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Siphons Work Due To Gravity, Not Atmospheric Pressure: Now With Peer Review
knwny (2940129) writes "Peeved by the widespread misconception that siphons work because of atmospheric pressure, physics lecturer Dr. Stephen Hughes, [in 2010] wrote a mail to the prestigious Oxford English Dictionary(OED) pointing out the error. To back his claim, Dr.Hughes tested a siphon inside a hypobaric chamber to check if changes in atmospheric pressure had any effect on the siphon and demonstrated that gravity and not atmospheric pressure was the driving principle. [This week, the] paper detailing his experiment was published in Nature. The OED spokesperson responded saying that his suggestions would be taken into account during the next rewrite."
This is the corrected link to the letter: http://www.theguardian.com/science/blog/2010/may/10/dictionary-definition-siphon-wrong
You win again, gravity!
rewriting history since 2109
>A straw with a hole in it cannot siphon.
A straw has two holes in it.
A straw with only one hole can't siphon.
I should use this sig to advertise my book ISBN-13 : 978-1501515132.
Exactly. You beat me to it.
It can if the hole is below the level of the higer reservoir. Otherwise the hose itself becomes the higher reservoir in which case it still siphons, just not in the direction you want.
that sucks!
I'm not very smart, but it seems to me that the difference in potential energy between masses even at small differences in height would be vastly greater than the work that the negligible delta pressure between those same two heights could do, so isn't this kind of obvious?
The 2012 Impact Factor for Scientific Reports is 2.927. For comparison, that of Nature is 38.597. Still impressive, but please lets be precise.
Does it suck or blow?
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Inside the tube it's not atmospheric pressure, as there is no gas in the tube of a proper siphon: it would be Fluid Pressure.
and a straw with three holes in it might work as a siphon, depending on the size of the third hole (and other related factors such as the viscosity of air)
I work for the Department of Redundancy Department.
A straw with a hole in it cannot siphon.
It cannot be atmospheric pressure, given that he demonstrated that a syphon works in a vacuum.
It seems to me that surface tension is enough to keep the liquid in the tube, even when the equipment is in a vacuum.
Seriously? If atmospheric pressure had any influence, it would do the opposite: The pressure at the lower end of the tube is higher than at the other end, so the fluid would flow upward. Obviously this doesn't happen.
Topology fail. Straw = 1 hole = donut = coffee mug.
He demonstrated no such thing. In fact, he demonstrated that the siphon stops working at sufficiently low atmospheric pressure:
When the pressure was reduced further the siphon broke into two columns - in effect becoming two back-to-back barometers.
You can't pull on one end of a column of liquid and drag the whole column up. Something has to push it from the bottom, unless its own inertia can carry it.
Saying "siphons work due to gravity, not atmospheric pressure" is like saying "fire works due to oxygen, not fuel".
Actually, the wikipedia article on siphons shows an experiment done by Pascal where two beakers of mercury were positioned with a siphon between them. But in this version, the siphon had a third tube projecting upwards from where the top of the bend in the siphon is. The whole thing, excepting the end of the upward projecting tube, was positioned under water. So there is no ability for a vacuum to form in the siphon tube since it is open to air. The mercury still moved from the higher beaker to the lower from the pressure of the water. From this experiment, it would seem that this guy has it wrong and it is the pressure that pushes the fluid up and through the siphon.
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A siphon (at least the kind in the article) generally means a u-shaped tube that pulls liquid up over the top and down again. I suppose a couple of bendy straws stuck together might work with a bit of tape, but holes are still a problem because it breaks the pressure seal and stops the slug of falling liquid from applying force to the container. I am sure in a couple of days we will all be able to see u-tube hypobaric siphon action on youtube.
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I'm very sorry that your grade school taught that.
If "the whole amount is bonded together", how do drips happen?
In vacuum, there wouldn't be any water, only vapor. So a water siphon actually can't work in vacuum.
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It cannot be atmospheric pressure, given that he demonstrated that a syphon works in a vacuum. Please RTFA before you post.
I'd be more impressed if the Oxnard English Dictionary accepted the change.
OK, here's what happens. There are two parts of tube to a siphon, the part where fluid flows upward and the part where fluid flows downward (maybe repeated multiple times, but let's stick to the simplest construction exemplifying the principle.) It is hardly surprising that gravity is what makes the fluid flow downward in the second part of the tube. The interesting bit is what makes the fluid flow upward in the other part, because fluids don't usually do that. The naive explanation is that the fluid in the other part pulls on the fluid in the upward part, but if that were right, then you could siphon with a half-tube, and that doesn't work. That's where pressure (but not necessarily atmospheric pressure) comes in. The fluid in the downward part would leave a "vacuum" where the two parts of the tube meet. This low pressure zone is the cause of the pressure difference, and the pressure of fluid in the upper reservoir pushes the fluid into the tube. Without a closed tube, this pressure differential couldn't form and the siphon doesn't work. The atmospheric pressure on the upper reservoir is part of the pressure which pushes the fluid into the tube, but it is countered by the atmospheric pressure at the lower end of the tube, which is actually bigger (because there's more air above the lower end of the tube). That's why atmospheric pressure is not involved, but pressure is. In the end it is of course gravity which causes the pressure, but the defining element of a siphon is that it requires a closed tube, and that makes pressure, not gravity, the key aspect.
You are completely incorrect. The liquid may need vapour pressure to remain a liquid, but a siphon manifestly does not require any pressure to run. All you need is a full U-shaped tube and a downward force. Gravity is convenient. The U-shaped tube is often filled by using atmospheric pressure to start the siphon, but this is not a necessary condition. The way the tube gets filled in the first place has no impact on the steady state operation of the siphon.
No doubt the confusion comes from the fact that raising water with a vacuum pump does require pressure. People learned some centuries ago that atmospheric pressure can't raise water more than about 10 feet. Simple siphons are commonly started with vacuum pumping.
If the top of the siphon is too high for a vacuum pump, some other method must be used, but the siphon action will work at much greater heights because, as the article points out, the siphon action itself does not depend on pressure. What are the height limits, I wonder? Redwood trees are about as tall as trees can get with the capillary action method they use to raise water. I expect siphons work at much greater heights than that.
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I wasn't using the topology definition. There's no gravity or atmospheric pressure in topology.
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Uh...
It is bonded together with Covalent Hydrogen bonds. Which are very weak. Thus, liquid. It doesn't mean they aren't bonded. It means the bonds are easily broken and have little effect (i.e. the substance doesn't change).
Atmospheric pressure is actually due to gravity.
i didn't realize there was any confusion about this. obviously it's because of gravity. it's like when you have a long chain suspended from above with both sides hanging down. when the two sides are the same length it is stable, but when one side becomes longer than the other then the weight of gravity pulls the whole chain down. duh?
Then that brings up the question of how the siphon actually pulls the liquid up and over.
Gravity pulls the liquid down on the back end, but the front end needs to be pulled up by something and that's the pressure differential penguinoid mentioned. So you are incorrect, as a siphon requires the pressure created by the gravity pulling the liquid down to pull the liquid up the front end. Or it can't siphon.
This can work in an environment without an atmosphere but with gravity, obviously, but not the point. You still need pressure. Just not explicitly atmospheric pressure.
Liquid pressure exists entirely independent from atmospheric pressure. This can be demonstrated from first principles. A siphon can be operated just fine in a total vacuum, although not with water, which would boil like mad.
One can also make a perfectly workable siphon using two immiscible fluids - e.g., oil and water.
You just disproved your own point. What Pascal's experiment showed was that it wasn't a vacuum that created the siphon ( a vacuum would be a difference in air pressure), but when one beaker was placed higher, gravity caused the mercury to flow from the higher beaker to the lower beaker. Even without the vacuum normally associated with a siphon.
Lots of mistakes there. In the experiment you are referring to, the whole thing was NOT "positioned under water". In fact, the mercury siphon and both beakers of mercury were positioned in a larger container exposed to the air. The siphon tube has an extra pipe exposing the top of the bend to the air as well. The outer container that contains the siphon is "slowly filled with water". Since the two beakers that make up the siphon containers both contain mercury the siphon tube is then filled with mercury from the lower beaker before the higher one because of the weight of the water appearing on the lower one first. The extra tube at the bend in the siphon prevents any compression of the air in it. With properly selected heights of the two beakers of mercury the siphon pipe can fill from the lower one first, over the bend and into the higher one and the mercury will flow "upwards" due to the weight of the water only being present on the lower mercury. However, as soon as the weight of the water is present over both containers of mercury then the flow will reverse and go "downhill".
Who said anything about water? I didn't, the person I responded to didn't, and the scientist in the story DID perform a siphoning experiment in a vacuum.
It's perfectly possible with mercury.
Well, obviously, since it is all due to gravity.
Excuse me, but please get off my Pennisetum Clandestinum, eh!
And what, pray tell, causes the water to go downward?
no way man. if you have a siphon that is 2 inches tall, there is no way there's a meaningful difference in atmospheric pressure between the top and the bottom. if that were the case you could hold a straw vertically and wind would rush through it.
it's like a chain hanging from a ladder, just gravity.
Oh thanks a lot. After reading this I tried pouring coffee into a donut hole to make a sort of coffee+donut breadbowl and it just made a mess instead. Topology fail is right!
A straw with a hole in it cannot siphon.
If the liquid has sufficiently high viscosity and surface tension, the siphon may still work. If the liquid has sufficiently low viscosity and surface tension, or if the siphon is too tall, the siphon will not work even without the additional hole.
Pure water, without any dissolved gasses, has a substantial tensile strength. It is not theoretically stable, but in practice it is. Enough so that a siphon will work in a vacuum.
Such pure water is hard to find, though.
a,e,i,o,u and sometimes w and y (at be if of up cwm by)
It's not atmospheric pressure, it's internally induced pressure due to buoyancy differences, which are normally created due to gravity and a connection that is rigid enough to withstand the internally induced pressure. If you have a closed system of two non-rigid containers connected by a rigid body, then the fluid will try to flow in the direction of its buoyancy. Helium balloons connected internally by a straw (even a curvy one) would try to fill the higher balloon, right?
So yeah, he's right that in the absence of gravity, a normal siphon will not work. But, if you took that siphon system on the ISS and put one end outside in space, and one inside, you'll have a siphon-like effect due to air pressure. Likewise, if you take two balloons of water with a rigid connector and submerge one in a pool of Hg, then that "siphon" will work against gravity. :D
The claim in the paper (linked from one of the first comments) is that it's the tensile strength of water that allows the siphon to work.
For the case of water I think that's garbage. Water doesn't have enough tensile strength to support more than a very low siphon. It's air pressure that allows siphons of usable height. Because of it's relatively high vapour pressure while a liquid it's going to be hard to prove anything either way using water though.
Mercury would be a better bet. It has a very low vapour pressure.
If you set up a siphon so that no mercury is flowing (source and destination reservoirs are at the same pressure) then you can make it flow either way by lifting or lowering the reservoirs relative to one another. (You can do this with water too)
I predict that if you were to then move the apparatus to a vacuum chamber, the mercury in the siphon tube would come out due to gravity and there would be a vacuum in the tube too. Raising and lowering the reservoirs would then not cause any mercury to flow either way.
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
First off, a hydrogen bond is not covalent.
Secondly, hydrogen bonding has nothing to do with the ability to siphon a liquid. If it did, you couldn't siphon gasoline, as, being a hydrocarbon, gasoline doesn't have any hydrogen bonds.
A straw only has one hole. It just happens to be a very long one.
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He DID demonstrate such a thing. However he also demonstrated what you describe.
Below the height at which two barometer columns form, the siphon works.
You can't pull on one end of a column of liquid and drag the whole column up.
But you can have a column of liquid higher than the pool is comes from, without any atmospheric effect. It's called capillary action. My reference to surface tension should have given you the hint.
We learned in grade school that it works because a lot of liquids, especially water, stick together. The water going downward pulls the water upwards because the whole amount in the hose is bonded together. THAT is how it works.
But if you fill a large diameter pipe with water then the water falls out of the pipe even if you keep the top end closed. Put a piece of card across the low end though and air pressure will hold the water in.
Based on looking at a drip, I'd guess that water doesn't have enough tensile strength to support anything more than a couple of mm of itself.
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
Huzzah! If only my high school physics teacher was still alive. We frequently argued this point.
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You can't pull on one end of a column of liquid and drag the whole column up. Something has to push it from the bottom, unless its own inertia can carry it.
If you have a fluid with high intermolecular attraction (like water), yes you can.
Who didn't understand that siphons used gravity to move fluids?
Dictionary definition:
"A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe."
This definition is correct as atmospheric pressure differences start the process. However the dictionary doesn't explain that gravity eventually takes over. Dr. Hughes sums up:
As any petrol thief knows, to get the liquid over the "hump" of the tube you have to suck the other end or, more pedantically, lower the pressure in your lungs to beneath atmospheric pressure by expanding them. Once the liquid has passed the highest point in the tube, the continuous chain of cohesive bonds between the liquid molecules in the tube, and the force of gravity, do the rest.
If it ain't broke, don't fix it.
Since he had to go to some length describing the troubles he had because the low pressure formed bubbles due to cavitation, etc. (remember he could not perform this at zero atmospheric pressure because the water would boil), why use water?
Why not use a liquid that will not boil in a vacuum, like (I think) mercury? That would very easily prove that atmospheric pressure is not required to make a siphon work (because there's no atmosphere!).
Take a flexible tube and dunk it in a bucket filled with mercury letting it fill up. Now, sealing the ends, keep one end in the bucket while lowering the other end to another bucket positioned substantially below the first. Pump all the air out of the chamber and unseal the ends. If the siphon works, it is definitely solely due to gravity (remember there's no air!).
Actually, not knowing what the intermolecular bonds are like between mercury molecules, will the siphon still work? If mercury molecules have little or no attraction between them (unlike water which has very strong intermolecular bonds as seen with its high surface tension and high boiling point), perhaps it would behave like discrete particles and there would not be any siphon effect. For example, imagine the bucket and tube to be filled with sand. Would there be a siphon effect? I don't think so because the grains of sand wouldn't "pull" on each other so the sand in the tube would just run out in both directions from the high point in the tube.
Another way to think of the intermolecular bonds is to think of a coiled chain which is held aloft. If a part of it is pulled over a pulley and a substantial length is allowed to dangle down the other side, the rest will be pulled up to the pulley and then down. Of course if all the links in the chain are broken (no intermolecular bonds) then the chain will simply fall away from the pulley on both sides.
Pessimism.
Moving water over a mountain is easy in a pipe. Say you have a reservoir at height, like a mountain lake, and you want to pump it to a city in the valley below. You need only get it over the ridge. Once the flow to the lower height starts, it will continue. The problem with your suggestion is that you can't get the siphon started. All this guy is saying is that the flow continues due to gravity. Which makes good sense. The atmospheric pressure at the lower basin is actually slightly higher than at the higher basin, so it's clearly not atmospherically driven.
Gravity pulls the liquid down on the back end, but the front end needs to be pulled up by something.
For low heights that can be surface tension.
Both work due to gravity. The difference is, in chain fountain, it's the link between beads that's pulling the next bead down and in siphon it's vacuum between molecules in the tube?
So what we call a siphon, which is just a simple hose, does not work below a couple hundred torr. What is proved here is that a specially constructed siphon can work at low pressure. What we need to see for the gravity hypnosis is that a specially constructed siphon cannot work at low gravity.
My take on this is that as gravity pulls water down out of the exit of the siphon, creating a vacuum in the tube, that then pulls water up from the reservoir. It is a compelling and reasonable theory. More experimentation is needed.
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They cover that in the paper and videos. At 40,000 ft equivalent atmospheric pressure, water begins to cavitate or boil inside the siphon, but the momentum of the water pulls the bubbles past the apex before they can stop the flow, resulting in a "waterfall" inside the tube. Slightly lower pressure decreases this effect, slightly higher increases it.
At some point around 41,000 ft equivalent pressure the bubbles form too quickly and touch all sides of the tube at or slightly before the apex, resulting in the flow stopping. However if you then increase the pressure again at a certain point (around 30,000 ft IIRC) the flow resumes. They discuss attempting the experiment in the future with an ionic liquid that won't vaporize.
If you think about it, this is the same phenomenon as the ball chain flowing out of a container (https://www.youtube.com/watch?v=_dQJBBklpQQ). Gravity pulls on the first ball, which pulls on the next, which pulls on the next. As soon as that pull is strong enough to lift the chain from the surface to the apex, a siphon effect begins that will empty the entire container.
IANAP, but it appears that water siphons work the same way. Once enough water flows over the apex sufficient that the force of gravity on that water exceeds the weight of the water prior to the apex the siphon will flow. The big tell-tale sign that any explanation involving the air pushing down on the surface of the liquid is wrong is the flow rate - it is almost completely independent of atmospheric pressure.
The one question I still have is why the flow stops at 41,000 ft. I would have expected a kind of spring effect, followed by the lower portion of the siphon slowly descending as water vaporizes off the pre-apex portion, allowing the water in the lower part to descend while maintaining the same vapor pressure. I'm sure it is my failure to understand, so if anyone can offer a better explanation please do so!
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I believe the limit on height is the pressure at which water turns from liquid to a gas at the ambient temperature. If it were to remain liquid at all pressures, then the water column could be lifted the height where the weight of the water equals the pressure of the atmosphere (which would be roughly 33' at STP).
Is it just my observation, or are there way too many stupid people in the world?
Mod Anon Informative... Siphoning works with all liquids within their vapor pressure limits regardless of surface tension or cohesion properties.
Yes it's an anecdote! Were you expecting original research in a Slashdot comment?
No that is incorrect. He showed that the atmospheric pressure value dropped out of the flow equations. The flow is caused by the difference in pressure generated directly from gravity acting on the columns of water. From what I understood, this is why the predicted cavitation, and break down of the siphon action, occurred when the pressure, not the pressure difference, at the siphon apex dropped to the vapour pressure of water.
The fluid in the siphon moves due to the relative differences in weight in the two siphon columns. The longer, heavier fluid column falls; the shorter, lighter fluid column is dragged up and over the top then falls in turn. You could see a similar thing with a chain or rope over the top of a pulley. The whole thing is driven by gravity.
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It is not theoretically stable, but in practice it is.
In theory, there is no difference between theory and practice.
In practice, there is.
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What force pushes the liquid up into the tube on the high side?
Note that in the experiment in TFA, at 0.18 atmospheres, the siphon stopped. .18atm is still well above the vapor pressure of water.
The maximum height of the rising leg of the siphon is, in fact, the same as the height the fluid would be in a barometer.
Much of this is a semantic problem more than anything. A siphon is driven by the potential energy differential between the pools, which is typically gravity. However, sufficient atmospheric pressure is also necessary for the siphon to work. I am discounting the case of liquid helium since it doesn't need a siphon at all to find it's way up and over the lip of a container.
There.
No. A water column height is proportional to temperature and pressure. Under standard conditions, you can get a column about 32 feet long before the water breaks to form a void. It is called cavitation, but in effect it is a local boiling effect. Boiling is when the vapor pressure of the water is at or above the local atmospheric pressure. Water vapor bubbles jump out of the water liquid. If that happened in the siphon tube, it would break the siphon, but again, the column would have to be pretty long before it happened
Behold, this dreamer cometh. Come now, and let us slay him... and we shall see what will become of his dreams.
re: the summary's title: One simple word would have needed all this hand-wringing. "Siphons Work PRIMARILY Due To Gravity [...]"
Also, help me out. Isn't reducing pressure at one end how siphoning is started? I understand gravity's role in moving the column of fluid along, but as pointed out, you need both gravity and pressure, right?
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http://www.britannica.com/EBch...
"The action depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure; a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight."
no way man. if you have a siphon that is 2 inches tall, there is no way there's a meaningful difference in atmospheric pressure between the top and the bottom. if that were the case you could hold a straw vertically and wind would rush through it.
it's like a chain hanging from a ladder, just gravity.
Well, not exactly. A column of water does not have the cohesion or tensile strength of a chain. Remember, vacuums don't "suck", rather fluid pressure differences provide pushes.
A mercury column in a sealed tube open at the bottom can be about 76cm in height, when under 1 atm of pressure. The volume above that height will be a vacuum (with a bit of mercury vapour I suppose). Can you get a mercury siphon to work in the atmosphere to lift over a hump greater than 76cm? No, because unlike a chain, the mercury would split at the top of the hump as soon as the height of the hump is higher than the 76cm corresponding to 1 atm of pressure. If you lower the atmospheric pressure, the max height of the hump will decrease.
With that said, it is the force of gravity on the fluid driving the motion, not the difference in air pressure between the two ends of the siphon pipe, so as long as the air pressure is high enough to prevent the fluid from splitting at the top of the hump, different air pressures will not have much effect on the siphon's operation - the fluid flow rate for example would be constant for all workable air pressures.
Of course I have not read the linked papers or watched the videos. Maybe I'm totally wrong and siphons work just fine in vacuums, but that has never stoped me from spouting off before, so why now?
It would be pedantic to accuse you of being pernickity.
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None of the physics is new. Just good old newtonian pohysics. So why not write out the equations of motion and see exactly what is happening?
Actually, no. The water will only go up the high leg of the siphon up to the height that the atmospheric pressure can support and no higher unless driven by a pump.
So let's say you use a pump to start the siphon. While you are pumping, the water flows (but that's not a siphon). Now, shut the pump down. The water on in the high leg will fall back to about 33 feet (the height it would reach in a water barometer). Meanwhile, the water in the lower leg will drain out until it is also at 33 feet.
It is an interesting thought experiment. It would be an interesting actual experiment for someone like Mythbusters that has the time and materials to set it up (a long transparent hose and a crane or tower > 32 feet).
Moving water over a mountain is easy in a pipe. Say you have a reservoir at height, like a mountain lake, and you want to pump it to a city in the valley below. You need only get it over the ridge. Once the flow to the lower height starts, it will continue. The problem with your suggestion is that you can't get the siphon started. All this guy is saying is that the flow continues due to gravity. Which makes good sense. The atmospheric pressure at the lower basin is actually slightly higher than at the higher basin, so it's clearly not atmospherically driven.
Sure, but you can't use "suction" to lift the water higher than about 10m. You can push the water over the 10m high side of the reservoir, but if you stop the pusher pumps, the "siphon effect" won't magically keep it going, the water will just drop down the pipes away from the top of the hump on both sides leaving just a bit of water vapour in the created vacuum. You will have created a big barometer.
...a 1.5 m high siphon was set up in a hypobaric chamber to explore siphon behaviour in a low-pressure environment. When the pressure in the chamber was reduced to about 0.18 atmospheres...
Atmospheric pressure isn't enough, but it's still required. In this experiment, 0.18 atmosphere is just enough for (in theory) a 1.8 meter siphon, had the guy attempted to get it to work at 2 meters, it would have failed because the atmospheric pressure needs to be high enough to hold the column of liquid.
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Are you a policeman?
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If the top of the siphon is too high for a vacuum pump, some other method must be used, but the siphon action will work at much greater heights because, as the article points out, the siphon action itself does not depend on pressure. What are the height limits, I wonder? Redwood trees are about as tall as trees can get with the capillary action method they use to raise water. I expect siphons work at much greater heights than that.
Just because you call your tall u-shaped tube a "siphon" does not mean it will behave differently than the tall u-shaped tube someone else calls a "barometer". Once your siphon hump is more than about 10 *meters* (10.3m or about 34 feet) high, the water falls down on each side of the hump, leaving a vacuum (with some water vapour) at the op. The air pressure sets a limit on the height of both suction pumps and siphons.
Some of these posts have shown me that gravity is the force involved. But it seems to be a bit of a trick. Atmospheric pressure is also due to gravity. So the air pressure, or water pressure in the mercury example, are ultimately due to gravity. The only example that tries to take air pressure completely out of the equation, by running the siphon in a vacuum chamber, will not work unless the fluid has insufficient tension capabilities. So it does seem to be a combination of both, as in a normal siphon the water in the tube is under pressure, not tension. The air pressure is pushing the water up the tube, it's not being pulled up the tube. The only reason the air pressure pushes the water up the tube is because the water in the downward side of the siphon has fallen down, due to gravity, and made less pressure at the top of the tube.
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For those who didn't read the article well: the paper actually does show that the flow stops when there isn't enough pressure. The water column still needs to be supported, and this happens by a combination of atmospheric pressure (the dominant force at 1atm) and molecular cohesion.
Also, NO, this paper does NOT show a water siphon working in a vacuum. (Reference is made to another study, but not at similat water column heights)
The key point being made here is that although atmospheric pressure is required to maintain a certain siphon height, the force causing the water to flow is due to the potential energy difference.
good point
Sucking can help, but it is not needed.
You can immerse the siphon tube in liquid, plug the ends, and then position the siphon so that the ends are in each of two reservoirs with different water levels. When the plugs are removed, the liquid begins to move.
No sucking.
The liquid within the siphon tube acts in a manner similar to a pump's piston. For the siphon to work a sufficient amount of liquid is required to be drawn by gravity to the receiving end to overcome the head pressure on the end from which the liquid is being drawn. Atmospheric pressure contributes only in as much as it can vary the head pressure that needs to be overcome for the siphon to operate. The idea of a pump's piston may be extended to the vessel from which the liquid is being drawn. Consider for a moment that this vessel is sealed but for the point from which the liquid is siphoned. As the liquid in the vessel is drawn down a vacuum is created which resists the drawing of liquid. The more liquid siphoned away the greater the vacuum and thus the greater the head pressure that must be overcome for the siphon to operate. Of course this could operate in the reverse fashion were a substance to be pressed into the vessel from which the liquid is drawn. As the pressure increases against the liquid the head pressure is reduced.
Two of my imaginary friends reproduced once
Nope. Strange how many people get this wrong, it's really not that complicated.
The water doesn't work like a chain, the cohesion of water is only just enough to hold a drop of water together, certainly not enough to pull a whole column of water along through a siphon. The motion is caused by gravity BUT atmospheric pressure is needed as well (as shown in the actual experiment that was referenced in the Slashdot summary and described in more detail in Nature). Here's how a siphon acually works:
Suppose you have a source reservoir and a destination water reservoir, with the water level of the destination lower than that of the source. The reservoirs are connected by a tube that goes from the source reservoir up to an apex above both water levels and then down into the destination reservoir. The tube is filled with water (you have to start the siphon somehow by filling it with water before it can work).
Now, if you would calculate the pressure at the apex starting from the inlet, it should be equal to atmospheric pressure MINUS the water pressure from the difference in height between the apex and the source reservoir level. On the other hand, if you calculate the pressure at the apex starting from the outlet, it should be equal to atmospheric pressure MINUS the water pressure from the difference in height between the apex and the destination water level. If the destination water level is lower, the latter value for the pressure at the apex is lower than the former. Of course there can only be one pressure at the apex, which will be in between these two pressures. It is lower than what you would expect when calculating from the inlet, and higher than what you would expect when calculating from the outlet, so the pressure gradient will suck water in from the inlet and push it out of the outlet.
But note the two times I wrote "MINUS" in bold capital letters. You can't go below zero pressure. When the atmospheric pressure is too low to push the water from the source reservoir up to the apex, the siphon breaks up.
That's exactly what happened in the experiment described in Nature. They tested it with a 1.5 meter siphon in a pressure chamber. The water in the siphon broke up when they reduced pressure to below 0.18 atmosphere, which makes perfect sense because at that point the pressure at the apex would start to approach zero. The siphon actually turned into a double barometer with vacuum (or a bit of water vapour, actually) in between.
So yes, the motion is caused by gravity but you DO need atmospheric pressure or it simply won't work. In fact, if you look at it a certain way, it's not even wrong to say that atmospheric pressure is pushing the water up to the apex and therefore making the siphon work.
The article says: "how could a siphon possibly work by a difference in pressure when atmospheric pressure is the same for the liquid at both ends of the tube?" It does work by a difference in pressure, just not a difference in atmospheric pressure. The liquid falling out of the exit end of the siphon causes a difference in pressure.
Mod Anon Informative... Siphoning works with all liquids within their vapor pressure limits regardless of surface tension or cohesion properties.
Is siphoning limited to liquids? Could you not siphon a gas as long as it were kept from leaking out of the apparatus, but the volumes of the upper and lower containers were not constrained?
For example, I would suspect that CO2 would follow the path of a siphon.
Gravity pulls one side down; pressure pushes the other side up.
Gasses expand to fill entire volumes. Siphons transfer from one reservoir to another. You can't really have a gas reservoir. I guess if you had extremes of densities where you had two gasses that would stratify like maybe Sodium Hexafluoride in a Helium environment it might sort of work, but eventually the gas would expand out of both containers and fill the environment to equilibrium regardless of any sort of conduit in the environment connecting them.
In any case... I have no idea really. Googling "siphon with gasses" returns a whole bunch of stupid things about siphoning gasoline. I guess that's one more argument in favor of British English.
Yes it's an anecdote! Were you expecting original research in a Slashdot comment?
it holds the water column together (so gravity can act on it), but to say that it is driven by air pressure makes no sense, because if I am not mistaken (?) air pressure should be greater at lower altitudes (no?), so that it should push it up the hose, which it presumably does to a minor extent.
Bukowski said it. I believe it. That settles it.
Actually, no. The water will only go up the high leg of the siphon up to the height that the atmospheric pressure can support and no higher unless driven by a
That is nonsense.
The water will go as high at *your* end as high it is at *the other end*. So if I have the other end 1000m above your place (like a pipe coming down from a lake in the mountain), it will either spray out at your end roughly 1000m high, or you can simply attach a pipe and feed it into a roughly 1000m high building ... or other lake. That has nothing to do with "atmospheric" pressure (and as you lack simple basic physics knowledge: already the romans (and I would not wonder if older cultures as well) had lead pipes (pipelines even) to distribute water based on this principle in cities ... 2500 years ago. I would be shocked if the law of physics had changed that much since then).
Cost free eBook I read (by iBook/Kobo/Amazon/ObookO/Gutenberg etc.): "The Green Odyssey" by Philip Jose Farmer.
The maximum height of the rising leg of the siphon is, in fact, the same as the height the fluid would be in a barometer.
That makes sense to me. If you took a 100m tall u-tube and filled it with water, and then inverted it, the water would fall down on both sides of the tube, creating a vacuum on the top. If you moved the basin on one side of the tube lower than the other, the water level on that side of the tube would fall by the same distance, but the water level on the other side of the tube would not fall at all. Nothing connects the two columns of water capable of transferring a force from one to the other - only a near-vacuum exists between them (in reality it would be steam in equilibrium with both columns at the vapor pressure).
Once your siphon hump is more than about 10 *meters* (10.3m or about 34 feet) high, the water falls down on each side of the hump, leaving a vacuum (with some water vapour) at the op. The air pressure sets a limit on the height of both suction pumps and siphons. ... .... ever heard about it?
That is nonsense.
You can have a U-formed pipe as tall as you want, if both sides have the same hight nothing at all will happen. If one side is higher water will flow out of the other side, until the water level is even
That has nothing to do with air pressure but with the weight of the water: hydraulics, you know
The air pressure sets a limit on the height of both suction pumps and siphons.
No it does not, or no country in the world had a working water distribution system. We use the principle of imbalanced "syphons" in so called water towers since centuries.
Cost free eBook I read (by iBook/Kobo/Amazon/ObookO/Gutenberg etc.): "The Green Odyssey" by Philip Jose Farmer.
> Under standard conditions, you can get a column about 32 feet long
Where "standard conditions" means standard AIR PRESSURE and temperature. At standard pressure it works fine. If pressure is reduced by 80%, it stops working at all. See the article for details.
The pressure at which the water starts to boil is more of a practical limit than a true one. The problem is that as the water boils it produces gas at a rate greater than your pump can remove it, so you reach a steady state at greater than zero pressure. If your pump was REALLY strong you could get the pressure lower, though all the water would eventually boil away.
The other bit of kinetics going on is with heat. Boiling is endothermic, and thus either needs a flow of heat from the outside or it will lower the temperature of the water. Eventually the column of water will start to freeze (which is exothermic), and the rate of boiling/sublimation will steadily drop as the temperature of the ice approaches absolute zero (if the tube is perfectly insulated).
Air pressure does not push the fluid up the tube ever. The liquids cohesiveness is pulling it up the tube. This video is actually running a siphon in a vacuum with good explanations of how a siphon really works.
The only time atmospheric pressure enter the picture as a driving force is the case when you suck the liquid throw the siphon tube initially.
You can't run a siphon of water over a height greater than atmospheric pressure is that water does not have sufficient cohesiveness to pull it over this height.
You are completely right. The experiment did show that the water column broke up when the air pressure was reduced enough.
Now syphons work due to gravity instead of atmospheric pressure. Thanks Obama.
Proverbs 21:19
Water doesn't pull (at least not much). Atmospheric pressure is pushing the water in from both sides, like two connected barometers. The barometer that has to push the water up over the lowest height difference (i.e. the side of the high reservoir) is the one that wins the fight, so water flows from that side to the other. If the height is too high, the barometers no longer meet and the siphon turns into a double barometer with vacuum or some water vapour in between. Boiling doesn't really have much to do with it, with mercury for example you'd just get a vacuum like the one you have in a mercury barometer.
I gave a slightly longer explanation, explained differently, here.
We hear you have a station in space, with air pressure, but zero gravity. Do you have a few minutes to settle an argument?
Yes, it is a BS factor, to some degree. But Scientific Reports is not Nature... that is for sure.
It would seem you have the common misunderstanding of how a siphon works. Pasted below is a section from the wikipedia article on siphons. Note how the siphon works even when there are air bubbles in the tube. Thus no cohesiveness needed.
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.[6][13] While water has been found to have a great deal of tensile strength in some experiments (such as with the z-tube[14]), and siphons in vacuum rely on such cohesion, common siphons can easily be demonstrated to need no liquid tensile strength at all to function.[4][6][13] Furthermore, since common siphons operate at positive pressures throughout the siphon, there is no contribution from liquid tensile strength, because the molecules are actually repelling each other in order to resist the pressure, rather than pulling on each other.[4] To demonstrate, the longer lower leg of a common siphon can be plugged at the bottom and filled almost to the crest with liquid as in Figure 4, leaving the top and the shorter upper leg completely dry and containing only air. When the plug is removed and the liquid in the longer lower leg is allowed to fall, the liquid in the upper reservoir will then typically sweep the air bubble down and out of the tube. The apparatus will then continue to operate as a siphon. As there is no contact between the liquid on either side of the siphon at the beginning of this experiment, there can be no cohesion between the liquid molecules to pull the liquid over the rise.
-- ssoorrrryy,, dduupplleexx sswwiittcchh oonn.. -Quote found on actual fortune cookie.
Mod this up insightful, I don't know who modded it down. Indeed, a siphon can't reach 33 feet. And in the nature experiment, it couldn't even reach 1.5 meters anymore once air pressure was reduced below 0.18 atm. So air pressure is needed for a siphon to work.
The total pressure acting on water in the pipe is
[P(air) + P(h1)] - [P(air) + P(h2)]
where h1 is the position of upper end and h2 is the position of lower end of the pipe.
If you want to siphon the liquid fast, either lift the upper end or lower down the lower end of pipe, which is the proof that gravity is in action.
Although, P(air) gets canceled in a regular siphon as in the equation above, if air pressure is different at two ends it will start affecting the flow. Obviously, in outer space P(air) will be zero and P(h1) and P(h2) will be very weak.
No, it is you who are incorrect. Even a hypothetical liquid with zero vapor pressure will not siphon over an apex higher than P(atmosphere) / (g * liquid density) above the liquid level in the upper reservoir. Otherwise, vacuum (instead of vapor) bubbles will form at the apex. If you set P to zero in the above formula, the apex cannot be higher than the liquid level in the upper reservoir, so you cannot satisfy the definition of a siphon. A true siphon manifestly does require pressure to run. As explained in TFA.
However, pressure is a necessary but insufficient condition. It is tempting to observe that atmospheric pressure seems to be be exerting work on the higher reservoir, and this is the base for the fallacy in the dictionary. This is a fallacy because an exactly equal amount work is exerted against athmospheric pressure by the lower level reservoir, and in total, there is no net pressure work because the volume stays the same. There is net gravitational work because there is a net flow of mass along the gravitational potential gradient, and that is the only true driving force, as Dr. Hughes correctly pointed out.
Now, real real-life liquids have interesting properties such as inertia, viscosity and the ability to form foam while evaporating, so it is not out of the question that a real-life syphon containing a well-chosen liquid, once set in motion, would continue working even if the apex is moved too high (or the pressure decreased too low) according to my formula. If the authors would have done more effort to study that regimen, then their paper would not have been so utterly dull and trivial. As it is, I was able to predict all their results (including the waterfall) by sitting in front of a sheet of paper for half an hour (yes, I did that before peaking at TFA - and getting surprised that my theoretically predicted waterfall actually materializes in reality). I could think of better things to do with a hypobaric chamber.
Then why did the siphon in the Nature experiment stop working when pressure was reduced below 0.18 atm? (For a 1.5 meter high apex).
Correct explanation here.
There is one condition:
P(air) > P( h_apex )
1 atm > densityOfWater * g * h
which means h < 1 atm / (density of water * g )
or h < 30 feet for water
For mecrucy it guess h < 3 feet for siphon to work
The siphon in the experiment stopped working when the pressure was reduced below 0.18 atm. The water was not boiling, the air pressure was just no longer enough to get the water over the 1.5 meter high apex.
Correct explanation here.
Liquid doesn't pull (at least not much), but you can get an effect much like a pull if there's enough pressure on the other side. The pressure at the apex is not negative, it's just lower than atmospheric.
BUT... boiling is not required. If the pressure is zero, the liquid can break apart, leaving voids filled with... nothing. I'm also not convinced that siphoning at pressure zero can never work. Mercury, for example, has a lot stronger bond between molecules than water. Maybe some liquid could pull through a tube, even without filling it, like a string. Can anyone provide such an example, or a good reason that no substance will work that way?
Two phenomena are at work:
1. Atmospheric pressure is needed to take the fluid till the apex. It will not affect the rate of siphoning but it is a necessary barrier that has to be overcome.
2. Once atmoshperic pressure has done its work, the rate of flow of fluid will be completely determined only by the difference between the heights of two ends of the pipe and the amount of gravitational force.
Exactly.
Interestingly, it seems possible that you could effectively pump the water at that point by heating the high side at the level of the water column, causing the water vapor (at 11 torr) to flow to the low (and cool) side. Of course, that's not a siphon, it's a heat engine.
I'm talking about the high leg. You're placing me on the low side.
You could have the gas in a flexible bladder, or my example of CO2 which is more dense than air.
Great explanation! Completely agree.
In fact, you could take it a step further and apply Bernoulli's equation to the fluid in the system. The difference in pressure between the source reservoir and the destination reservoir is exactly offset by the effective pressure loss due to friction between the flowing liquid and the tube wall.
The difference in pressure between the source reservoir and the apex will include part of this friction pressure loss (proportional to the length of the tube from the source to the apex relative to the overall length) + reduction in pressure due to the velocity of the fluid in motion (static liquid is at a higher pressure than liquid in communication with it at the same elevation but in motion -- the principle behind the pitot-static tube).
This is why flow through the siphon can be regulated by raising or lowering the apex of the tube. Make it higher, the total friction pressure loss is increased and the velocity must decrease to offset that loss. At some point, the tube is sufficiently long that the flow is slowed to a standstill.
licet differant, aequabitur
>A straw with a hole in it cannot siphon.
A straw has two holes in it.
A straw with only one hole can't siphon.
But it is just one hole open from top to bottom.
A bendy straw could be used as a siphon.
In a darn fine vacuum water would boil and the siphon
would fill with water vapor and stop.
As others above indicated.... you need both.
As you minimize both to zero you get no siphon action.
That is something Bill Nye the Science Guy could demonstrate
if he ever got a free ride to the ISS.
Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't. Mark Twain.
In fact it's not even wrong to say that air pressure makes a siphon work
Wouldn't that be wrong if a siphon works in a vacuum? http://science.slashdot.org/co...
Honestly, Who in the world could have ever though that anything but gravity was the cause.
Now we have to go through a whole scientific method for this instead of publicly shaming stupid people?
Any 6 year old can do a scientific demonstration of gravity causing a siphon to work. 2 cups and a 2 foot length of tubing.
Do not look at laser with remaining good eye.
In the vacuum, what would happen if it had empty space in the upper part of the tube? Would the liquid would just fall out of the lower end with no bubble going up the lower opening to fill the space? It would be interesting to see if that's the case.
This was all part of the standard curriculum in my high school physics class, decades ago.
/* No Comment */
This is an example of capillary action, not a siphon.
http://en.wikipedia.org/wiki/C...
and a straw with three holes in it might work as a siphon, depending on the size of the third hole (and other related factors such as the viscosity of air)
Add location to this as well.
If the third opening was under water on the head end
no problem.
If the third opening was on the down stream end
it could limit flow volume by admitting some air but
note that when the location of the bottom end extends
more than about 32 feet the weight of water tends
to pull a vacuum and perhaps trigger harmonic hammering
and cavitation like actions.
Some big water systems do use siphons. I wonder if they have
a pump and other tricks to eliminate bubbles that might limit
flow over hill and dale.
of the bottom end
Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't. Mark Twain.
Perhaps you misunderstood, due to my poor wording. I should have said "inverted U-formed pipe" or maybe an "n-formed pipe".
If that isn't what caused your "this is nonsense" statement, then perhaps you need to review simple mercury barometer construction
https://en.wikipedia.org/wiki/...
Note the second paragraph on Siphons which discusses the maximum height:
https://en.wikipedia.org/wiki/...
Here is another reference for the maximum height of a suction pump:
http://mysite.du.edu/~jcalvert...
People do not use suction pumps to raise water beyond 10m in one stage. They can use various pump designs to push water from the bottom to much higher heights, but you can't "pull" it up more than about 10m without changing the local air pressure.
I would be interested to see any references or examples to the contrary.
It seems that the tensile strength of the liquid would allow some fluids to work in a vacuum. Water, due to it's very strong tensile strength, will still work in a siphon in a vacuum. If there was a low tensile fluid in the siphon, it would do like you said. Perhaps the gap between two separating parts of the fluid would itself be a vacuum or near vacuum as some of the fluid might evaporate due to low pressure in the forming gap. Again from the wikipedia article:
in the laboratory, some siphons have been demonstrated to work in a vacuum – see vacuum siphons – indicating the tensile strength of the liquid is contributing to the operation of siphons at very low pressures.
And the paragraph on vacuum siphons mentioned says this:
Experiments have shown that siphons can operate in a vacuum, via cohesion and tensile strength between molecules, provided that the liquids are pure and degassed and surfaces are very clean.
I have found this discussion very insightful as I have had to think and learn a lot more about what makes a siphon actually work than I have bothered to do in the past. It's nice to increase one's understanding of the world.
-- ssoorrrryy,, dduupplleexx sswwiittcchh oonn.. -Quote found on actual fortune cookie.
Never in my 42 years have I heard anyone explain siphons working from atmospheric pressure. Obviously it is gravity. The same atmospheric pressure exists at both ends of the hose.
...when it is a textbook problem in fluid mechanics at the introductory level? Bernoulli's equation, conservation of flow, physical conditions, end of story (within the minor tweaks introduced by viscosity and Poisieulle's formula).
Oh, wait, because somebody did the umpty-zillionth practical experiment of running a siphon and managed to publish the results.
Yeah. Sure. That makes sense.
Or, perhaps it makes no sense at all. It might make sense as a science fair project, though, for some bright high school student.
rgb
Even when the experts all agree, they may well be mistaken. --- Bertrand Russell.
The one question I still have is why the flow stops at 41,000 ft. I would have expected a kind of spring effect, followed by the lower portion of the siphon slowly descending as water vaporizes off the pre-apex portion, allowing the water in the lower part to descend while maintaining the same vapor pressure. I'm sure it is my failure to understand, so if anyone can offer a better explanation please do so!
I think it does. The time scale is just staggeringly different. Watching a water surface dry, and one with low area versus the volume, at that, is a boring activity. Put some table salt in a glass and fill an identical glass with water. Put some lid over it all. The equilibrium state will be more water in the initially empty, salt-containing, glass, than in the one originally containing water. Why? Because of the change in boiling enthalpy. But that change, and the formation of a water film or drops on all other surfaces in the enclosed volume, is immensely slow anywhere near room temperature.
I think in most cases the flow itself would keep bubbles in check. Bubbles move at a fixed speed up a liquid. As long as the liquid is moving faster than that speed through the siphon, bubbles shouldn't be an issue.
But I don't know the dynamics of what happens if an air pocket manages to form at the top. It may or may not dissipate on its own. Or it may grow, slowing and eventually stopping the siphon action.
I work for the Department of Redundancy Department.
> As some liquid pulls out and follows the force of gravity; a suction is created, and water molecules that are adhering follow the flow this creates.
That fact that you can siphon a gas shows that "molecules adhering" has nothing to do with it. A fun way to see this for yourself is to put some dry ice in water, then siphon off the CO2. The cold CO2 isn't MUCH heavier than air, so the siphon doesn't flow very fast, but it does flow.
Gravity pulls the fluid out of the low side, creating low pressure in the tube. The higher atmospheric pressure then pushes fluid into that low-pressure tube from the upper reservoir.
> After pressure is reduced by 80%; the substance ceases to be a proper liquid -- in essence, it loses the properties of water.
Which doesn't matter. Try the dry ice CO2 experiment to see for yourself.
Now I have to go get some dry ice and see if I can set up a CO2 siphon. It would be fun to try it with sulfur hexafluoride, but my grocery store still refuses to stock that.
If you can set up a siphon for a dense gas -- and my intuition, which is admittedly out of its depth here, tells me it should work -- that would seem to argue against the "pulling on water" explanation.
If "the whole amount is bonded together", how do drips happen?
Failure to use protection.
Thanks, I'll be here forever, fuckers.
But seriously, because bonds can be broken. In this case, the force of gravity overcomes the force of the bonds which hold the water together.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
The experiment did show that the water column broke up when the air pressure was reduced enough.
But that only tells us what happens when siphoning water, which is really strange stuff. And in fact, water breaks up when the air pressure is reduced enough. That is ultimately what makes a "test" of this nature just jerking off. Sure, if you reduce atmospheric pressure a lot, liquid water stops acting like liquid water. But uh, we knew that. On the other hand, they didn't have to build a lot of expensive equipment or anything, so it seems like a fairly harmless sort of masturbation.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
Right, I was wrong.
I like to think of it, using his barometer reference, as a tug of war where one side has a natural advantage (elevation). Imagine it so that the two barometers are pushing against each other (where the siphon works) and the top one is winning due to it's advantage causing the liquid to be transferred down.
It's really quite something.
You can't pull on one end of a column of liquid and drag the whole column up. Something has to push it from the bottom, unless its own inertia can carry it.
And yet, the conclusion of the experiment is the reverse of yours. The experiment showed as expected that water will not siphon when the pressure drops too far, because of the properties of water. But supposedly it also showed that it's not atmospheric pressure that drives the siphon. I think that's because the speed didn't change as the pressure changed. As you likely know, flow rates are related to pressure, just as pressure drop is related to flow. If the siphon is powered by atmospheric pressure, then at half the atmospheric pressure the siphon should run substantially slower. As per TFA, "In the first run there was little change in flow until the siphon reached 25,000 feet (37.60 kPa, 0.37 atm),". IOW, although the atmospheric pressure dropped to just over a third, the flow remained basically unchanged. This is the opposite of the result you would expect if the siphon actually were driven by atmospheric pressure.
This experiment, in fact, shows that the siphon is not driven by atmospheric pressure. Rather, it reminds us that water does not remain liquid at very low pressures, such as those created inside a hose while trying to run a siphon at very high altitudes — or indeed, simply running a very long siphon. Nothing is pushing meaningfully from the bottom; instead, when you suck on a straw you are pulling on the water molecules near you by creating a pressure differential, and those water molecules are pulling on the water molecules beneath them. Or so says science, anyway. And Nature.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
I am surprised that there are people that this is not obvious to. Sure, atmospheric pressure keeps the liquid together and from forming bubbles, but the movement is pure gravity...
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
Now I have to go get some dry ice and see if I can set up a CO2 siphon. It would be fun to try it with sulfur hexafluoride, but my grocery store still refuses to stock that.
If you can set up a siphon for a dense gas -- and my intuition, which is admittedly out of its depth here, tells me it should work -- that would seem to argue against the "pulling on water" explanation.
Well, pulling on water would work over short distances I would think, but the surface tension of water isn't THAT strong.
A siphon works due to a combination of gravity and air pressure. It doesn't work in general without both being present. The slashdot summary is pretty weak, as TFA more-or-less points this out (if you lower the air pressure sufficiently the water columns on both sides of the siphon separate and flow stops).
In a siphon the water on the one side falls due to gravity. That creates a reduction in pressure on that side of the tube, which means the other side of the tube has atmospheric pressure on one side, and less than atmospheric pressure on the other, which causes its fluid to rise. However, atmospheric pressure can only lift it so far - a tall enough siphon tube will just create a vacuum at the top and the water will not flow over the top..
No. it's the pressure that pushes the mercury up to the top of the tube, but the reason why it then flows down the other side is because the weight of the mercury on the down side is higher. The diagram makes it obvious that this must be the case because the water pressure in the lower beaker is obviously higher than the water pressure in the higher beaker.
All I want is a secure system where it's easy to do anything I want. Is that too much to ask ~~ Randall Munroe
I think in most cases the flow itself would keep bubbles in check. Bubbles move at a fixed speed up a liquid. As long as the liquid is moving faster than that speed through the siphon, bubbles shouldn't be an issue.
But I don't know the dynamics of what happens if an air pocket manages to form at the top. It may or may not dissipate on its own. Or it may grow, slowing and eventually stopping the siphon action.
Folks with aquariums know about bubbles.
Since flow is partly a function of the cross section flowing a bubble
at the top of a siphon reduces the flow. It may be possible to
have a small pipe on the top and a longer but smaller siphon
in a position to pull from the top and drain the bubble.
The most interesting siphon is the inverted siphon. Used by the
Romans -- this was quite the engineering effort of the age.
Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't. Mark Twain.
I actually had a problem like this on a final exam in Fluid Mechanics back in the late 1960's. The required answer was the flow rate through the siphon for which there is an equation that uses parameters such as the pipe diameter and the difference in elevation between the inlet and outlet and some other things. However, the problem was given in such a way that it was not at all obvious that the siphon (which was a water siphon on Earth) at one point exceeded more than 33 feet above the inlet point. If you READ the entire problem carefully and CHECKED for this NECESSARY condition you had the answer (zero flow rate) with NO calculations needed -- a real time saver on the slide rule!
I'm not sure you do need atmospheric pressure. Experiments have been done at my university with an ionic liquid as the working fluid in a siphon that is in a vacuum chamber. An ionic liquid has virtually no vapour pressure so remains liquid even under vacuum.
It was demonstrated that the siphon continued to work, even at high vacuum (i.e., as good as you can get with a standard vac pump used on a lab fume hood).
Here's a video of a siphon working in a vacuum (10^-5 mbar) (and also a link to the paper featured as a result of the work).
You *do not* need an atmosphere, or atmospheric pressure to make it work.
https://www.youtube.com/watch?...
You just need a liquid that won't boil off under vacuum. The only reason the Nature experiment failed was because the working fluid was water, which boils at low pressure.
No, that "correct" explanation is wrong.
You do not need an atmosphere.
This video demonstrates a high vacuum siphon.
https://www.youtube.com/watch?...
No, you do not need any atmosphere at all. This has already been proven (and demonstrated on video!) yet people still seem to think "common sense" prevails.
https://www.youtube.com/watch?...
Without atmospheric pressure, there's nothing to prevent the water from boiling, nor to push the water column up beyond the miniscule bit water tension/cohesion would give. You could do it in a vacuum if you had a liquid that doesn't boil in vacuum, and limit the height of the siphon to whatever was allowed by cohesion and capillary action, and perhaps needing to limit the rate of drainage. Some people might still qualify this as a siphon, others might not.
If this still confuses you, consider the equivalent of a siphon created by pulling a portion of a long rope up, over a pulley, and down below the level of the rest of the rope. Then gravity will pull the rope up and over the pulley and down to a lower level, like a siphon. A liquid siphon functions the same way, powered by gravity, but atmospheric pressure pushing the water up takes the place of the rope's tension. More atmospheric pressure will allow for the siphon to go over a taller hump, or increase the maximum flow rate for a given diameter siphon.
Pretty much any scientist knows all this. Apparently the news was because the Oxford English Dictionary wasn't written by a scientist.
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TFA demonstrated that when atmospheric pressure fell too low, the siphon stopped working. When it was increased again, it re-started. At no point was the atmospheric pressure below the vapor pressure of the liquid (though that condition did exist inside the tube at some points).
In the video, they produced a fairly exotic liquid that substitutes weak ionic bonds for atmospheric pressure. If that's fair game, so is a magnetic liquid in free fall proving that siphons operate on magnetism. Or for that matter, a steel chain.
But most liquids do need atmospheric pressure over and above their vapor pressure.
The title is wrong, full stop. Siphons work because the weight of the fluid in the lower side of the siphon causes the pressure of the fluid at the top end to drop. Atmospheric pressure then pushes the fluid into the top end of the siphon. This is obvious and (as far as I was aware) was what's always been meant by "siphons work due to atmospheric pressure".
Claiming it's "not due to atmospheric pressure" is wrong.
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Now you're changing your argument.
TFA demonstrated that the siphon stopped working when the pressure fell too low because of the properties of the fluid being siphoned, not because a siphon requires atmospheric pressure to work.
An analogy would be that I can demonstrate that F=ma is not true for high values of F and small values of m because air resistance starts to affect the result. This doesn't mean that the equation doesn't work at these values, just that the experiment cannot measure the data under those conditions.
The water siphon experiment is the same - it stops working at low pressure, but not because you need pressure for a siphon to work. It's simply not possible to take data because the water boils off.
The ionic liquid experiment demonstrates that you do not need pressure for it to work at all - since it operates in UHV. The ionic liquid has cohesion in the same way that water does - it just has more of it due to the physical properties of the liquid. However, it is clear that gravity is the most important part, since you can siphon almost any liquid (like gasoline, which has very little cohesion compared to water or an ionic liquid) as long as you have a change in elevation.
There's no "substitution" of weak ionic bonds for pressure, because it's not pressure that is driving the water siphon. There happens to be pressure, purely because there's an atmosphere, but it's not why the siphon works. The water siphon works because of the cohesion of the water and gravity... just like the ionic liquid version under vacuum.
Ionic liquids aren't really "exotic", they're just uncommon to non-chemists. Almost any melted salt is an ionic liquid. If you make it with large, oddly shaped diffuse ions then it tends to be liquid at room temperature. They flow like other liquids. They can be decanted, they have surface tension, they work as solvents. There's no "cheating" or substitution going on. It was just used because it has a low vapour pressure and can thus go beyond the range capable with water.
You could do the same experiment with a liquid metal, such as mercury (convenient) or any other metal that you can keep liquid long enough to test it if you can stop it solidifying, although even mercury has a vapour pressure and will boil off in a high vacuum so you'd have to be careful about repeating the experiment.
Yes, and what of a chain and magnet providing potential energy? Still a siphon? It acts more or less like one.
I am simply arguing the other side of the coin. You argue that very specific liquids (not just uncommon to non-chemists, uncommon in conditions on earth's surface) can be siphoned without atmospheric pressure and gravity drives the thing (at least I presume you argue that). I argue that that with atmospheric pressure you get a much more generally usable siphon.
If we're going to accept the corner cases, might as well accept the steel chain with a magnet for potential energy as a siphon and then gravity cannot be claimed as driving force either.
However, it should be noted that even ionic liquids may have a non-zero vapor pressure and still need an atmospheric pressure above that to work. It's just that that pressure is typically considered a hard vacuum. (one extreme case deserves another :-)
It's sort of like the case of a light bulb. You may argue that a break in the circuit means the light goes out. That is so frequently true that you will get little argument. I might argue that if the wires are long enough and the frequency of the power source is high enough it will stay lit so you are wrong. My argument is technically correct, but it has to be acknowledged to be an extreme corner case and largely irrelevant.
I argue that that with atmospheric pressure you get a much more generally usable siphon.
No, you're arguing that an atmosphere is *required* for a siphon to work when it clearly does not.
The use of an ionic liquid to prove that is not an extreme corner case, it's simply making use of the properties of a particular substance to test a hypothesis.
Elemental sodium is very rare on the earth's surface, but I wouldn't call an experiment to test how it reacts with water or with air to be an extreme corner case - there's no doubting that it's a controlled experiment. You can't dismiss the results of the experiment because you think it is too niche. It's one of several experiments carried out into the function of a siphon and the conclusions are not drawn just from that one case.
That is what has been done here, along with other measurements using different liquids and conditions to determine how a siphon works and the variables that affect it. It has been determined through multiple experiments by different groups that an atmosphere is not needed. While it has an effect on the liquid that you are siphoning in terms of its physical properties, it has no function in the mechanism of the siphon itself as expected.
As has been pointed out, a siphon moves liquid from high to low elevation via an uphill pipe. The outlet of the siphon is at higher pressure than the inlet. If pressure affected the siphon then it would be an inhibitory one - however, this effect is not seen at varying pressure - i.e., you'd expect to see the effect diminish as the pressure dropped if it had an effect at all. The fact that you don't see this at different pressures and that the liquids behave the same regardless of pressure (up to the limit of that particular liquid's vapour pressure) demonstrates that it has no function in the way a siphon works.
Your comparison to a magnet and chain is not relevant. You're introducing new forces (and while ionic liquids contain ions, they're not magnets), and the chain itself is a contiguous object. It "acts like" a siphon, but then a piston driven by steam extends in the same way that a solenoid driven by electricity does. Does that mean the solenoid is driven by pressure because it acts like the steam piston?
I am trying to generalize the definition of a siphon and see where we stop calling it a siphon. Since you don't seem interested, I'll stop.
And why are all drops exactly the same size? Oh that's right, they're all bonded together.
Look at the replies to that post you referenced, explaining why it's wrong.
Granted, the video of that experiment does show that, if you have an extremely cohesive liquid (an ionic liquid, comparable to a bunch of magnets much stronger than water molecules), you can get it to siphon up a few cm using the cohesive force. But I bet they couldn't get it up to even 10 cm or so.
A normal siphon, under atmospheric pressure, can siphon water up to 10 m which also happens (coincidentally?) to be the height of water that corresponds to one atmosphere of pressure. In the Nature experiment referenced in the article, the water in the 1.5 m siphon broke up when they reduced the pressure to about 0.18 atm. How can you then say that atmospheric pressure has nothing to do with it?
I'll give you a car analogy:
Suppose we see a bunch of cars traveling up a 2000 meter mountain. I would say that their engines are pushing them up the hill. You would say that, no, their engines have nothing to do with it. You would then show an experiment where a car traveling at high speed would cut it engine and then coast up and down a small 20 meter high hill. See, cars don't need engines!
Yes, you can siphon up a very small height (just two centimeters or so) using an extraordinarily cohesive liquid. That doesn't imply that siphoning has nothing to do with atmospheric pressure.
And how high could you pull up the liquid? I bet it wasn't more than a few cm. At least it was in the experiment I saw on youtube. And, like you said, that was using an extremely cohesive ionic liquid.
I really wish someone would try that experiment with an apex more than 10 cm above the liquid surfaces. I'm pretty sure the fluid will break up.
Using atmospheric pressure, though, you can siphon ordinary water up to 10 m high.
For such a pedantic dialogue as this thread, I was hoping to see someone write "the air pressure and internal fluid tension set a limit ..." before I reached my pettifogger deFUDer saturation point, but it was not to be.
The water in the experiment was still way above boiling point: at 0.18 atmosphere, water boils at a temperature of more than 50C. Of course the pressure up in the tube gets lower than 0.18 atm, but the whole point is indeed that the column breaks up when the pressure in the tube approaches zero. Not because the water boils, but simpy because it won't stay together. Unless you have a particularly cohesive fluid like some ionic fluids. You can use those to actually siphon up a few cm in a vacuum because they can stand a small amount of tension (negative pressure) without breaking up. But you'll get nowhere near the 10 meters you can get under atmospheric pressure.
The ionic liquid they used for that experiment is a particularly cohesive liquid that even stays together under a small amount of tension (negative pressure). They only managed to get it up a few cm, though. Too bad they didn't try higher, I'm sure the fluid would break up at around 10 cm or so, probably even lower. Certainly nowhere near the 10 meters you can siphon water up to using atmospheric pressure.
It's really just like a drinking straw: when you suck, you reduce the pressure in your mouth (still a positive pressure, just less than normal) so that the atmospheric pressure pushes the liquid into the straw. In a siphon, gravity is providing the suction. But you can't suck (much) more than atmospheric pressure because the fluid breaks up when the pressure reaches zero. Except for very special liquids that can go slightly below zero, but not much.
Yes, it shows that you can siphon an extremely cohesive ionic liquid up to a very small height of just a few cm. The liquid would probably break up if you tried more than 10 cm or so, and you certainly wouldn't get anywhere near the 10 meters you can siphon ordinary water up to under atmospheric pressure.
The liquid is special because it is so cohesive that it can actually stay together under negative pressure. That is quite extraordinary, but certainly a very limited effect. Normal fluid breaks up as soon as you try to pull it apart, and so will even the ionic fluid if you tried only a little bit more height.
The water in the experiment was still way above boiling point: at 0.18 atmosphere, water boils at a temperature of more than 50C. Of course the pressure up in the tube gets lower than 0.18 atm, but the whole point is indeed that the column breaks up when the pressure in the tube approaches zero. Not because the water boils, but simpy because it won't stay together.
It's not simple at all. There's gases in the water. When the water approaches its boiling point, the gases begin to escape more rapidly. That itself may be enough to break the siphon.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
It's not that much more cohesive than water - in fact, it is less. The surface tension of the ionic liquid used is less than that of water, yet the siphon still works.
The fluid breaks up due to a lack of hydrostatic pressure if you change the elevation too much - with more liquid in the reservoirs, you could siphon across a larger height without breakup of the fluid.
Here's the conclusion of the paper:
"Although this experimental setup is a special example of a siphon, liquids with low or near-zero tensile strengths can be easily demonstrated to function in siphons at a normal positive pressure. It is therefore concluded that whereas cohesion does have a part to play in most siphons, the underlying principle is most readily explained in terms of gravity and hydrostatic pressure differential without regard to the mechanism of atmospheric pressure or cohesive force."
In the video, they said that the ionic liquid is like a bunch of little magnets sticking together but still able to move around each other. They stick together so well that the fluid wouldn't even evaporate in outer space. How can it be less cohesive than water, then? I know water molecules are little bipoles too, but water certainly does evaporate in outer space, so I would assume that the cohesive force is a lot less, no?
And the part about the lack of hydrostatic pressure... Do you mean the tubes have to be inserted deeper into the liquid? I don't think that matters at all, since the pressure in the tube as it passes through the surface will always be the same as the pressure above the surface. It doesn't matter how deep or wide the reservoirs are.
I do understand the conclusion: some siphons can work using the tensile strength of the fluid instead of (or in addition to) atmospheric pressure, but that doesn't mean you shouldn't at least mention the requirement.
The video was giving a layman's description of cations and anions - i.e., the components of an ionic liquid.
The actual measured scientific data demonstrates that the ionic liquid used has a lower surface tension than water - most liquids do, since water is one of the most cohesive liquids there is. That's just a measurable fact.
Water hydrogen bonds, and the physical properties of water are very strange, but it evaporates in outer space because the vapour pressure is higher than that of the ionic liquid. There are a number of factors that go into the resulting vapour pressure, of which cohesion is one. The properties of the ionic liquid give it a near-zero vapour pressure, but a lower surface tension (and cohesion) than water. It's just how it turns out.
Hydrostatic pressure is the pressure exerted on a fluid by the weight of the fluid above it - it's why the pressure at the bottom of the sea is high - there's a large amount of water above pressing down. The hydrostatic pressure at the bottom of the siphon reservoir is created by the weight of the liquid pressing down. The deeper that reservoir is, the higher the hydrostatic pressure will be, and the higher you will be able to siphon. The level of the surface of the liquid above the opening, along with gravity, determines the hydrostatic pressure (assuming that the pressure of the gas or lack thereof above the liquid is fixed - i.e., in a vacuum it is zero, for a normal siphon it is close enough to equal to not matter, but ever so slightly higher at the lower siphon exit).
I think that when people say that a siphon works by gravity, they mean that gravity is the driving force that makes the water flow through the tube. You have shown that you need ambient pressure to make a siphon work, but that is similar as saying that you need a tube to make a siphon work. Saying that a siphon works by gravity and ambient pressure is equivalent to saying that a gas engine works by burning gas and sparks.
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For such a pedantic dialogue as this thread, I was hoping to see someone write "the air pressure and internal fluid tension set a limit ..." before I reached my pettifogger deFUDer saturation point, but it was not to be.
Good point, but since I was talking about a height of "about 10 metres" for water (not the most accurate of heights) and the internal fluid tension supports I would guess less than a centimetre, I figured the internal fluid tension was more of a rounding error than anything that needed to be explicitly stated. But epine is correct, the internal fluid tension does add some (small for water at least) height to the effective max for suction pumps and siphons.
What? You'd be right to say a water siphon requires atmospheric pressure but that's a special case of siphon. The concept of a siphon itself requires only gravity to explain its operation. Your car analogy is simply not a fair analogy; siphon operation has nothing to do with momentum.
The siphon stopped working at 018 atm. It's perfectly predictable that it would function as the pressure was lowered until at a certain point it would stop working, although it would be tough to calculate the exact point. Of course it needs both gravity and air pressure to operate.
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Was this ever under any doubt? That's what they taught us in junior high.
Star Trek transporters are just 3d printers.
That's pretty close to the point. the water at the apex will cavitate.
Star Trek transporters are just 3d printers.
The textbook example for this is trees. They can lift a column of aqueous solution way into the air by dint of capillarity, temperature effects, surface tension, and various other effects, which AFAIK still aren't completely understood.
Star Trek transporters are just 3d printers.
Two things basic physics should teach us: you can't push on a rope, and you can't pull on a fluid.
Star Trek transporters are just 3d printers.
That's totally incorrect. There is no "surface" involved. (What surface do you think is involved?)
It would seem, though, that we're beyond the realm of basic physics here. Capillary action and surface tension both involve "pulling on a fluid". I don't think of siphons as operating in that realm, but I'm no hydrodynamicist.
Like a lot of these questions, the answer is the same as a multiple-choice question when you were in school: All of the above.
Assume that only one thing is acting, and you will find yourself "up the preverbial polluted waterway without a means of propulsion."
So if you dig a hole in the ground, have you wasted your time?
Or are you confusing topological definitions with more useful definitions that can distinguish between a teapot, a doughnut and a straw?
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the "pulling on water" explanation
Just cut the tube across its length, such that it becomes something like a canal without cover. Now try the siphon. Pulling on water applies equally in a cut open "tube", but siphon does not work.
So playing with gas shouldn't be necessary - though it might be fun.
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You can observe siphon action with various non-fluid objects - you can accomplish the same with a string of beads and a couple of jars, for instance.
I would have gone with a train analogy