Slashdot Mirror
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
A straw with a hole in it cannot siphon.
You win again, gravity!
rewriting history since 2109
Have they ruled out Peer Pressure as well? I wonder if Peer Review can help there.
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?
A bullet may have your name on it, but artillery is addressed to " Whom It May concern"
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.
The article discussed in the paper was not, in fact, published in Nature. It was published in Scientific Reports, which is an online journal published by the Nature Publishing Group, the publisher of Nature and many other journals.
Seriously, people thought that ?
Obviously members of the "the world is flat" club.
I didn't know that
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.
Gravity pulling on the liquid creates a pressure differential -- but only if there's atmospheric pressure.
Don't waste your vote! Vote for whoever you want, unless you live in a swing state it won't matter anyways
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.
More Caffeine. NOW
The pipe/tube that is the siphon, rose to a height of more than 10 meters. 10 meters is the height that you cannot "suck" water above, because of it weight is greater than pressure pushing. Basically, the water would turn to a gas crosses the apex. If it still flowed, then I would "believe" more fully. Based on what I understand, if gravity is the sole component, then moving water over a mountain would be easy.
I'd be more impressed if the Oxnard English Dictionary accepted the change.
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.
Intellectual Property is a monopolistic, selfish, and defective concept. It is "tyranny over the mind of man"
Atmospheric pressure is actually due to gravity.
Actually combination of gravity and surface tension of the fluid. Gravity pulls the fluid down the tube, surface tension makes it want to stay together so it's friends go along for the ride.
Well, obviously, since it is all due to gravity.
Excuse me, but please get off my Pennisetum Clandestinum, eh!
Water doesn't suddenly boil in a vacuum, it would still be liquid water. Water, in liquid form, is not compressible, so its size would even expand. In the absence of ambient air, if left long enough, the energy in the system should dissipate, and after a few minutes the water turns in ice at an astonishing rate. You can find many youtube videos of people using vacuums to freeze water.
The article actually says that in a vacuum the siphon breaks and you are left with two water columns.
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.
Huzzah! If only my high school physics teacher was still alive. We frequently argued this point.
-- This sig is only a test. If this were a real sig it would say something witty. --
this was in the news 4 years ago with arguments going back and forth about whether it was gravity or atmospheric pressure...
See this register article entitled siphon wars
http://www.theregister.co.uk/2010/05/17/siphon_retaliation/
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.
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?
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?
We'll have to have a "gravity siphon" and a "barometric pressure siphon". Or, probably more accurate, a totally different class as the two mechanisms and forces are totally different than each other.
"Physicists Prove the Moon Drives the Tides"
"Scientists Discover Light Is Made of Oscillating Electric and Magnetic Waves"
"It's Official: The Earth Is Not Flat"
some still calling this weather? http://www.youtube.com/results?search_query=weather+manipulation+WMD
that English majors know literally nothing about (per the new definition of the word "literally") but let's raise a massive shitstorm over this one.
There.
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."
Another of these BS "factors" that push people to pad their references with as much BS as possible.
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?
In order to get a syphon flowing, at least everytimne ive done it, you have to suck on one end to get the fluid going (AKA create a pressure differential). After that, gravity and fluid tension do the rest.
I thought that would be obvious tho, somebody wrote a damn paper on it... lol. Too much time on this guys hands.
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).
...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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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.
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 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.
Agreed. This Slashdot article is very misleading in referring to the experiment being published in Nature.
Gravity pulls one side down; pressure pushes the other side up.
where do you think the atmospheric pressure comes from
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.
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).
Actually redwoods exceed the hieght of capillary action, hence there location, and novel technique of retrieveing liquid from low laying clouds...
Now syphons work due to gravity instead of atmospheric pressure. Thanks Obama.
Proverbs 21:19
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.
Yeah, either way its NPG, so probably bullshit.
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.
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
Are you talking about a U or an inverted U?
Nicely discussed in this introductory textbook:
http://books.google.ch/books?id=OtxEAAAAIAAJ&focus=searchwithinvolume&q=siphon
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.
I'm talking about the high leg. You're placing me on the low side.
Trees don't use capillary action to raise water. That's also a widespread misconception. Best explanation of the process I've ever seen:
http://www.youtube.com/watch?v=BickMFHAZR0
Negative pressures, like negative temperatures, are one of those fun little physics areas that seem counter-intuitive at first.
Atmospheric pressure keeps the liquid liquid.
Why would you think differently?
When you look at the difference in air-pressure between the top and bottom, its negligible. The difference in water pressure on the other hand it significant.
It's covered in basic fluid mechanics.
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.
This was all part of the standard curriculum in my high school physics class, decades ago.
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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.
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.
even simpler way to start (with no nasty backwash) is to fill the hose with water. totally submerge it... then lower one end so it's below the surface level of the water.
I was getting some work done on my deck (3 stories up). the drain got clogged with debris, as a result, I ended up with about 6 inches of standing water.
dropped the garden hose in the water.. got all the air out of the hose, and dropped one of the hose about 1 story lower than the deck. water emptied in about 20 minutes - almost directly into the storm drain.
...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.
I believe j-beda was talking about an inverted U-shaped tube, with the open ends at the bottom.
> 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.
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.
Without a big to do.
Not to be a fuck head here but don't he fly city size shit in the air ... like so 1984 ago .
Agree! LOL!
must (eventually) blow...
e.g black hole http://en.wikipedia.org/wiki/Hawking_radiation
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.
Rampant carbon sequestration destroyed the Dinosaurs' tropical paradise. I'm here to help repair the damage.
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.
A siphon can form without pressure - TRUE
A siphon can form without cohesion - TRUE
A siphon can form without gravity - FALSE
All of these forces have an effect on siphons. The most important force is gravity - THE END
I don't suck!
In Soviet Russia, dot slashes YOU!
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.
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.
Star Trek transporters are just 3d printers.
Burn the witch! Burn the witch!
https://www.youtube.com/watch?v=8F4i9M3y0ew
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."
A straw, a standard coffee mug, and a donut all have just one hole. So actually, a straw with one hole (the standard kind) can siphon ;)
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.