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Broken siphon and gravity

  1. Nov 15, 2011 #1
    Dr. Hughes, an Australian physicist has taken exception to the Oxford Dictionary’s definition of a siphon, http://www.telegraph.co.uk/news/worldnews/australiaandthepacific/australia/7709513/Physicist-spots-99-year-old-mistake-in-Oxford-English-Dictionary.html" [Broken] claiming that a siphon works by gravity rather than by atmospheric pressure. Many people still disagree with this view. I thought that if there was any one forum where the matter could be probably be decided one way or the other, it was probably Physics Forums. And to help decide this issue there is no better place to start than with the broken siphon. Given below is a diagram of a broken siphon. At the beginning of the experiment container B is four fifths full of water and container A is filled with a little bit of water which is retained by closing pipe d. Pipe c extends from the bottom of container B to near the top of container A, pipe d leads from the bottom of container A to the atmosphere. When pipe d is opened, water begins to flow from pipe d to the atmosphere, and water flows from container B into container A through pipe c. This continues for a short time after which the flow of water stops. So how does a siphon work ?
     

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  2. jcsd
  3. Nov 16, 2011 #2
    Hhhhmmm...well, I am no expert, but I will contribute with my 2 cents.

    I personally always have thought that it is gravity that makes a siphon work and that's why the "second" half of the siphon needs to be longer (have more water free to fall down) than the "first" half.

    I bet experiments can be conducted with same hose diameter and various fluids of varying density and see if the atmospheric pressure alone accounts for the amount of flow or needs the weight of the fluid itself to account for it...siphon water, siphon mercury...
     
  4. Nov 16, 2011 #3
    gsal .
    I beg to disagree, I claim that gravity has absolutely nothing to do with the working of a siphon! And that means that Dr. Hughes of the University of Technology in Brisbane was wrong!! And also that that the Oxford Dictionary was wrong in changing its deifinition of how a siphon actually works! Or at least it was wrong in its new definition of a siphon and how it works.
     
  5. Nov 16, 2011 #4

    NascentOxygen

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    If it "works by gravity" does it follow that we could increase gravity (by placing the apparatus in a centrifuge) and notice a change in its operation? If it "works by atmospheric pressure" could we place it in a sealed vessel and increase or decrease the air pressure and observe a predictable change?
     
  6. Nov 16, 2011 #5
    Theoretically a siphon should work in a vacuum, at least that's what everyone says, although personally I have my doubts. And, unfortunately yes, a siphon should work better in a centrifuge, although then we would have to deal with the problem of cavitation which would make any calculations awfully difficult.
     
  7. Nov 16, 2011 #6
    McQueen: o.k., you "beg to disagree", please offer your reasoning.

    Nascent: No, you will not notice a change on the siphon operation if you increase gravity since both sides of the siphon would be subjected to the same gravity...just like a balance scale would work just as well here no earth as in mars.
     
  8. Nov 16, 2011 #7
    gsal,
    Maybe you are right, because if there is no gravity and no atmospheric pressure, what difference would a centrifuge make ???
     
  9. Nov 16, 2011 #8
    Along with the experiments offered earlier, here are another two.

    Get in a pressurized chamber with two water tanks at "different heights" and no gravity...would you be able to siphon? No, because there is no gravity...then, again, you could argue that the pressures are the same on both sides.

    O.k., now.

    Bring that chamber back into the influence of gravity, but make sure to pressurize it enough so that the influence of gravity is negligible...can you siphon? I would say definitely yes.

    any thoughts?
     
  10. Nov 16, 2011 #9

    NascentOxygen

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    Cavitation! :surprised :surprised I'm talking about x1.5-x2 not x20 g ! If there is a dependency, a small increase in g should reveal it.
     
  11. Nov 16, 2011 #10
    Gsal,
    I admit I have been toying with this topic BUT I wish to state that the affront of Dr. Hughes of the University of Technology , Brisbane in claiming that the Oxford Dictionary was wrong in its definition of how a siphon works, and THEN giving a wrong explanation, was what led me to this deception. The answer is ludicrously simple ! AND it has nothing to do with gravity, although admittedly gravity does have a part to play in the speed at which a siphon works. To put it in a nutshell it is a combination of hydrostatic pressure and atmospheric pressure that make a siphon work. . Look again at the diagram of the broken siphon. A normal siphon will work even when the end of the siphon is just lower than the water level in container B and once again it is hydrostatic pressure that is the key. However in the case of a broken siphon, follow my reasoning:
    Water flows out of pipe d creating a partial vacuum in container A and atmospheric pressure then forces water out of container B into container A through atmospheric pressure. But as soon as an equilibrium is reached the water stops to flow, either into container A through pipe c or out of container A through pipe D. WHY ?? The answer is simple if the vacuum in container A can support the hydrostatic pressure of the column of water in pipe c then OBVIOUSLY it can also support the hydrostatic pressure in pipe d, since it is shorter than the column of water in pipe c! Remember that hydrostatic pressure has NOTHING to do with area it has only to do with the height of the column of water. The American poet Longfellow was bemused by the fact that even an ocean of water could be supported by a narrow column of water PROVIDING its height was greater than or equal to that of the ocean! So hydrostatic pressure is the key. Bring pipe d below the level of water in container B and water will continue to flow out of container B into container A and out through pipe d until all the water is gone! Lower pipe d still further and the flow of water will be faster ! So I would like to recommend to the moderators at physics forums to write to the Oxford Dictionary, requesting that the word gravitation be removed from their definition of how a siphon works and to substitute instead atmospheric pressure and hydrostatic pressure. Dr. Hughes explanation of how a siphon works featured in all the major newspapers and it is a blatantly wrong explanation.
     
    Last edited: Nov 16, 2011
  12. Nov 16, 2011 #11

    russ_watters

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    Most of the explanations so far have been wrong. A siphon requires both gravity and air pressure. You might want to do a search because we've discussed this before. But briefly:
    A siphon will not work in a vacuum. Without air pressure, it cannot stay filled (consider a barometer).

    A siphon works due to the pressure difference due to the difference in height between two columns of water. The pressures are there due to the actions of the two water columns against atmospheric pressure. You can't just say it is one or the other.

    Trya this: calculate the pressure at the top of a normal siphon, working from each direction.
     
    Last edited: Nov 16, 2011
  13. Nov 16, 2011 #12
    Russ Waters,
    Let us take the ideal situation , say in near orbit to earth, where atmospheric pressure is absent, but gravity, albeit weakened, still exists. Will a siphon work? According to Newton, a resounding yes! Why? Even this weakened gravity (of the earth) would create a force differential between the hydrostatic pressure existing in the system (referring to my diagram) of tube c + (water) in container B and tube d coming out of container A. End result no flow of water, if tube d is above the level of the water in container B! Lower the tube d below the level of the water in container B and Lo and behold water will flow out of tube d! Lower tube d still further below container A and water will flow faster. So yes, gravity has nothing to do with the siphon except for raising or lowering the speed at which water flows. And this holds true even if it were talking place in a vacuum, although I do admit that if the experiment were far enough away from an appreciable gravitational force the force differential might be so slight as to be almost indiscernible. To summarize, it is hydrostatic pressure that drives a siphon. On earth it is the force of atmospheric pressure and hydrostatic pressure which together govern the working of a siphon. Gravitation has nothing to do with it !!! I therefore repeat my central point that Dr, Hughes was wrong in stating that a siphon works by gravitational force.
     
  14. Nov 16, 2011 #13

    russ_watters

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    Both his explanation and yours are a mess. His because you can't pull on a liquid. Yours because you just defined a scenario with neither gravitational force (in orbit) nor air pressure.

    Don't overcomplicate this. Draw a normal siphon, put some numbers on it and solve for the pressures in it.
     
  15. Nov 16, 2011 #14
    Russ waters,
    How can YOU (emphasis there) calculate the difference in pressure if gravitational force has nothing to do with it, since you follow Dr. Hughes and claim that a siphon works due to gravity. OK! since you insist, Take a simple situation of two beakers one filled with water to a height of 10 cms and the other to a height of 5 cms. Insert a tube in the 10 cm beaker, suck out all the air in the tube so that it fills with water and immerse the other end of the tube in the 5 cm beaker.What happens ? The hydrostatic pressure in the 10 cm beaker is 100 Kg/cm2 and the hydrostatic pressure of the water in the 5 cm beaker is 50 Kg/cm2. In the absence of atmospheric pressure what do you expect to happen? I will tell you what does happen, water flows from the 10 cm beaker into the 5 cm beaker till both pressures are equalised. Coming back to your reference to my explanation of how a siphon works being a 'mess' ! I would greatly appreciate an apology! Personally, as far as this topic is concerned. I think that you have to get your facts right.
     
  16. Nov 16, 2011 #15
    O.k., so maybe it is a combination of both atmospheric pressure and gravity, but I would insist it is probably gravity that has the most influence.

    The things is that we should be clear how it is we think gravity is involved or not. Certainly, it is gravity, in the first place, the reason why we have atmospheric pressure...but I myself was not using atmospheric pressure to explain involvement of gravity...I thinking more along the line that it is the weight of water on the "second" half of the siphon that allows the water to flow.

    McQueen: you mention that it is hydrostatic pressure that helps, I think this is what I had in mind..after all, if it was not for gravity, you wouldn't have hydrostatic pressure...then, again, I regress (sp?) maybe the involvement of gravity in this context is the same that produces atmospheric pressure.

    russ: you can't pull on liquid? Sure you can...how do you think I collect gasoline from the neighbor's truck every friday night? With a hose, a little vacuum and I can pull liquid against gravity.

    By the way...if you have to tanks of water at different height and you bring the typical siphon, empty, and put it in place...would water start flowing all by itself? No!...in order for a siphon to work, it needs to be jump-started by filling with water in the first place...the little atmospheric pressure differential alone cannot pull this off...you need the weight of the water on the "second" half of the siphon for it to work.
     
  17. Nov 16, 2011 #16
    gsal,
    Look I have said before and I will say it again GRAVITY has nothing to do with how a siphon works!!! Gravity only regulates the speed at which a siphon works AFTER all other forces, (i.e., atmospheric and hydrostatic) have been dealt with, and it is only when there is an imbalance between these forces that gravity comes into play. As a matter of fact I would have agreed with you BEFORE I actually did the experiment! BUT since you seem to be an adept with siphons, WHY don't you do the experiment as illustrated in the diagram I posted? You will think it is black magic but will soon see that gravity has nothing at all to do with how a siphon works. All it needs is some lengths of small diameter plastic tubing and two PVC water bottles. You can make the top bottle ( which has to be air tight) airtight by just using a soldering iron to make the holes in the bottle cap, fixing the two tubes in them as illustrated and maybe use window putty or some such thing to seal it, and you are all set to go! Lets hear what you have to say, after doing the experiment!
     
    Last edited: Nov 16, 2011
  18. Nov 16, 2011 #17
    McQueen:

    I followed your explanation in posting #10, that's fine and I agree with it.

    I just don't know if it is a matter of semantics, now, that makes us be or not be in agreement.

    The reason why I say a siphon works thanks to gravity is because it is gravity that's pulling the water in the "second" half of the siphon, which in turn pulls the water up from the higher tank. Period.

    I don't think atmospheric pressure helps here; in fact, you could even say that atmospheric pressure is working against the siphon and is unable to contain it..think about it...if anything, atmospheric pressure is ever so slightly higher at the surface of the water of the lower tank and weaker at the surface of the higher tank. How is that supposed to make the siphon produce a flow?

    So, you say "poteyto", I say "potato"...would you say that hydrostatic pressure is due to gravity, would you say that hydrostatic pressure is due to the weight of the water? Then, we are in agreement. I think.

    Please comment/confirm.
     
  19. Nov 16, 2011 #18

    russ_watters

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    I said no such thing. Please, McQueen, calm down, slow down and pay more attention to detail. There are pieces of your explanation that are correct, but seemingly due to your aggression, you're being argumentative and contradicting yourself in places.

    The key issue that brings in air pressure is that you cannot pull on water.
     
    Last edited: Nov 16, 2011
  20. Nov 16, 2011 #19

    russ_watters

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    Incorrect. You can only push on a liquid. It has no structure to grab onto to pull it!

    When you prime a siphon, you "suck" on one end, but thats not a scientific description. What you are really doing is reducing the pressure in your mouth enough for the atmosphere to push the liquid up the tube. So in the absence of an atmosphere, the liquid cannot go up the tube.
     
  21. Nov 17, 2011 #20

    russ_watters

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    Attached is a diagram of a typical siphon, with one modification: a valve is added to turn a dynamic situation into a static one to make the analysis easier (possible). I suggest when the issue is how a typical siphon works, we look at a typical siphon.

    My siphon shows a 1m height difference between the vessels (higher on left) and an additional 1m to the top of the tube. Questions:

    1. What is the pressure at points 1 and 2 (at the top of the siphon, on either side of the valve)?
    2. What happens when you open the valve (qualitatively)?
    3. Repeat #1 and #2 with half gravitational force (but full air pressure).
    4. Repeat #1 and #2 with half air pressure (but full gravitational force).

    and a couple more:
    5. Repeat #1 and #2 with no gravitational force.
    6. Repeat #1 and #2 with no air pressure.
     

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