excatly what I try to make a point of. vacuum will distribute all differences in pressure in the whole volume, because there is no distance to move in.

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#### Mee_n_Mac

##### Guest
spacehugo":x5oidpkr said:
the tube is up in space and closed in both end and totaly empty, in one end there is a device registering any difference in pressure and in the opposite end there is a valve to let pressure in. we also have a enclosed container which contains atmosperic pressure (760 mm mercury). when that pressure is released from that container into the tube thru the valve, there is nothing inside and there is no distance because pressure will distribute all over at the same amount. also keep in mind that no matter how much "nothing" you have, even the smallest particle will make it "something". if you have something, whatever you will put in, it must blend, before distributed all over in the same amount . this is the essential point in this question

If we have an evacuated tube and another at 760 mm mercury, and they are rigid, what does being in space have to do with the question ? If you're asking when pressure starts to rise at the end of the tube, 300 km from the valve that seperates the empty from the full tube, then as DrR says ... it all depends. I have no idea what you mean when you say "there is no distance because pressure will distribute all over at the same amount". In the final steady state condition you will have equal pressure throughout the 2 tubes but gas molecules initially exposed to the vacuum will have built up some velocity when they reach the far end. They will bounce off the end and you'll have pressure waves running back up (and perhaps down and ...) until equilibrium is reached.

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#### origin

##### Guest
spacehugo":11alhjld said:
"Finally, this has nothing whatever to do with the "speed of vacuum" which as a term makes no sense at all. The vacuum is not moving". Excatly what I try to make a point of. vacuum will distribute all differences in pressure in the whole volume, because there is no distance to move in.

Trying to figure out what you are saying is a very frustrating guessing game.

If you are saying that you think the pressure will instantaneously increase in the whole vacuum chamber the answer is: No, absolutely not.

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#### davamanra

##### Guest
First, "vacuum" is somewhat of a misnomer. It is actually a lack of pressure and/or matter and energy, relative to the surrounding environment. If I haven't forgotten too much of what I learned in physics, I believe the upper limit for the "speed of vacuum" in this context would be the speed of sound. Of course it has to be remembered that the speed of sound is relative to the differential in pressure. In near vacuum (actually a better way to phrase it would be as pressure differential approaches zero) the speed of sound would be close to zero, but as pressure differential approaches infinity, the speed of sound approaches the speed of light. As I said it's been a long time since I've done physics, so correct me if I'm wrong!

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#### DrRocket

##### Guest
davamanra":3vcpx9jv said:
First, "vacuum" is somewhat of a misnomer. It is actually a lack of pressure and/or matter and energy, relative to the surrounding environment. If I haven't forgotten too much of what I learned in physics, I believe the upper limit for the "speed of vacuum" in this context would be the speed of sound. Of course it has to be remembered that the speed of sound is relative to the differential in pressure. In near vacuum (actually a better way to phrase it would be as pressure differential approaches zero) the speed of sound would be close to zero, but as pressure differential approaches infinity, the speed of sound approaches the speed of light. As I said it's been a long time since I've done physics, so correct me if I'm wrong!

No the speed of sound is related to stiffness in a solid and temperature, pressure and ratio of specific heats (commonly called "gamma") in gasses. In gasses is actually relatively insensitive to pressure, other things being equal.

http://en.wikipedia.org/wiki/Speed_of_sound

The speed of sound in a gas is actually the speed of propagation of a pressure wave in the limit as the magnitude of the wave (pressure differential) goes to zero.

In a vacuum the notion of sound and of speed of sound becomes essentially meaningless. The theory on which it is based is continuum mechanics, the discipline in which materials are treated as continuous and in which the fact that they are really composed of atoms is ignored. In a vacuum there are ideally no atoms or molecules and in practice there are relatively few. If the vacuum is nearly perfect, as in this thought experiment, then the continuum model breaks down. If there are literally no molecules then there is no medium and there is no sound, so the speed of sound is not a meaningful concept.

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#### origin

##### Guest
I think it is intresting how fast sound propegates in water. If you are above water and you hear a boat motor you can automatically determine the location of the boat because the sound will reach one ear before the other ear and your brain immediately determines the sound is in the direction of the ear that first heard the sound. Under water the sound is traveling so fast that your brain cannot determine if there was a difference in the times that the sound reached each ear. So if you are underwater the direction of the sound of a boat motor cannot be determined, it seems to come from all directions at once.

That makes it all the more amazing that dolphins can use echolocation, they certainly can process information quickly.

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#### spacehugo

##### Guest
First, I know that pressure travel thru a medium (gasses,liquids etc.) at different speed depending on its ability to be compressed (or decompressed), but when there is nothing to travel thru, like in vacuum, why is it that the differences in pressure not instantly will distribute in the whole volume containing vacuum ?

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#### origin

##### Guest
spacehugo":249fect4 said:
First, I know that pressure travel thru a medium (gasses,liquids etc.) at different speed depending on its ability to be compressed (or decompressed), but when there is nothing to travel thru, like in vacuum, why is it that the differences in pressure not instantly will distribute in the whole volume containing vacuum ?

Instead of looking at the mathematic and causing confusion lets look at this on a purely empirical basis. Imagine if you have a beach ball full of air and you took it to outerspace. Now puncture the beachball do you think that the air molecules will instantly be distributed throughout the universe? Of course not. Will they be instantly distributed between the moon and earth? No, of course not. The molecules will move from the area of high pressure to the area of low pressure and the velocity will be only due to the difference in pressure. The molecules are moving quickly but at most thousands of feet per second.

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#### MeteorWayne

##### Guest
And no matter (hahahaha) what, they can't move faster than the speed of light Even light can't do that!

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#### spacehugo

##### Guest
yes but vacuum is no matter, it is empty. and as you stated " no matter can travel faster than light". if you have a vacuum inside a sealed container, inside there is nothing( all over in the container)(except from these virtual particles which jump in and out of existence in all over the universe). and if you let one particle inside, you have the hole volume of nothing inside disturbed. now you have a volume inside with something in it.

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#### MeteorWayne

##### Guest
I don't understand your point. If you have container with a vacuum and open it to the outside envirenment, the mass flow of the particles that fill the vacuum would be described by standard fluid dynamic flow equations.

Your last post does not appear to be related at all to your original post.

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