QUT physicist corrects Oxford English Dictionary (w/ Video)

Aren't both explanations right, but both lacking the other half? As far as I can tell, gravity pulls liquid in the lower leg down, creating a lower pressure in the top of the siphon, thus allowing atmospheric pressure to push water up the upper leg.

I'm also fairly sure that the conceptual chain model is completely wrong - I don't think any amount of tensile strength in the liquid is required, from hydrogen bonds or otherwise. Gravity and pressure will take care of everything.

Of course, atmospheric pressure is the result of gravity, but the gravity itself is a result of geometry of omnipresent tachyon flux - so it should be definitelly more correct to say, sifon works because of tachyons for not to keep the layman people disorientated... >:-\

I agree with Saraphim: it seems obvious that there would be no significant siphon without both gravity and atmospheric pressure.

But since Dr Hughes is content to ascribe the entire effect to gravity and intermolecular attraction, I would like someone with access a laboratory to perform a rate-of-flow experiment at 1 bar, 0.5 bar, and 0.1 bar, and post the video to YouTube.

Call me an old empiricist, but I think Dr Hughes should just do the experiment.

..rate-of-flow experiment at 1 bar, 0.5 bar, and 0.1 bar, and post the video to YouTube..

Sifon could be working even at zero pressure due the insintric cohesion of fluid. But it would require very clean pipes without holes, outgassed fluid, etc... In real cases the presence of atmospheric pressure is necessary for having sifon to work.

Siphons purely by shape and gravity do not function- an original catalyst is necessary, such as a change in pressure great enough to force liquid past the arm in enough volume to allow gravity and the "molecules pulling on each other" to take over and keep the siphon functioning. Why is this not mentioned?

Think About it: you use atmospheric pressure to start the syphon of, however once the syphon is flowing the atmospheric pressure on both sides of the tube is (almost) equal and therefore negate eachother. the only effect left is gravity, just like water flowing down a stream. Atmospheric pressure is actually very slightly higher on the lower end of the tube which would tend to cause the opposite effect. Think about it its obviously gravity once the syphon is running!

@ Sazzle: I'm not referring to the effect of an atmospheric pressure delta between the two container surfaces.

Imagine first just the bend and the descending pipe, the end of which is submerged in the lower container, preventing air from leaking up the pipe, such as occurs when you dump out a soda bottle.

In my thinking, the pressure delta that "pushes" the liquid through the tube is between the surface of the top container and _the vacuum that would be created_ if the liquid in the down-pipe were to separate mid-length.

In your model, what is to keep the level at the top of the bend from simply dropping (vacuum!) down both sides of the bend? If it were the "hydrogen bonds", then an equal volume of water would have to sit on a table like pudding, right?

As I said before, my intuition says both gravity and air pressure are required. But I would like to see the experiment.

Think About it: you use atmospheric pressure to start the syphon of, however once the syphon is flowing the atmospheric pressure on both sides of the tube is (almost) equal and therefore negate eachother. the only effect left is gravity, just like water flowing down a stream. Atmospheric pressure is actually very slightly higher on the lower end of the tube which would tend to cause the opposite effect. Think about it its obviously gravity once the syphon is running!

Okay, but what would happen to the flow if air pressure was removed?

A syphon would work in a vacuum, it has nothing to do with atmospheric pressure, it is purely gravity. In fact it probably works even better in a vacuum. The only way atmospheric pressure would affect a syphon is if the source and receiver reservoirs were in separate closed containers where the change in liquid volume would lower or raise their individual air pressures. A syphon won't work in zero G.

Since air pressure is not the general cause, while being in some cases, involved, it is itself an effect of gravity. No gravity = no atmosphere, no atmosphere = no air pressure.

A siphon would not work in a zero-atmospheric-pressure environment, because the water would boil away before we could do our experiment. However, if we had a liquid that would not boil away in zero environment, the siphon would work in a vaccuum, if there were still gravity.

Pressure becomes important only if the vessel being drained does not have any way for air to enter into the vessel. If it does not have any way to enter, then the external atmospheric pressure will eventually equal the force of gravity operating on the liquid, stopping the siphon.

W

There is both gravity and atmospheric pressure involved. Gravity from the top of the tube to the bottom of each side, plus atmospheric pressure on both sides. The atmospheric pressure pushes the water to the top of the tube, and the height difference between the two side keeps the water moving the right way.

If the top of the tube was higher than about 10 metres above the outside water levels you would get a vacuum forming in the top of the tube from the weight of the water on both sides of the peak. This cavitation would prevent the siphon from working.

Continued from my previous comment...

If you had a lower atmospheric pressure the siphon would still work, but you would not be able to raise the top so high before the liquid cavitated. Zero atmospheric pressure would not allow the liquid to be raised to any height. Hydrogen bonds may make a trivial difference to the exact numbers.

A siphon would not work in a zero-atmospheric-pressure environment, because the water would boil away before we could do our experiment. However, if we had a liquid that would not boil away in zero environment, the siphon would work in a vaccuum, if there were still gravity.

Correct

A siphon would not work in a zero-atmospheric-pressure environment, because the water would boil away before we could do our experiment. However, if we had a liquid that would not boil away in zero environment, the siphon would work in a vaccuum, if there were still gravity.

Correct

Incorrect, for the reason explained above.

Remember the Magic Glass trick you did as a kid? The one where you fill a glass to the brim with water, slide a playing card over the top while not letting in any air. Hold the card on with your hand and turn the glass upside down. You take your hand away and the card sticks to the water, and the water does not fall out of the glass.

That is the atmosphere holding the water up - the exact same force that lifts the water to the top of the siphon. No atmospheric pressure would mean that the water would fall out of the glass, just as it would mean the water could not get up a siphon. Same thing.

Im think it might work in a vacumn, but I'm not sure. I'd guess differnt liquids would behave differently.) I think what's confusing people about the necessity of atmos pressure is WHY its needed. It may play a crucial role in 'pulling' the water over the hump by creating a lower pressure, or not. because there's a whole other way of looking at it- the air can only help in the hump if there's air present. If the tube is completely filled with water then air cannont be playing a role, can it? (I doubt that the gases usually 'mixed' in water matter here) The idea of water 'cavitating' sounds plausible, but water may act just like air in reducing it's pressure at the hump, thus 'pulling' the water through. Water is almost completely uncompressable but it's pressure does of course vary. I'd guess its this pressure variation alone that makes it work, And why I lean toward it working in vacumn. I know nothing about hydrogen bonds so dont know there relevance.

I give up.

Go back to school kids, and this time do science.

This is a little confusing for a layman!

Is Wikipedia also incorrect, by the way?

Just take a look at http://en.wikiped.../Siphon. The siphon can be fully explained by Bernouilli's law for an inviscid flow, like I vaguely remember from my fluid dynamics course 25 years ago. So pressure, gravity and flow speed are all playing a role. It is just better to state that a siphon is working via Bernouilli's principle.
In order for the water to flow, the end of the tube has to be lower in position than the beginning of the tube (see video 2). Video 1 is confusing. I have the impression that the water is flowing out of the tubes instead of flowing in. This would be only possible by using a pump.

I hope this means gravity mills are a viable concept in low earth orbit.

Syphon, sifon, do you people misspell words on purpose? The correct spelling is peppered all over this page. Maybe you should get a copy of this dictionary for yourselves.

Also bravo to the guy that pointed out water would boil away in zero atmosphere and nullify that experiment. I hadn't thought of it until you mentioned it - very true!

For the vacuum experiment one would have to use a liquid with a low vapor pressure. My vote is for mercury at 235 K, just above its freezing point. Cyberguy's impression that even in a column of short height any liquid would form a vapor bubble which would destroy the siphon is correct, so long as he is planning on ignoring capillary action. If the mercury is attracted to the siphon wall and not repelled (I'm guessing a metal tube would work), then the column wall will partially support the weight of the mercury. Just make sure the tube is thin and that it's ascending arm is very short.

The trick is, siphon could work even for the atmospheric pressure bellow vapour pressure of fluid. The tensile strength of water is variously quoted, but it is no less than 3,000 psi.

Imagine a small bubble of gas in water; the pressure in this little sphere is given by

P = S*pi*2*R/(pi*R^2) = 2*S/R circumference*tension/area of
equatorial plane)

where S is the surface tension in dynes/cm (about 75) and R is the radius in cm.

Water molecules are of the order of, say, 10^-8 cm, so let's size our arbitrary bubble at 10^-7 cm, a roughly molecular level hollow sphere.

The calculated pressure is then of the order

2*80/(10^-7) dynes/cm2 = 1.5*10^9 dynes/cm2 which translates into about 2*10^4 psi.

http://iopscience...0/8/018/

Watter in vacuum evaporates and freezes fast, but in large volume it's quite maintenable - after all, like the liquid ammonia or even liquid nitrogen at room pressure, which I'm dealing with often. In can be poured from flask to flask easily. The interesting demonstration of the fact, for siphon effect atmospheric pressure isn't crucial could serve a superfluid hellium - for such fluid a whole surface of vessel can serve as a siphon.

Everyone should go back and read what "Cyberguy" has offered above as explanation. Remember that experiment with the 30-meter glass tube from which all gases have been removed - a vacuum - place one end in a swimming pool and remove the cap from beneath the water's surface. Water will rush up into the tube and replace the vacuum BUT ONLY to a certain height, about 10.3m at sea level. Barometric pressure, inches-of-mercury and all that jazz.

Okay another question, why does the draining end of a siphon always need to be lower than the intake end? Isn't this where the difference in pressure from the intake and outflow really comes into play? It is not the atmospheric pressure which is the same at both ends.

From that statement it would appear that the atmospheric pressure would not be needed. And it would not be if water molecules were cohesive enough.

Even leaving water evaporation in a vacuum out of the equation isn't there still another problem? Beyond the point of molecular cohesiveness wouldn't the fluid simply drain out both the intake and outflow ends with no siphoning taking place? Wouldn't the outside vacuum simply be transferred into the neck of the siphon stopping the flow of the fluid?

Seems we are kind of missing the basic fact that atmospheric pressure INCREASES as you decrease in elevation. Thus, the atmospheric pressure is HIGHER at the lower end of the tube, and if AP was the motivating force, the fluid would flow uphill.

Therefore, gravity must be the motivating force.

Everyone should go back and read what "Cyberguy" has offered above as explanation. Remember that experiment with the 30-meter glass tube from which all gases have been removed - a vacuum - place one end in a swimming pool and remove the cap from beneath the water's surface. Water will rush up into the tube and replace the vacuum BUT ONLY to a certain height, about 10.3m at sea level. Barometric pressure, inches-of-mercury and all that jazz.

Thanks, that pretty much sums it up, you need atmospheric pressure as well as the force of gravity.

Seems we are kind of missing the basic fact that atmospheric pressure INCREASES as you decrease in elevation. Thus, the atmospheric pressure is HIGHER at the lower end of the tube, and if AP was the motivating force, the fluid would flow uphill.

Therefore, gravity must be the motivating force.

Gravity alone cannot explain how a siphon works. As CSG88 pointed out a siphon at sea level will not work beyond 30 some feet in height. Go to a high enough elevation and it will not even work beyond 10 feet in height. The atmospheric pressure is needed to force the fluid up into the intake side of a siphon. When the weight of the fluid in the intake equals the atmospheric pressure a limit on the height of a siphon's ability to work is reached. Gravity alone cannot be the answer, if it were there would not be any limit as to how high a fluid could be siphoned.

Water will rush up into the tube and replace the vacuum BUT ONLY to a certain height, about 10.3m at sea level. Barometric pressure, inches-of-mercury and all that jazz.

I can understand your way of thinking, but some commercially available siphons doesn't require some suction or pressurization at all. They contain small centrifugal pump at the bottom.

http://www.homede..._400.jpg

The maximal "column of water" that can exist without the aid of a pump at 1 atmosphere is 10.3 meters in height. No pumpless-siphon can operate if the rising leg the water must travel will exceed an altitude differential in excess of that value.

Curiously similar problem is the "train-over-the-mountaintop". Find us some friction-less track and no-drag-wheels and the train will pull itself over the mountaintop in the direction where the output side is the lower elevation - with NO ENGINE in sight.

When imagining the train in my prior post, please think of it kinda like the water, a very large supply of train cars exists on either side of the mountain (i.e., on the level plain). If the East Plain is at 800' above sea level and the West Plain is at 200' above sea level and there are thousands and thousands of cars then the train will move over the mountain, regardless that the peak is 2,500 feet above sea level, in the direction from East-to-West.

The velocity achieved by our (frictionless) train will be proportional to the elevation difference between the East and West plains.

O.K. I'll throw in my two cents. I think it needs atmospheric pressure and gravity. At the top of the bend, the water on the longer side flows out to the destination. When it does so, it leaves a void, a vacuum where it was. Atmospheric pressure pushes water from the source to fill the void and the whole sequence is repeated. If there was no pressure, the system wouldn't care if a void was left by the evacuated water since any vacuum is as good as another as far as fluids are concerned. What motivation does water in a vacuum have to fill another vacuum? None, as far as I know. The gravity provides the energy to create the vacuum at the top, and the pressure fills in the vacuum. Hydrogen bonds? Come on, those are negligible, it is pressure that keeps the water chasing itself through the pipe.

Even siphon in vacuum could still work due the water cohesion and due the inertia of water column in motion. It would be just unstable.

Ladies and gents, Let me explain.
Atmospheric pressure only serves to initialize/start the siphon. A low pressure (imagine sucking on a hose to create a near vacuum) at the downstream end of the siphon is made sufficient enough to allow atmospheric pressure (constant on the upstream end of the siphon) to overcome the gravitational head (the distance water must be force up from the surface to the crest of the highest point of the siphon tube). Once the siphon has been initialized then gravity does the rest. The difference in the elevation makes the liquid naturally seek equilibrium. Look up Bernoulli’s equation and you will get the gist of it. I believe without the hydrophilic property of the liquid you would encounter cavitation.

Ladies and gents, Let me clarify.
Atmospheric pressure only serves to initialize/start the siphon. A low pressure (imagine sucking on a hose to create a near vacuum) at the downstream end of the siphon is made sufficient enough to allow atmospheric pressure (constant on the upstream end of the siphon) to overcome the gravitational head (the distance the liquid must be forced from the surface to the crest of the highest point of the siphon tube). Once the siphon has been initialized then gravity does the rest. The difference in the elevation makes the liquid naturally seek equilibrium. Look up Bernoulli’s equation and you will get the gist of it. I believe without hydrophilic property of the liquid you would encounter cavitation.

Ladies and gents, Let me clarify.
Atmospheric pressure only serves to initialize/start the siphon. A low pressure (imagine sucking on a hose to create a near vacuum) at the downstream end of the siphon is made sufficient enough to allow atmospheric pressure (constant on the upstream end of the siphon) to overcome the gravitational head (the distance the water must be forced from the surface to the crest of the highest point of the siphon tube). Once the siphon has been initialized then gravity does the rest. The difference in the elevation makes the liquid naturally seek equilibrium. Look up Bernoulli’s equation and you will get the gist of it. I believe without hydrophilic property of the liquid you would encounter cavitation.

waylifdotcom - wrong
waylifdotcom - wrong
waylifdotcom - wrong

Let me repeat, if you were correct a siphon would work at any height beyond 30 feet. The fact that it will not proves you wrong.

Three strikes you are out!

I didn't intend to universally define a siphon. I wanted to help explain what atmospheric pressure had to do with the function of one in the real world. You are correct that in general and on earth atmospheric pressure will not overcome the shear weight of a column of 30' of water. Or any liquid for that matter as it relates to its density. The actual mechanics of it moving a liquid from one place to another is based on gravity as demonstrated by Bernoulli. Atmospheric pressure (which is partially governed by gravity anyway) only limits its functionality.

I also wanted to apologize to everyone for the earlier triple post. I am sure everyone has received the SQL bandwidth exceeded error.

This article is just plain wrong. What's it doing on physorg?

I am so sure that the Oxford dictionary will erroneously make this change without consulting any scientists and ignore scientific facts to change the definition from correct to incorrect. It seems some of you have as little common sense as scientific insight.

Although I didn't specify a liquid when I said a syphon would work in a vacuum, water would still probably work because it wouldn't vaporize quickly. And the spelling syphon is an accepted spelling of the word according to my dictionary. A vacuum or lower pressure is not required at the end of a hose to start the action, there are 2 other ways to get it going with just a hose as people who have syphoned gas can tell you. How do you think the ones in the picture are started?

there are 2 other ways to get it going with just a hose as people who have syphoned gas can tell you. How do you think the ones in the picture are started?

The old "dip the whole hose in and cover the end with your finger" method definitely requires atmospheric pressure to work.

There seems to be a lot of armchair physicists in here. The prevailing 'wisdom' is that atmospheric pressure is not required for a siphon to work.

That notion is incorrect.

Both gravity and atmospheric pressure are required in order for a siphon to function. Excepting of course the possible capillary action of an incredibly small-bore tube, but I'd argue that that is no longer a siphon device.

With only atmospheric pressure, a siphon does not work.

With only gravity, a siphon does not work.

Water cannot pull itself through a siphon tube, rather gravity pulls the water on the 'down' side of the tube, causing a pressure drop within that 'down' side of the tube.

That pressure drop, results in an imbalance of pressure between the 'down' side and the 'up' side. The water on the 'up' side is pushed by the atmospheric pressure up and over the 'hump' in the tube.

Without gravity pulling on the 'down' side and atmospheric pressure pushing on the 'up' side, a siphon doesn't work.

ZeroX is right. I dare any of you to use atmospheric pressure to explain how Helium II (helium in a specific superfluid state) siphons itself out of any open container by forming a thin Rollin film -- without the need for a topological structure equivalent to a tube.
http://en.wikiped...lin_film

Yep. I aggree. I started designing an experiment on paper, priming the hose outside a vacuum, with a tap in the tube to prevent any flow. Then put the whole lot in a Bell jar and reduce the pressure. I then reallised that as the pressure approaches zero, the level of the liquid in the "up" side of the tube would drop towards the surface of the source container - just as the level of a mercury column would drop - meaning that the device wouldn't start when the tap was opened.

Temple is exactly correct, and explains what I said yesterday. In fact Saraphim, in the very first post, got it right also.

I am truely amazed at the uneducated armchair postings in this forum. This stuff is really trivially simple. Get a grip people!

Let's hope the Oxford English Dictionary does not replace the incomplete description of how a siphon works from 100 years ago with Dr. Hughes clearly ridiculous explanation of hydrogen bonds pulling the fluid up the intakes side of a siphon.

Oh well, it is their right to be wrong for the next 100 also if they so choose.

The atmospheric pressure is the same at both ends of the pipe in any real world example like the one in the picture at all times. ERGO it has no effect.

The atmospheric pressure is the same at both ends of the pipe in any real world example like the one in the picture at all times. ERGO it has no effect.

If atmospheric pressure has no effect, why is there a limit as to how high a fluid can be siphoned?

Atmospheric pressure is what forces the fluid up into the intake tub replacing the fluid gravity is pulling out of the outflow tub.

A siphon will not work without gravity.
A siphon also will not work in a vacuum.

A siphon will not work in a vacuum. In a normal siphon arrangement, the tube is above the initial water or fluid surface. For example, the siphon tube will be thrown over an embankment. There is no reason for the fluid to go uphill over the top of the siphon (gravity works against it) and then downhill below the initial fluid surface in a vacuum.

To start a siphon effect the tube must be filled with water and then dumped over the enbankment. Gravity will cause the fluid in the downhill side to drop out of the tube causing a partial vacuum which, by atmospheric pressure, fluid will be pushed over the top of the enbankment. In a vacumm the fluid would never be pushed over the top. So it is really a combination of both gravity and atmosheric pressure which causes the system to work.

The Oxford American, oddly enough, seems to get it right: "a pipe or tube used to convey liquid upward from a container and then down to a lower level by gravity, the liquid being made to enter the pipe by atmospheric pressure."

I do wonder about ZeroX's assertion that cohesion is enough to make a siphon work with less dependency on ambient pressure. As a practical matter, I don't think it would, because of dissolved gas in the water. But if you had 100% water, might not cohesion be enough to do the trick?

Hooray hooray hooray, three cheers for Oxford American!!!

Cohesion could not lift water even a millemeter except in a very very skinny tube. But that is not based on the principle of siphoning. It is based on capillary action, an entirely different principle.

I don't mean cohesion as the lifting force. Gravity would handle that. But we need something to prevent a vacuum from forming at the top of the siphon once the lift becomes high relative to ambient pressure.

If the siphon is very narrow it could work just by capillary action.

If the siphon is of any width, it will do nothing at all until it is primed.

That means first you must evacuate the air. No water will move anywhere while the up-side of the siphon is full of air at equal pressure. When the air is evacuated to create a vacuum in the tube, atmospheric pressure alone is sufficient to prime the siphon (ie move the water up over the hump) up to a certain height, given by Cyberguy as ~10.3 meters.

Once primed, the water is not pushed up the up-side by pressure. It is pulled up via cohesion.

A siphon taller than ~10.3 meters cannot be primed with atmospheric pressure alone. You must physically pump the water up the up-side to get it flowing over the "hump". But again, once it is primed, the siphon will continue to work due to the cohesion of water, and the driving force is gravity.

...

... stupid character limit.

There will be a limit on the height of the up-side, when the weight of the water in the up-side exceeds the cohesive (tensile?) strength of water. Above this height the water would separate to form a cavity of vacuum - thus the water would vaporize and create a pocket of water vapour with pressure equal to the atmospheric pressure at the base of the siphon. Past this point the siphon would not work.

I do not know how high this would be, but it would depend on the width of the siphon - the narrower it is, the higher it could be. Needless to say it can reach far higher than ten meters, once primed. Ten meters is simply the limit beyond which you need a pump to prime the siphon.

And once primed, the siphon operates via gravity. The atmospheric pressure makes no contribution to its continued flow.

Yeah, so I guess this would only be true in an idealized circumstance ie no dissolved gasses, no roughness on the interior of the siphon, no impurities of the water, etc. - all of which are likely to render the tensile strength of water moot under practical circumstances. So in normal conditions, with these many imperfections, it would actually be the pressure that keeps the liquid "together" inside the siphon. The limiting height then would indeed be that of atmospheric pressure; or less, I suppose, because of the vapour pressure of water.

This means there's really two kinds of siphons, and two siphon effects at work. One, in practical day-to-day siphons, relies on pressure to maintain the flow. The other, in idealized siphons, conditions are controlled enough that tensile strength will continue to drive the flow (even without any pressure - or negative pressure). These, you could indeed make to vastly taller heights as I described... I wonder if it would be possible to make one...

When it comes to raising the liquid up the pipe, you guys are really complicating a very simple concept.

There is no "tensile strength of water" in its liquid form - apart from surface tension which is very weak! Try pulling on two ends of a stream of water. There is no tug-of-war possible with liquid water, because it has no tensile strength to speak of.

The lift on the up side of a siphon is 100% due to the difference in pressure between the outside air pressure and the pressure in the liquid at the top of the column. You DON'T have to idealise anything to understand this. You DON'T have to consider pure and impure water as two separate cases.

The pressure in the top of the column can drop to zero if the column is raised high enough, in which case we get a cavity. Yes, some water vapour will occupy that space, but it will not be at atmospheric pressure, but zero.

Capillaries can raise water higher than 10.3 meters - trees do this. But that is a different phenomenon.

There is a very simple way to demonstrate that atmospheric pressure plays no part in forcing the water through a siphon. Simply fill a hose with water and insert each end into one of two level containers of water. Nothing will happen. The water will remain in the hose and will not flow in either direction.

Then lower one container and water will start to move from the higher container to the lower one. The only change is in hydrostatic pressure - atmospheric pressure is (virtually) unchanged at both ends. Therefore it is the hydrostatic pressure that is driving the transfer, not atmospheric pressure.

Then lower one container and water will start to move from the higher container to the lower one. The only change is in hydrostatic pressure - atmospheric pressure is (virtually) unchanged at both ends. Therefore it is the hydrostatic pressure that is driving the transfer, not atmospheric pressure.

Try it when the walls of the containers are higher than 30 feet above the level of the water.

Try it when the walls of the containers are higher than 30 feet above the level of the water.

All that tells you is that the hydrostatic pressure in the short leg is exceeding the atmospheric pressure and therefore allowing a vacuum to form at the top of the pipe. It does not mean that atmospheric pressure is forcing the transfer of water. In fact, technically the atmospheric pressure is going to be higher (just slightly) at the lowest end of the pipe and so if atmospheric pressure was forcing the transfer of water then it should run backwards!

So many points of view on how a syphon works. Many of them partially correct as a syphon uses lots of different rules depending on the syphon. Capillary action can prime and help run a syphon. Atmospheric pressure will help to run a syphon such as an open ended pipe with the liquid dropping out the end. the atmospheric pressure would act on the surface of the liquid at the top of the syphon but only on the much smaller exit at the end of the hose.

Gravity must have the main effect, surface tension and tensile strength, electro static charge will all play some part. Friction will also have its part to play in preventing a syphon working.

What would happen is a vacuum with a syphon set up no atmospheric pressure two vessels different hights and a primed syphon between them would it work? Harder to test would be remove gravity and a vacuum to se if once in motion the hydrostatic, surface tennsion, electrostatic or what ever would be enough to keep things going.

The atmospheric pressure does not MOVE the water through the siphon (yes, gravity does that), but it does force the water to the top of the siphon (on both sides) thereby getting the water "over the hump".

If there is insufficient atmospheric pressure for the height of the pipe, the water does not go all the way up the inside of the pipe and there is a gap in the liquid inside the pipe.

Ignore capillary action. We are not talking about that.

So - a siphon needs both atmospheric pressure to get it up to the top of the pipe, and gravity to move the liquid. See? Simple.

wttmartin9 wrote: "a syphon uses lots of different rules depending on the syphon. Capillary action can prime and help run a syphon. Atmospheric pressure will help to run a syphon such as an open ended pipe with the liquid dropping out the end. the atmospheric pressure would act on the surface of the liquid at the top of the syphon but only on the much smaller exit at the end of the hose."

Wrong, wrong and wrong.

To everyone who thinks that liquid water has a tensile strength, please enlighten me.

What is the tensile strength of liquid water?

Following on from my previous post - Liquid water actually does have a tensile strength due to the hydrogen bonds, but that does not apply to siphons - only capillaries.

Water does have a tensile strength, and not just in capillaries, although the capillary tensile strength is larger than in bulk water. Tensile strength calculated using static stressing has reported values as high as 250 bar (~250 atm) (http://www.jstor..../53521). Ordinary water has a lower limit mostly because of impurities that act as nucleation sites as well as small gas bubbles. If water had no tensile strength then it would be impossible to raise water to any height in an enclosed column, let alone all the way up to the 10.3 m limit.

I also find CyberGuy's assertion that it's atmospheric pressure that pushes the liguid up the short arm to the top quite funny. If this was possible, every time you popped a straw into your favorite drink it would form a fountain that emptied your drink onto the floor, table, armrest, lap, etc.. Obviously there's only enough pressure to lift the fluid to the same height inside the tube as outside the tube.

Cyberguy is right on. For all of you that still do not believe atmospheric pressure is needed for a siphon to work try siphoning as sea level first and then at 10,000 feet. The siphon will work up about 33 feet at sea level but less than 33 feet at 10,000 feet. Now think about that, why would that be true?

The answer is simple atmospheric pressure is needed along with gravity for a siphon to work.

Imagine a fish-tank on the left that is full of water and closed on the top, so the water reaches the top.
Imagine a fish-tank on the right that is lower, also closed on the top and filled completely.
We connect the tanks with a purged hose that goes higher than the upper edge of the left tank.

Now we simulated vacuum conditions.

Where is the flow?
It goes somehow against my feelings, but there is no flow if there is no molecular connection outside the tanks to fill up gaps.

I suppose the water in the hose is now like a hook hanging from an edge like shown below where e is the edge and m is any mass. The dots are air or vacuum as you wish. If the hook is strong enough it will remain motionless.
/\
/e \
.e..|
....|
....|
....|
....M
....M

The energy came from gravity because the water flowed to a lower level than the rise over the lake's embankment," Dr Hughes said.

The energy comes from the sun that evaporated the water.

If you start the siphon by sucking on the longer leg, you create a difference in atmospheric pressure (like a straw) however if you fill the entire tube with the liquid before siphoning you don't need that difference in atmospheric pressure. So gravity can do it all by itself, but the usual method involves creating a vacuum while holding the longer tube lower than the shorter tube and once the longer tube is full to below the longer tube then gravity takes over to continue the flow.

barakn wrote: "I also find CyberGuy's assertion that it's atmospheric pressure that pushes the liguid up the short arm to the top quite funny. If this was possible, every time you popped a straw into your favorite drink it would form a fountain that emptied your drink onto the floor, table, armrest, lap, etc.. Obviously there's only enough pressure to lift the fluid to the same height inside the tube as outside the tube."

barakn, you are being extremely stupid. A straw is OPEN at the top, so has equal atmospheric pressure on both ends. So of course it does not pour liquid from the top.

A siphon, on the other hand, is CLOSED to the outside air at the top. Remember we are only talking about how the liquid gets UP the tube - because we all understand the downward side. There is a pressure difference between the top of the siphon and the outside pressure. It is this pressure difference that raises the liquid.

Think of it this way. People often start a siphon by sucking on the lower end of the hose so liquid at the other end is drawn in, over the hump and down, and starts to flow.

The liquid on the down side is, in a way, maintaining that original suction to draw in more liquid. The liquid still flows in because this pressure difference (the suction) is still happening. Only it is caused by the weight of water on the downward side, not by a person any more.

The important pressure difference is NOT between the two open ends of the tube (which are both at atmospheric pressure), but between the TOP of the tube and the intake. This is what keeps the water being drawn in and up.

barakn wrote: "every time you popped a straw into your favorite drink it would form a fountain that emptied your drink onto the floor"

Imagine you push a straw under water so it fills with water, then put your finger over the top of the straw so air cannot enter. You then lift the straw partly out of the water, keeping your finger over the top end.

The straw will be full of water, above the level of the outside surface. What do you think keeps the water up the straw? It is atmospheric pressure!

The atmosphere pushes on the surface of the water, and this is the pressure in the tube at the bottom where it is level with the outside surface.

The pressure at the top of the tube is less, due to the weight of the water in the tube pulling down.

This is the pressure difference that keeps the water up the tube.

The atmospheric pressure pushes the water to the top of the tube.

Cyberguy, 5 days ago.

The important pressure difference is NOT between the two open ends of the tube (which are both at atmospheric pressure), but between the TOP of the tube and the intake.

Cyberguy, today.

Well, which is it? Your vacillation is an indication you don't have a firm grasp of what's going on.

No vacillation, my friend. I have a 100% grasp of the situation.

Both statements are saying exactly the same thing.

It is easier to explain by drawing a diagram, but here I have to use words. Work with me here, and concentrate. I know it's hard for you.

1. In any column of liquid subject to gravity, such as in a siphon or in the ocean, there is less pressure at the top and greater pressure at the bottom due to the weight of the liquid. Think about this carefully, and satisfy yourself that this is always true.

2. Consequently, in a siphon there will be a lower pressure at the top (that is the the highest point, at the hump) inside the tube compared to the pressure at lower points inside the tube. Do you agree? If you disagree, go back to step 1.

3. This is a stunningly clear example of a pressure difference. Do you continue to agree?

(Continued below)

4. In a pressure difference, one side pushes more than the other side. In the case of a siphon, it is the atmosphere that is greater than the pressure inside the tube at its highest point. Do you still agree so far?

5. Hence it can be said that "The atmospheric pressure pushes the water to the top of the tube" - because it is the greater of the two pressures. True?

6 And it can also be said that "The important pressure difference is NOT between the two open ends of the tube (which are both at atmospheric pressure), but between the TOP of the tube and the intake". This is just a simple re-statement of all my preceding points.

So the two statements are making the same point. what step above do you disagree with?

dudes stop being so stoopid. The notion of atmospheric pressure being involved is borne because you people think that the only way to create a siphon is to suck air out the other end of the tube in order to start it off!!

lets set the stage: two containers with water with pipe linking them (filled with air at this stage)

how to create a siphon?

step 1: one water level has to be lower than the other

step 2: water from the HIGHER container needs the water to travel to the APEX of the pipe (highest point) before it can begin falling down the other side

how to achieve this? well either...

1. you suck at the lower end
2. you pump it up just till the water arrives over the apex of the pipe, from the higher end. Point to note - no atmospheric pressure needed here, (and for those who will inevitably not understand - the pump can be removed after the water goes over the top)

gravity & surface tension (hydrogen bonds) takes care of the rest!!

also note that the reason for pumping it over the top of the apex is primarily to overcome gravity!!

sounds like you fellas sucking too much?!

and btw Stephen Hughes was my lecturer at uni too.. he's a very intelligent bloke! he'd know what he's talking about!

Dakaptin - Pumping is just a way of increasing the pressure in the liquid at the bottom to overcome the existing atmospheric pressure inside the air-filled tube. It's just a special case of the pressure difference that raises the liquid.

You write "surface tension (hydrogen bonds) takes care of the rest!!"

Then explain why a siphon cannot work when there is more than one atmosphere's difference between the peak and the intake (about 10.3 meters in the case of water). Your hydrogen bond theory fails to explain this.

also, if you are debating that it is a pressure difference created in the tube as a result of water exiting the tube from one end that continues to drive the water flow, then clearly you people are stoopider than I thought. pressure differences inside the tube conspire to follow hydraulic pressure.

Also with atmospheric pressure: Put a flow meter to the system and increase the height of the higher one. you whould not see an increase in flow with the same tube (unless you go a few hundred metres where there is a meaningful diffenece in atmospheric pressure). this proves atmospheric pressure is unimportant in the scale of 100m or so difference. If atmospheric pressure was a meaningful factor in this system then you would see a larger difference at the small scale and be able to plot a linear relationship of flow vs height in this scale of few metres.

If hydrogen bonds were the cause of raising the liquid, then a siphon, once primed, would work the same irrespective of outside air pressure.

This is, in fact, not the case.

A siphon at altitude can lift water over less of a hump than at sea-level, also disproving the hydrogen bond idea.

Note: 10.3 meters is how deep a diver has to swim to experience two atmospheres of pressure. This is purely due to the weight of water - no hydrogen bonds involved in this. It is no coincidence that this is also the maximum distance that water can be raised inside a siphon.

And if hydrogen bonds raise the liquid, how do you raise a liquid that does not possess hydrogen bonds such as mercury?

And why can you only raise mercury to a height inversely proportional to its weight, compared to water? It is the atmosphere stoopid, not hydrogen bonds!!!

Dakaptin - you are getting confused, and not reading or understanding what I write.

I am NOT talking about the difference between the inlet and outlet pressures. I agree they will both be about the same pressure, about one atmosphere (give or take negligable differences).

I am comparing to the highest point that the tube reaches, the top of the hump about halfway between the two ends. That is where the lowest pressure is, inside the tube.

When the internal pressure at that point reaches near zero, that marks the maximum height that the liquid can be raised to. This is proportional to the atmosperhic pressure outside the pipe.

@dakapin: I belive this is what is confusing you about pressure. The air pressure is the same at the intake and outflow of a siphon. But there is a pressure difference in the siphon that is needed for a siphon to work.

As gravity pulls the fluid out the outflow tube the pressure is reduced all the way up to the neck of a siphon. This is the low point in fluid pressure in a siphon.

Here is where the difference in pressure come into play. The fluid pressure at the neck of any working siphon must be less than the atmospheric pressure at the intake of the siphon. This atmospheric pressure is what pushes the fluid up the intake to the neck of a siphon for gravity to pull it down the longer outflow tube.

The fluid pressure at the neck of the siphon cannot be less then the 0 pressure of a vacuum. This is exactly why a siphon will not work in a vacuum.

Hydrogen bonds have absolutely nothing to do with siphoning. Only gravity and atmospheric pressure are needed.

I beleive wiki! go see it here's a section:

"Vacuum siphons

However, the above height limitation assumes that a liquid cannot take a negative pressure. In practice, liquids such as water and mercury exhibit a property known as tensile strength and are able, under certain conditions to take negative pressures. One example is in tall trees, where the water is pulled up from the roots further than 10 meters, the conventional limitation imposed by gravity and atmospheric pressure.

Surprisingly, experiments have indeed shown that siphons can operate in a vacuum, provided that the liquids are pure and degassed and surfaces are very clean.[29][30] However, typical practical siphons make little or no use of liquid tensile strength to achieve their effect, instead relying on air pressure.

It may not be possible for a siphon to operate in a vacuum if the liquid does not adhere to the surface of the tube."

on an ending note: look up barometers as well

surprisingly, experiments have indeed shown that siphons can operate in a vacuum, provided that the liquids are pure and degassed and surfaces are very clean.

Yep, as ZeroX already said May 10, 2010. It basically means, Dr Hughes is right.

@Alizee - In an ordinary siphon the special conditions you mention do not exist. Look at the photo at the top of the page. Does it look to you as though the water is pure and degassed and the pipe surfaces are very clean? Yet it siphons! So the principle of the siphon is not hydrogen bonds in this case now is it?

I reiterate, the water is pushed up by the difference in atmospheric pressure and the pressure in the pipe at the highest point.

@Dakaptin: "look up barometers as well".

I don't need to look it up - I already know about them. The principle that they work on is atmospheric pressure, not hydrogen bonds. Just as I have been saying all along.

Sheesh...

And if hydrogen bonds raise the liquid, how do you raise a liquid that does not possess hydrogen bonds such as mercury?

Apparently you need lessons on the fundamental properties of liquids too. All liquids consist of atoms or molecules that are loosely attracted to each other one way or another. It doesn't have to be hydrogen bonding. In the case of mercury, being a metal, it is classically pictured as positively-charged metal ions in a sea of electrons. Since the ions are attracted to the electrons, the mercury can exhibit tensile strength due to chains (albeit temporary chains) consisting of ion-electron-ion-electron-ion....

Then why can a liquid not be raised higher than 1 atmosphere in height, except in very special conditions? This is, after all, the principle of the barometer.

If atomic bonding had a significant effect, then a baromenter wouldn't work.

In summary, I am not denying that hydrogen bond forces, or the equivalent in mercury, exist. I am simply saying that this is not the basic principle by which a siphon works.

The fact is (except in highly idealised circumstances)atmospheric pressure determines how high a siphon (or barometer) can lift a liquid. This shows unequivocally that the atmosphere pushes the liquid up the tube.

Hydrogen or ionic bonds have no significant effect in an everyday siphon.

Yep, as ZeroX already said

Didn't you admit somewhere that Alizee and XeroX are two accounts used by the same user?

@dakaptin wrote - "I beleive wiki!"

Good. Read this: http://en.wikiped...i/Siphon

Selected highlights:

"gravity pulling down on the columns of liquid on each side, causes reduced hydrostatic pressure at the top of the two columns"

"An occasional misunderstanding of siphons is that they rely on the tensile strength of the liquid to pull the liquid up and over the rise. ... common siphons can easily be demonstrated to need no liquid tensile strength at all to function. "

"The maximum height of the crest is limited by atmospheric pressure, the density of the liquid, and its vapour pressure. When the pressure within the liquid drops to below the liquid's vapor pressure, tiny vapor bubbles can begin to form at the high point and the siphon effect will end."

"For water at standard atmospheric pressure, the maximum siphon height is approximately 10 m (33 feet); for mercury it is 76 cm (30 inches)."

I'm right, you're wrong. Suck it up.

The science is settled then? Or should we vote or seek a consensus?

I take it that you are speaking ironically. If so, you are 100% right - appeals to authority are obviously not science.

Dakaptin was using appeals to authority ("I beleive wiki!" and "Stephen Hughes was my lecturer at uni too.. he's a very intelligent bloke! he'd know what he's talking about!"), and not addressing my specific comments regarding experimental results that disproved his and others incorrect ideas.

I explained the principle of the siphon every way I knew how, and getting getting responses that were simply plain wrong. My previous post was just a different style, with a bit of juvenile fun thrown in at the end for my own satisfaction.

It worked for me.

Why do you think that pressure in a hose cannot exist regardless of atmospheric pressure? It seems kinda silly to talk about siphoning when you are not dealing with a liquid, so why would you even bring up the liquid boiling away? It seems like people are making this too complicated.

Container full of liquid which has a hose from near its bottom to exit its side to an empty container which is higher.

Now we lower this empty container below the full one.

Where do we need atmospheric pressure to keep the syphon going?

Lyte,

Container full of liquid which has a hose from near its bottom to exit its side to an empty container which is higher.

Now we lower this empty container below the full one.

Where do we need atmospheric pressure to keep the syphon going?

At the surface of the water in the higher liquid container. Try this: Enclose the higher container so that new air can't replace the volume of air and as the air pressure in the upper container decreases due to lowering water surface level, so will the pressure it exerts on the liquid. The siphon will eventually stop working long before the upper container is emptied.

CSharpner: And if we do this test in vacuum?

CSharpner: And if we do this test in vacuum?

I don't know. Would be difficult considering water evaporates in a vacuum, creating its own atmosphere. But, how about NOT in a vacuum, just as I suggested above?

Lyte,

Sorry, I misread your posting. Yours does not have a "hump" that it goes over. In a vacuum, it would certainly all pour into the empty container.

If there's no hump (where the water is forced uphill), then we're not discussing the same thing (though, I do appreciate your using that as an example to stress a point).

In a siphon scenario, try my example.

I'm not siphoning water here, and yes initially there is no "hump" in my example, but when the full container empties enough the hump will form.

If we use (any)liquid that does not evaporate at 0 atmospheric pressure and we are in vacuum. doesn't my example verify that the only thing you need is gravity.

I guess by "the hump will form", you mean that after it gets going, you're going to position it so that there /is/ a hump.

Good question. I'm not sure what would happen. I speculate that it would stop working. That would prove atmospheric pressure would not be needed, otherwise, it proves AP is not needed. Hast this experiment been done and if so, what are the results (links?)

Now, let's go back to my example WITH an atmosphere. This experiment HAS been done and the flow does decrease (if not stop altogether). This would indicate that AP /is/ involved for a siphon to work.

That would prove atmospheric pressure would not be needed, otherwise, it proves AP is not needed. Hast this

Typo corrections:
"That would prove atmospheric pressure would not be needed, otherwise, it proves AP *is* needed. *Has* this"

Atmosphere is definitely needed. Read the posts above, or read this: http://en.wikiped.../Siphon.

@Lyte wrote "Container full of liquid which has a hose from near its bottom to exit its side to an empty container which is higher.

Now we lower this empty container below the full one."

Taking your example, and ignoring effects around the liquid evaporating, in a vacuum a siphon stops when the water level falls below the highest level in the hump.

Sorry Cyberguy, your own link proves that Atmospheric pressure is NOT needed. Read the section "VACUUM SIPHONS"

Yes. And it says this "However, typical practical siphons make little or no use of liquid tensile strength to achieve their effect, instead relying on air pressure."

In other words, no air pressure = no siphon.

Aye, But as long as siphon can work without air pressure relying on liquid tensile strenght alone, the Oxford English Dictionary statement about atmospheric pressure being the operating force in a siphon is not correct. If the conditions are right gravity can make it happen all by itself.

Aye, But as long as siphon can work without air pressure relying on liquid tensile strenght alone, the Oxford English Dictionary statement about atmospheric pressure being the operating force in a siphon is not correct. If the conditions are right gravity can make it happen all by itself.

But, we haven't established that it can, and in regular, every day siphons, it's definitely air pressure + gravity that makes them work. Even if you can find a special case where you can get one working without the aid of air pressure on the source side, the everyday siphons still use air pressure on the source side to get the liquid up to the hump.

Liquid tensile strength is NOT the basic principle by which a siphon operates - it is air pressure and gravity.

You are saying that because we can artifically create a special case that does not use air pressure, air pressure is not involved in all the other cases either.

That logic is totally screwed. If you believe that then your brain needs a check-up.

The previous post was @Lyte

Lyte,

Surprisingly, experiments have indeed shown that siphons can operate in a vacuum, provided that the liquids are pure and degassed and surfaces are very clean.[29][30] However, typical practical siphons make little or no use of liquid tensile strength to achieve their effect, instead relying on air pressure.

That's from the vacuum siphon section you referenced on the wikipedia link.

I think you're spending an awful lot of effort on the special case. Standard, every day, NON-special-case siphons NEED air pressure to operate.

I understand your point, but air pressure is due gravity, so saying that gravity is the operating force rather than air pressure is correct in my book.

Suppose we construct a syphon with a closed cock (tap) at the high point of the hump, giving access to the atmosphere. Start the syphon however you like, then when it is flowing, open the cock to the outside air. Will the syphon continue to run? If, as some here propose, the tensile strength of the fluid is what pulls the water up the inlet side, then presumably the syphon will continue to run happily. However I doubt it!

@Lyte - Yes, gravity causes atmospheric pressure. But you have switched position, and you are now conflating two causes in your new explanation.

A few posts ago you were saying it was the tensile strength of the liquid that was getting the liquid up to the top of the hump, not air pressure. Now you are saying gravity, because air pressure is due to gravity.

The gravity term that was being discussed before was only about the lower hose outlet compared to the inlet, which causes the liquid to flow towards the outlet. This was being considered separately to the outside air pressure.

You are now fudging the difference. You are not being honest with yourself.

At first I didn't understand the air pressures role in siphon, I admit. I thought it was about tensile strenght + gravity. But to defend Dr. Hughes, I still think gravity is the operating FORCE in siphon, air pressure falls to the "right conditions" along with the rig setup.

Siphon can work without air pressure (in some extreme cases), but it cannot work without gravity.

(Yes Dr. Hughes is wrong saying "The column of water acts like a chain with the water molecules pulling on each other via hydrogen bonds,")

Fair enough.

And I learnt that water has a tensile strength that - in certain circumstances - can be much stronger than just the meniscus skin of surface tension. Who would have thunk it? ;-)

Dr Hughes is more right than the current dictionary definition. Just that gravity is not the complete story.

Lyte - I respect you for admitting where you were mistaken. Most people can't do that. Kudos to you.

hang on a moment ... whats this about siphons not working without gravity? Of course they can. Take the classic spinning donut shape in space (no gravity) filled with air so that the air in forced to exert pressure on the external walls. Then construct a siphon on the wall orientated toward the spin center. No problem, it will work fine. No gravity involved. Theres the centrifugal force and air and thats all - no gravity. It may be that these types of space craft have been said to exhibit 'artificial gravity' but its not gravity. It does not matter how the pressure from air is engineered. Only that you have air pressure. Gravity is just a one way (common) way to give air pressure but is not the prime cause of a siphon effect. Air Pressure (regardless of how) is all a siphon needs to work.

OK, so for a syphon to work, you need either an accelerated reference frame, or gravity in addition to ambient pressure from some less dense fluid (usually air).

I'm getting a feel for this now. From the above, I can see that you could syphon liquid mercury over a 1.4m hump (i.e. 2 atmosphere's worth of mercury) so long as the whole apparatus was submerged under 10.4m of water.
The _weight_ (not mass) of the water plus the atmosphere allows the mercury to form a column 1.4m tall, allowing it to pass over the hump.

@DaveBruce - I hadn't thought about mercury siphoning under water, but yes, you are 100% correct.

Well, I'm glad we're all in agreement now that regular siphons need air pressure and gravity (or a gravity substitute, like a spinning space ship, or acceleration).