# Buoyancy, pipes, pressure

1. May 3, 2015

Hello, the known presenter "Vsauce" on youtube had a video in which he talked about a " space straw" basically a pipe or circular enclousure the goes from a higher pressure area to a much lower one like a straw from surface of water at sea to outer space.

Now , my question is If I were to put a pipe that would go down to almoust the bottom of the ocean ,a deep place like the mariana trench for example and the other side of it would stick a few meters above water surface , would the pressure from the water combined with a sufficiently small diameter result in water flowing out of the pipe above water surface much like in a fountain?

2. May 3, 2015

### Staff: Mentor

How would the diameter influence anything?
You would just constrain the motion of water, but (neglecting water currents and so on) nothing moves anyway.

3. May 3, 2015

### Staff: Mentor

The pressure is the other half of the equilibrium with gravity already. Dropping a pipe into the ocean doesn't change that.

4. May 4, 2015

Ok, but then why would in the space straw analogy he said that if we had a straw just a little below water and all the way up to open space , the difference in pressure would mean water would go up that straw some length , isnt this the same situation of pressure difference, just different pressures?

Since space has vacuum and here at sea level there is pressure which is more than vacuum of space such a straw can be considered a vacuum cleaner with the vacuum of space being the " motor".
I thought if this works then since there is greater pressure at the bottom of the water reservoir and less pressure up at water surface it should be the same , could you explain more please why the first one works but this one doesnt?

5. May 4, 2015

### Staff: Mentor

The "space straw" idea is wrong too, for the same reason.

Last edited: May 4, 2015
6. May 4, 2015

well in that case to not stir up any more confusion here is the link to the video

at the start he soon starts to talk about pressure difference atmosphere and the straw , please watch it and tell me would the water be pushed up a little bit (he mentions the approximate height) the straw or would nothing happen at all in which case he is wrong ?

thanks.

7. May 4, 2015

### nasu

The water will be pushed up only if the air in the straw will be evacuated somehow.
He is a little ambiguous but he doesn't say that the water will be sucked into space. Not that is will raise in the space straw by 10 m. Just that it won't rise more than that.

8. May 4, 2015

### sophiecentaur

I'm afraid I couldn't handle ten whole minutes of that guy on the vid.
Isn't the point of this thought experiment that the straw has no air in it - therefore no pressure.?
Under that condition, the pressure on the water surface outside the straw would surely push water up inside until the total pressure at 'sea level' inside the straw is the same as the pressure at sea level. outside There would be a column of about 10m in the straw. This column would be producing the same pressure as the atmosphere outside the straw.
This is the same idea as that of Gasparo Berti, who was the first (?) to state the limit to the height of a syphon.
There's nothing under the Sun.

9. May 4, 2015

### nasu

You can't mean that. Unless you play on the meaning of "under".
I suppose you forgot the "new".

10. May 4, 2015

### sophiecentaur

New new new new new - silly old sod!

11. May 4, 2015

Oh right I got the picture , I forgot the very important aspect of the difference in pressures also means they have to be isolated to stay different.

Ok so here would go a pretty wild guess, If a good vacuum can make water go about 10m above sea level , then would instead of using a straw which extends all the way up to space I could build a pipe with a shield inbetween so there would be two pipes in a pipe.have the pipe higher than those 10 meters and evacuate all air in the chamber above the pipe.(for throught purposes say achieve space vacuum) now the inner pipe would be made such that one side of it has a bigger diameter and the other one has smaller one.
If I'm correct the same level of vacuum will achieve different heights of water columns in different diameter pipes correct?

So if I have this two diameter pipe in one enclousure and a sealed vacuum chamber above where the water level ends and a small opening at the top of the smaller diameter pipe , would the sucked up water in the smaller diameter pipe flow down to the larger diameter pipe were the level of water should be lower and would this continue as long as the same vacuum is kept at the above chamber?

a little far off maybe but I'm keen on knowing the answer to this

12. May 4, 2015

### jbriggs444

You lost me on all of your pipes within pipes, some with the headspace evacuated and some not.

The device you describe is (presumably) a barometer. Mercury is traditionally used as the operating fluid because it has a lower saturated vapor pressure than water and because the required mercury column is less than one meter high.

Alternate designs for a water-based barometer (aka a "weather glass") allow some air to remain in the sealed head space which you would otherwise evacuate.

http://en.wikipedia.org/wiki/File:MercuryBarometer.svg

Edit: missed the following bit:

No, not correct. That is a version of the classic "poke a hole in the bottom of the boat and have the water squirt higher than the sides fallacy"

Last edited: May 4, 2015
13. May 4, 2015

### nasu

How is this related to OP? Were you after a perpetual motion machine after all?

14. May 4, 2015

### LURCH

Just a little side note: Something resembling perpetual motion actually is possible here, if you replace the straw with a "space elevator". Your elevator would have to extend well beyond geosynchronous orbit (say, 45,000 miles; or 75,000 km), and it would have buckets that come down one side, dip into the ocean, and climb up the other side. So long as a full bucket dumps its water at the top of the elevator every time an empty bucket fills up at the bottom (so the mass above geosynch is always greater than the mass below it), the conveyor would behave like a siphon, requiring no power source except at start-up. It would not, of course, be true perpetual motion, but it would sure look like it to anyone on the ground.

Not very helpful for understanding the vacuum-related problems of the OP perhaps, but an interesting thought experiment nonetheless.

15. May 5, 2015

jbrings444, well first of all the idea was simple , two pipes each one has different diameter but their in a single enclosure , and when those pipes reach a certain height there is a vacuum chamber at the top of them sealed together with the pipes.The reason I say that the same fluid should achieve different heights in each pipe given the same vacuum which they both share is also the same logic that applies if I would puncture a hole in your boat, a smaller hole would yield a higher inrushing water column a larger hole would yield a lower inrushing water column if the weight of the boat is the same in both situations. It doesnt have to be higher than the water level around the boat but there must be difference in height with different diameters and equal pressures.
If this reasoning is wrong tell me.

yes LURCH that is an interesting idea, an there is nothing perpetual motion here , because then again solar power can be considered perpetual motion because our lifetimes are so short compared to the sun that it's almoust perpetual. :D

by the way would this space elevator bucket thing work because when the bucket of water reaches certain height the gravitational pull from the centrifugal force due to its rotation would become larger than the weigth of the bucket of water just above sea level so it would be able to pull each next one up?
If so then this device would get it's energy from the rotation of earth itself.

Ok but back to my question , is these two pipes with tha vacuum above them had different diameters and the water levels at each one of them would achieve different heigths , then if I were to take water from the smaller diameter pipe and dump it into the larger diameter lower water level pipe would the water always keep the same heigth in the first pipe , assuming the vacuum is kept ?

I apologize as this sounds a little tricky but that's the best I can explain.

16. May 5, 2015

### Staff: Mentor

Yes, it is wrong. Whether static or squirted through a hole, the height of a colum of water is equal to the pressure divided by the weight density. Or the other way around: p=mgh. Period. It does not depend on the shape of the container or hole.

Using Lurch's barometer: a barometer gives the same reading (same fluid column height) regardless of the column's diameter.
This thread is on shaky ground instead of already locked because I can't decide if you are really trying to find a perpetual motion machine. Part of the problem is you don't know what a perpetual motion machine even is. Despite the name, it isn't about how long a machine runs it is about whether or not the machine violates the laws of physics -- most often conservation of energy:
http://en.wikipedia.org/wiki/Perpetual_motion

Last edited: May 5, 2015
17. May 5, 2015

### LURCH

That's right. At geosynchronous orbit, gravity and centrifugal force are balanced (for any object orbiting once every 24 hrs). Anywhere above that, centrifugal force is greater. And yes, that is exactly why it's not perpetual motion, because it is being powered by Earth's rotation.

But regarding your OP, all you've got there is an extremely expensive barometer that's impossible to build with any currently known materials. Once you've evacuated the tube, you could maintain that vacuum by either extending your tube out to space, so the open end is in a naturally occurring vacuum, or you could just cap it off. Either way, the guy is correct; the water would go up about 10 meters and stop, if the straw were evacuated first (which I notice he doesn't specify) .

18. May 6, 2015

Well that sounds interesting , so you are saying that if I had two pipes one with the diameter of 10cm and the other of 1km the same pressure would push the same fluid at equal heigths in both pipes?

19. May 6, 2015

### jbriggs444

Yes. That is correct.

20. May 6, 2015

### Staff: Mentor

Yes: Hydrostatic pressure, p= ρ g h