Science Questions

Would a siphon work in a vacuum?

Sun, 11th Apr 2010

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Raymond asked:

Would a siphon work in a vacuum?


The simple answer to that is, no, it wouldn't work.  The general way that siphons work in general is that, essentially, you've got air pressure pushing down on both ends of your pipe.  One end is higher than the other, then that can push push it on both sides, push the water up to the top and then it carries and going.  If there wasn't any air pressure, then you could form a bubble at the top of the tube and that bubble would expand and expand until the two levels of water are same level as that their two tanks.  Okay, so that's the simple answer.  The somewhat more complicated answer is that water has something called cohesion.  The water molecules do stick together as related to the surface tension.  And, therefore, as long as you don't form any bubbles, water can actually be pulled and actually will stay stuck together even at a negative pressure, even at zero pressure, or even as you can pull it apart, which is the reason why trees can actually go higher than about 30 meters which is the air pressure which will push the water up, or 10 meters, which push the water up.  And so even with a very, very thin tube and until you get a bubble, it would work for a bit but not for very long.


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Ray asked the Naked Scientists: Will a siphon work in a vacuum? What do you think? Ray, Tue, 23rd Feb 2010

I suspect that it will not. Vacuum does not pull. Pressure pushes. Vern, Tue, 23rd Feb 2010

No syhprum, Tue, 23rd Feb 2010

We discussed "how does a siphon work?" previously; the comments there should provide a comprehensive answer to this question too chris, Tue, 23rd Feb 2010

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