What Forces Must a Siphon Overcome to Work?

[McQueen]

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" 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 anyone 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 ?

 

[gsal]

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...

 

[McQueen]

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.

 

[NascentOxygen]

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?

 

[McQueen]

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.

 

[gsal]

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.

 

[McQueen]

gsal,
Maybe you are right, because if there is no gravity and no atmospheric pressure, what difference would a centrifuge make ?

 

[gsal]

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?

 

[NascentOxygen]

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.

Cavitation! I'm talking about x1.5-x2 not x20 g ! If there is a dependency, a small increase in g should reveal it.

 

[McQueen]

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.

 

[russ_watters]

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:

Theoretically a siphon should work in a vacuum, at least that's what everyone says, although personally I have my doubts.

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.

 

[McQueen]

Russ Waters,

A siphon will not work in a vacuum. Without air pressure, it cannot stay filled (consider a barometer).

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.

 

[russ_watters]

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.

 

[McQueen]

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.

 

[gsal]

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.

 

[McQueen]

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! Let's hear what you have to say, after doing the experiment!

 

[gsal]

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.

 

[russ_watters]

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.

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.

 

[russ_watters]

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.

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 that's 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.

 

[russ_watters]

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.

 

[russ_watters]

You had a similar typical siphon:

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.

No. You just calculated the hydrostatic pressure at the bottom of each beaker. This is irrelevant. The hydrostatic pressure that is relevant is the pressure at the top of the siphon. You need to consider in this analysis the height of the siphon.

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.

In the absence of atmospheric pressure, the siphon empties itself out into each beaker, leaving the siphon "filled" with a vacuum. There can be no continuous flow because there is no air pressure to keep the siphon filled.

Please look at the operation of a typical barometer for an explanation of why this is: http://en.wikipedia.org/wiki/Barometer

 

[gsal]

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. (but with air pressure?)
6. Repeat #1 and #2 with no air pressure (but with gravitational force?).

Hhhmmm...

I thought I could make things easier if the tube had an I.D of 2/sqrt(pi) cm, meaning, an internal cross section of 1 cm^2, also, I would like to assume that this experiment is taking place at a very convenient spot on the side of a hill where the atmospheric pressure is exactly 10,000 mm of water at the water surface of the higher vessel and just about the same at the lower vessel...but I am not sure if any of this helps.

...having said that, I still don't like this experiment; in particular, I don't like that valve!Answers (or attempts):
1.- I still don't quite know...am I supposed to count the pressure from the atmosphere? and substract that of the column of water? Pressures...at 1: it should be 10,000-1000=9000? and at 2, 10000-2000=8000?

2.- The water in the siphon flows to the right, thanks to a pressure differential of 1000 mm of water courtesy of gravity influence on water column.

3.- Pressures...at 1: 10,000-500=9500; at 2: 10,000-1000=9000. Open valve, water flows to the right, thanks to a pressure differential of 500 mm of water courtesy of gravity influence on water column.

4.- Pressures...at 1: 5,000-1000=4000; at 2: 5000-2000=3000. Open valve, water flows to the right, thanks to a pressure differential of 1000 mm of water courtesy of gravity influence on water column.

5.- Pressures:...at 1: 10,000; at 2: 10,000.0001 ...would this make the water flow from the lower vessel to the higher one? Does this prove that the siphon works thanks to gravity?

6.- Pressure:...at 1: -1000; at 2: -2000 ... water will flow from the top vessel to the bottom ...just like it is supposed to do. Does this prove that the siphon works thanks to gravity?

are these self-fulfilling answers? does this prove that the siphon works mostly thanks to gravity...just like I have been saying all along?

 

[McQueen]

Russ,
Taking the experiment you did in #20 and supposing the whole system is in a vacuum. The height of liquid in one beaker is 10 cms and the height of the liquid in the other beaker is 2cms. Since the experiment is being conducted in a vacuum, it is no use just sucking on the tube to create a greater vacuum. Instead fill the tube with liquid and then immerse the ends to the depths as shown in your diagram. Surely the hydrostatic pressure in the beaker with 10 cms depth which might be 10 gms/cm² will be greater than the pressure in the beaker with a depth of 2 cms of liquid? Just a query. Even if gravity is taken into account, the molecular attraction of the liquid molecules should overcome this, since gravity is a very weak force and the liquid should flow from the beaker with 10cms into the beaker with 2 cm depth. Alternatively instead of taking the siphon over the lip of the beaker it was connected at the bottom THEN liquid would surely flow from one beaker to the other, till both the levels were the same. I still maintain that gravity has very little to do with the working of a siphon and it is mainly due to difference in hydrostatic pressure. BUT don't mind me, I aplogise if you thought I was getting too vehement. Sorry. P.S I was assuming here that both beakers were on the same level!

 

[McQueen]

Russ, Re your post #21, I have not stated that there will be a continuous flow of water. What I said was that water will flow out of pipe (d) creating a partial vacuum in container A causing water to flow into container A from container B through pipe (c) because of atmospheric pressure. But that soon an equilibrium is reached and water ceases to flow. IF pipe (d) is lowered below container B then container B will be almost emptied of water, because water will flow continuously. I know this for a fact because I have done the experiment.

 

[McQueen]

Incidentally taking gsal's point of a tube with a 1 cm² area. Suppose we had the Atlantic Ocean on one side and the 1 cm² area tube on the other. Separated by a considerable slice of land and if the 1 cm² tube was buried in the ground so that it had the same depth as the Atlantic Ocean and the two were connected. Then the level of the water in the 1 cm ² area tube would never rise over that of the height of the Atlantic Ocean. This is known as the hydrostatic paradox! Quite amazing right ? The Atlantic Ocean on one side all those billions of tons of water combined with the atmospheric pressure still cannot push the water in the 1 cm² area tube above its own level. This is what has led me to state that hydrostatic pressure plays an important role (exceeding that of gravity) in the working of a siphon. For one thing it explains why 'water always finds its own level'.

 

[gsal]

McQueen:

You have not answer my question in post #17...what do you say?

 

[McQueen]

Gsal,
It goes without saying that without gravity nothing would work. I was attempting to come down to causative factors. For instance in my example of the broken siphon, if gravity were the determining factor, water should flow out of container A under the force of gravity, there is seemingly nothing to prevent it from doing so. Why doesn't the water flow, I have attempted to give an explanation, obviously in this case the force of gravity cannot compete with other factors or forces, so I felt any explanation of exactly how a siphon works would be clearer using a broken siphon than just using an ordinary siphon. You might have noticed that the example I have given of the broken siphon is a variation of heros fountain.

 

[McQueen]

Hi,
Here is a youtube link that demonstrates what I am saying: http://www.youtube.com/watch?v=70Smyiu09rY&feature=related"

 

[McQueen]

Hi,
I am a bit surprised that no-one has commented on the video link ! Surely this experiment proves conclusively that (a) minute variations in atmospheric pressure affect the working of a siphon ( all according to Newton’s Laws) (i.e., when a vacuum is created in the top most bottle through the flow of water out of the bottle, atmospheric pressure pushes water out of the lower bottle on the table into the top most bottle ) (b) gravity seems to have nothing to do with the question of how a siphon works. Surely more important is not the question of how a siphon works but that of the question of when a siphon will work! Or to put it more simply ‘ what constraints stop a siphon from working! Here is my take on the situation. IF the pipe leading out of the top most container into the lower most container is held above the level of the container on the table , no water will flow. WHY? Obviously, it has to do with hydrostatic pressure. I have performed the experiment myself and have noticed that, at the start of the experiment, since atmospheric pressure is equal at the ends of both pipes, water flows out of the pipe leading out from the top most container ( since it is at a higher level), but it soon stops. And again the question is why? Common sense tells us that when the water stops flowing out of the container on the table into the top most container , the water in the pipe connecting the two containers should drop back into the container on the table leaving the connecting pipe empty, this does not happen, also the pipe connecting the topp most container and the atmosphere ( lower most container) remains full of water, but that water does not flow out of the pipe ( from the top most container) and into the atmosphere. Why ? Common sense tells us that gravity should pull the water out of the pipe leading from the top most container through the pipe leading to the atmosphere, yet this does not happen! Obviously the vacuum created in the top most container is strong enough to support the hydrostatic pressure of the water in the pipe leading from the water container on the table into the top most container, thus retaining water in the pipe. But the weight of the water in the pipe leading out of the top most container ( if it is shorter than the pipe leading from the container on the table to the top most container) will be easily supported by that same atmospheric pressure, since it is shorter ( less hydrostatic pressure) !. Not until the pipe leading out of the top most container is lowered below the level of water container on the table will water start to flow. Ergo, Hydrostatic pressure rules, not gravity!

 

[tony_physic]

Gravity is required for a siphon. One it provides the air pressure which is required. Secondly it provides the force to move the liquid.
From the diagram the water in pipe d falls under gravity, reducing the level in flask A. This lowers the air pressure in A, Providing the pressure difference between the outside and that inside of A is greater than the weight of water in pipe c water will be forced up c into A, allowing more water to fall from d. When the weight of water in pipe c is equal to the pressure difference between that in A and the outside the flow will stop