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How does siphon work?

  1. May 7, 2012 #1
    How does SIPHON work? especially with the simple apparatus in transferring liquid from a container to other container. Would you please help me understanding the mechanism behind this. Then principles are used here.. Thanks :)
     
  2. jcsd
  3. May 7, 2012 #2
    Wikipedia has a quite detailed description about its working.

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

    If you get confused with something in that, you can post questions about it here :smile:
     
  4. May 7, 2012 #3
    "In practical siphons, atmospheric pressure pushes the liquid up the tube into the region of reduced pressure at the top of the tube in the same way as a barometer, and indeed the maximum height of a siphon is the same as the height of a barometer, because they operate by the same mechanism. The reduced pressure is caused by liquid falling on the exit side."

    how come?
    is it necessary not to immerse the end at the lower container the liquid?
    why you need the "initial pump", what's that for? what happens if we introduce that initial pump? thanks :)
     
  5. May 7, 2012 #4
    Do you know http://en.wikipedia.org/wiki/Bernoulli's_principle]Bernoulli's[/PLAIN] [Broken] Principle?

    In a very simple sense, higher fluid velocity results in low pressure than slower moving fluids.

    Also, read 'Theory' under siphons in wikipedia. Thats where the main -how-siphon-works- is hidden.

    As gravity pulls down the fluid, the velocity of the fluid increases, resulting in lower pressure.
     
    Last edited by a moderator: May 6, 2017
  6. May 7, 2012 #5
    \Comes in, probably annoying other people

    First, assume we have the initial "pump." Now we've got liquid flowing out of one end of the siphon. Assuming a rigid tube and liquid that remains at constant pressure, since some liquid is leaving the tube, something has to come in to fill the space. In this case, the liquid coming out is sort of "sucking" in some more liquid into the tube. Sure, the rigidity of the tube and the liquid being constant pressure were annoying assumptions, but the concept still applies.
     
  7. May 7, 2012 #6
    I don't think Bernoulli's principle plays a role here.
    Since atmospheric pressure acts equally on both arms of the tube, it does not pump water form the higher to the lower container.It is actually the difference of pressure in the two unequal arms of the siphon that pumps the water out.The height of the water column above the surface of the liquid in the upper container is lesser than the column in the lower container.
    Water is pulled down in the column on the left with lesser force than the pull in the column on the right, so the resultant force pulls water into the lower container.
     
  8. May 7, 2012 #7
    Of course, I do agree with your reason about the difference in pressures being the cause for water pumping. But as to why.... doesn't Bernoulli play a role(along with the initial difference in forces and gravity) in maintaining the pressure difference due to the moving water?
     
  9. May 7, 2012 #8

    sophiecentaur

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    Bernouli's principle doesn' t come into the basic syphon principle because a syphon works even at zero velocity. Bernouli is all about the effect of motion.
     
  10. May 7, 2012 #9
    I see. Just to clarify, does it effect the siphon after the flow has started?
     
  11. May 7, 2012 #10

    sophiecentaur

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    Pressure is affected by the velocity of a fluid so the Bernouli effect will be there. But the two effects are distinct and, in Science, being able to look at separate phenomena one at a time helps with understanding situations better.
     
  12. May 7, 2012 #11

    OmCheeto

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    Ha ha! I love this place. I didn't realize I'd missed the great siphon debate.

    Perhaps we need to look at this from the 'frictionless disks in a u-tube' analogy.

    pf20120507great_siphon_debate.jpg

    The above image is at Tinitial
    Initial air pressures are atmospheric.
    What will the image look like at Tfinal?
    And what will be the pressures between the disks?

    The image is not drawn to scale. The tube above and below the disks extends to 100 meters.
    Tube diameter is 1 meter.

    ps. I just made this up. It is not an actual homework assignment, that I'm aware of. Recommendations for parametrization will be most humbly accepted. :smile:
    pps. Must now go to the river.
     
  13. May 8, 2012 #12
    Aye, thanks.
     
  14. May 8, 2012 #13

    Andy Resnick

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    For some reason, this topic flared up in 'Physics Today' recently:

    http://www.physicstoday.org/resource/1/phtoad/v64/i4/p11_s1 [Broken]
    http://www.physicstoday.org/resource/1/phtoad/v64/i8/p10_s3 [Broken]
    http://www.physicstoday.org/resource/1/phtoad/v64/i11/p10_s2 [Broken]

    Personally, I think it's refreshing that something so 'obvious' is anything but.
     
    Last edited by a moderator: May 6, 2017
  15. May 8, 2012 #14

    russ_watters

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    Personally, I find it pathetic that supposed experts can get such a simple concept wrong/obfuscated.

    I guess someone needs to put a siphon in a vacuum chamber on youtube to demonstrate.
     
  16. May 8, 2012 #15
    I asked the very same siphon question to my professor last year (I am still in high school) who gave me the same answer that I posted in the thread. I'm just glad I got the concept cleared up now :smile:
     
  17. May 8, 2012 #16

    Andy Resnick

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  18. May 8, 2012 #17

    sophiecentaur

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    Excellent demo. A real Myth-buster.
    It does require a special kind of liquid to work in the first place though - as they make clear. The vapour pressure of the liquid needs to be low enough to avoid the formation of cavities at the low pressure point at the top of the inverted U.
    There have been claims that you can get a water siphon to work well over the '10m limit' but I find that difficult to believe / explain.

    To be fair to the dictionaries, the fact that practical siphons need atmospheric pressure to prevent boiling makes the conventional description excusable.
     
  19. May 8, 2012 #18

    russ_watters

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  20. May 8, 2012 #19

    russ_watters

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    Er, well - to prevent boiling and to push the liquid up the tube! Typical liquids do not have much in the way of "tensile strength", so they can't pull themselves up a tube. As the wiki mentions, you can easily demonstrate that tensile strength is irrelevant to a normal siphon by starting a siphon with a bubble in it.
     
  21. May 8, 2012 #20

    sophiecentaur

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    Too right.
    And, if most liquids have very low tensile strength, then the tensile strength explanation is not actually relevant to most liquids. It is an interesting slant on the whole thing and adds a healthy amount of confusion to the situation.
    Could turn out to be a bit of a big-endian and little-endian clash here.
     
  22. May 8, 2012 #21

    OmCheeto

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    When I first read the "tensile strength" argument of liquids, I once again thought this was another April fools joke:(adding water to rocks makes them melt at a lower temperature.)

    After all, I can reach into a sink full of water and pull the water apart very easily.

    But then again, if I fill a hypodermic syringe with water, put my finger over the little end thing, and pull on that big-endian thing, I feel a tension.

    So something is going on. :rolleyes:

    Anyways, my faux homework problem was based on the PDF I briefly looked at yesterday: www.phys.uhh.hawaii.edu/documents/TPT-final.pdf
     
  23. May 9, 2012 #22

    sophiecentaur

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    What you are feeling when you do that is the effect of atmospheric pressure on the back of the piston (the bit that's in the air). You have to fight against that to pull the piston out. You can,in fact, 'beat' a piston with a very small diameter and pull it out against the vacuum which forms inside. (The pressure times the small area gives a small resulting force). Any tensile force that you may be experiencing is very small - the same sort of force that pulls the edges of a water surface up the side of a glass. These forces are sufficient to prove embarrassing for small insects who want to get out from under a water surface and useful for pond skaters, who spend their lives standing on the top of the water. It is only at that scale of weight / forces that the inter molecular forces in water become significant.
    If you did the same syringe thing up in space, you could easily pull the plunger out and the water would start to bubble as the pressure inside became less than the vapour pressure.
     
  24. May 9, 2012 #23
  25. May 9, 2012 #24

    A.T.

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    What I don't like about the wiki is : "atmospheric pressure is the driving mechanism".

    For equal pressure siphons (be it 1bar or vacuum on both ends) gravity (acting on the fluid) is the driving mechanism. Equal pressure on both sides cannot drive anything. It just creates an offset (equal on both sides) to gravity. But it is the differential weight of the columns which causes the flow, and thus "drives" it.
     
  26. May 9, 2012 #25

    sophiecentaur

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    It all boils down to gravity in the end, of course. No g, no AP.
     
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