A physicist has demonstrated that the operating force in a syphon is gravity, and not atmospheric pressure, and has corrected a 99-year old incorrect dictionary definition.
In 2010, eagle-eyed Dr Hughes spotted the mistake, which went unnoticed for 99 years, which incorrectly described atmospheric pressure, rather than gravity, as the operating force in a siphon.
Dr Hughes demonstrated the science of siphons in a paper published yesterday in Nature Publishing Group journal Scientific Reports.
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Physicist's Experiment Corrects Dictionary
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(Score: 1) by dmewhort on Thursday April 24 2014, @10:28PM
Turn it upside down, do the contents still flow out the small end?
Doug
(Score: 1) by Snow on Thursday April 24 2014, @10:29PM
Is a syphon possible without any air pressure? I'm thinking no. The syphon tube has to be filled with fluid in order to work. Without any air pressure, you couldn't simply suck the air out, so you would have to pre-fill the syphon tube. But without any air pressure, what would prevent a void from forming in the syphon tube? I think that the tube would just drain, but without the syphon effect.
If I'm correct (and I may not be) then air pressure is a requirement for a syphon, but gravity is the primary force... Just a thought.
(Score: 5, Informative) by Anonymous Coward on Thursday April 24 2014, @10:38PM
Google is your friend: http://www.youtube.com/watch?v=8F4i9M3y0ew [youtube.com]
(Score: 1) by Toaster42 on Thursday April 24 2014, @10:57PM
I was just thinking about this video. It blew my mind so much when I first saw it. I had always trusted the dictionary ;-)
All higher forms of thinking come from neural connections built by solving the kinds of problems encountered in math.-Md
(Score: 3, Informative) by frojack on Friday April 25 2014, @12:16AM
BUT, they had to use a liquid that has a certain level of cohesion, sticks to itself, for the down hill side to pull liquid up the up-hill side. They specifically said water wouldn't work in a vacuum.
Its not clear if all liquids have this property. Some may simply turn to gas to fill the void in the top of the pipe.
Still it seems obvious that air pressure can't have any bearing on this, because pressure is the same (or less) on the uphill reservoir as the down-hill reservoir.
No, you are mistaken. I've always had this sig.
(Score: 0) by Anonymous Coward on Thursday April 24 2014, @10:57PM
Uh, I think that's exactly covered in the article. The scientist compares the atmosphere to the car's wheels and gravity to the engine.
(Score: 1) by Rich26189 on Friday April 25 2014, @12:08AM
Here's how I'm thinking of it.
Simplifying assumptions: Water and "U" shaped tube, up side down of course, and one leg shorter than the other. Both legs are straight.
Water though fluid is not elastic and won't stretch or compress very much if at all. Once the syphon action is started flowing (into the "higher" open end of the short leg and out of the "lower" open end of the longer leg) it would continue. The water in the longer legs 'feels' a greater downward effect of gravity. (Think of balancing a rope over a pulley, sort of but don't get too caught up in the analogy.) The water in the tube would not stretch but pull the water up through the shorter leg of the tube.
Thought Experiment: The syphon tube has unequal length legs but the longer leg has been coiled such that its open end is now higher than the open end on the shorter but straight leg. The syphon action is started, "in" the open end on the high side and "out" the open end on the low side. Would it continue to flow on its own? I say yes, but I could be wrong.
Rich
(Score: 3, Insightful) by TheLink on Friday April 25 2014, @03:10AM
Siphoning doesn't require liquids: https://www.youtube.com/watch?v=_dQJBBklpQQ [youtube.com] :).
Of course people might not consider that as siphoning but the principle is still there
Siphoning does require stuff to "stick together" whether by stickiness or atmospheric pressure or other means.
The direction stuff flows depends on which part of the "chain" pulls (or is pushed) more (assuming the other things are similar enough- if you can have stronger "gravity" at one side, stuff can flow differently).
(Score: 0) by Anonymous Coward on Friday April 25 2014, @05:04AM
You're correct in that an air pressure that causes the liquid to remain a liquid throughout the siphon is a requirement, yes, but in that case you could also say that a correct temperature is also a requirement.
I do wonder, however: with a sufficiently high tube and sufficient gravity, could one, regardless of air pressure, cause a siphon to stop working for similar reasons?
(Score: 2) by wonkey_monkey on Thursday April 24 2014, @10:43PM
I can't make head nor tail of that. Waterfall? Broke into two columns of water? Whut?
systemd is Roko's Basilisk
(Score: 4, Informative) by TheB on Thursday April 24 2014, @11:20PM
The "waterfall" is due to the separation of gasses from the water near the peek in the siphon tube.
Video of the waterfall from the article.e xtref/srep04741-s3.mov [nature.com]
http://www.nature.com/srep/2014/140422/srep04741/
(Score: 2) by frojack on Friday April 25 2014, @12:20AM
The water is turning into a gas at that altitude, breaking up the flow.
At some altitude, the tendency to break up into a gas may be strong enough to fill the tube with water vapor preventing any effective suction.
No, you are mistaken. I've always had this sig.
(Score: 4, Informative) by c0lo on Thursday April 24 2014, @11:35PM
RTFA, pal. Otherwise, for any others having troubles (and/or being too lazy to RTFA), some simplistic explanations:
* one can't hard-boil an egg over approx 10000m/30000ft - the water boils at 65-70℃ insufficient to coagulate the protein in the egg;
* on the other side, water stays liquid at over 300℃ in the hydrothermal vents [noaa.gov] because of the high pressure of the water on top of it.
Supplementary, keep in mind the pressure varies within a column of water.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2, Insightful) by wonkey_monkey on Friday April 25 2014, @08:24AM
I did RTFA, dude, as should have been obvious since I quoted it. I tried to understand it; I was not successful. Sorry if it seems "lazy" to you that I then dare to ask for clarification.
Given the basis of this story, it should hardly be surprising that some people don't have quite the same grasp of the mechanics of siphons as you do.
systemd is Roko's Basilisk
(Score: 2) by c0lo on Friday April 25 2014, @02:05PM
Oh, shit. I thought that's elementary.
Still having troubles in understanding a siphon at normal atmospheric pressure? Imagine a chain over a pulley: if one of the arms is longer than the other, that arm will get pulled more by gravity, thus it will drag the shorter arm over the pulley.
Nothing different with the liquid, except that the molecules that make the "chain of liquid" will lose cohesion if the ambient pressure is too low (or the temperature is high enough).
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by wonkey_monkey on Friday April 25 2014, @10:52PM
An elementary understanding of siphons is one thing. Knowing what a siphon expert means when he writes about a "waterfall" appearing with no further clarification is another.
As for your analogy:
http://en.wikipedia.org/wiki/Siphon#Chain_analogy [wikipedia.org]
systemd is Roko's Basilisk
(Score: 2) by c0lo on Saturday April 26 2014, @12:42AM
Yes, I knew it, but anyway, thanks for the notification.
As any type of model, any analogy will ultimately be flawed, by the very nature of the analogy definition [oxforddictionaries.com]. This doesn't make the use of an analogy invalid; on the contrary it's a great opportunity to examine what are the limits of analogy and what are the specific details that make the analogy break (and also a great risk in accepting the analogy as valid and move on with a flawed understanding. Good for you that you didn't.)
In this case:
"That is, under most practical circumstances, dissolved gases, vapor pressure, and (sometimes) lack of adhesion with tube walls, conspire to render the tensile strength within the liquid ineffective for siphoning.".
Grok this and you understand why that "waterfall" was formed: water needs external pressure to stay tensile, a chain doesn't; however, there are liquids [wikipedia.org] with a very low vapor pressure - resistant to evaporation, doesn't lose tensile strength - which "siphons" very well [youtube.com] in vacuum.
"A further problem with the chain model of the siphon is that siphons work by a gradient of hydrostatic pressure within the siphon, not by absolute differences of weight on either side" - well, the analogy can be "forced into correctness" to account for this specific difference. But indeed, unless one can't deal with gradients and integration, would be a waste of time to do so.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 1) by TheB on Thursday April 24 2014, @11:23PM
Isn't a siphon effected by both gravity and pressure difference?
Are they saying that the force of gravity is the dominant force causing the water to flow, or are they saying pressure difference has absolutely no effect?
What about other fluids with different specific gravity.
How does siphoning of lighter than air gasses work?
I thought that this was all well understood.
(Score: 4, Informative) by c0lo on Thursday April 24 2014, @11:46PM
Isn't the atmospheric pressure still an effect of the gravity?
It simply doesn't. Nor it does work for air or other heavier-than-air gases.
Siphoning works only when the medium subject to siphoning is cohesive and can bend with a low energy cost; liquids are an example, but you can describe as "siphoning" what happens with a chain over a pulley for cases of different lengths in the two "arms" of the chain delimited by the pulley: the longer arm weights more and pulls the shorter arm up.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 1) by TheB on Friday April 25 2014, @10:01AM
Thanks for the reply.
I forgot about the need for cohesion. You are right gases don't siphon.
How about a light liquid in a heavy liquid environment?
Yes it is. It isn't the only potential source of a pressure difference.
My reading of the article led me to believe that Dr Hughes was claiming that atmospheric pressure had no effect on siphoning.
From reading the paper, I see he is claiming that a low pressure area at the peek of the siphon isn't causing the flow. Cohesion alone is. He however isn't claiming the pressure difference has no effect, and at the end mentions that the flow would be greater if there was low atmospheric pressure.
The article makes more sense to me after reading the paper.
Shouldn't the summery state that siphoning is a product of gravity/cohesion. Both the (atmospheric pressure/low pressure zone), and cohesion theories had gravity. Only the dictionary ignored it.
Source > article > summery > TLDR poster.
(Score: 1) by Renevith on Friday April 25 2014, @06:00PM
http://www.merriam-webster.com/dictionary/gasses [merriam-webster.com]
"plural gases also gasses"
If you're going to be pedantic, try to be correct next time. Or better yet, just save the snark and stick to the topic? Your post was very informative but I was totally distracted by the unnecessary and inaccurate correction. Thanks.
(Score: 2) by c0lo on Saturday April 26 2014, @12:00AM
This I'll do. My apologies.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2, Informative) by sjwt on Friday April 25 2014, @12:54AM
But the pressure is HIGHER at the lower point, it would work in reverse if it relied on the difference in atmosphere. IIRC I was taught its about converting potential energy into kinetic energy, you need a difference of greater than the energy loss though friction to keep it going.
(Score: 1) by urza9814 on Friday April 25 2014, @02:54AM
The lower pressure can exert more force though if it's acting over a larger area.
Put a straw into a cup of water. Put your thumb over the end of the straw and pull it up, so that it holds the water in the straw as you lift it. Then release your thumb. The water in the straw will of course drop, but it *should* stabilize slightly above the water level in the glass, because the surface area of the water in the glass is larger so the atmosphere exerts more force.
At least that's what was demonstrated and explained on...some science show I watched recently ;) I think it may have been an episode of Connections which I've been watching after seeing someone here refer to it (BTW, Thanks to whoever that was! Great show!)
(Score: 2, Informative) by Anonymous Coward on Friday April 25 2014, @03:21AM
WHAT. THE. FUCK.
The water level in the straw is higher because of the surface tension of water in inside edge of the straw pulling it up.
By your logic, if we put a tube on the sea surface, water will sprout out because the sea surface is millions of time larger so the force should be so much greater that water should shoot up the tube like a sprout!
(Score: 2) by rts008 on Friday April 25 2014, @08:39AM
Pressure difference is caused by gravity. Think Earth's atmosphere, and the relationship to altitude/ air pressure and distance from Earth's core/'strength' of gravity.(hint:in a 'siphon', low pressure is at the 'high' end of the U, and high pressure is at the 'low' end...by your reasoning(pressure diff's a major component), siphons should flow from low levels to higher levels of the U)
Lighter than air gases work in a similar manner, except gases can readily compress compared to liquids. Thus you see both higher pressures in the low end(relative to the high end, compared to liquids), and more mass in the low end(higher gravity), so the 'siphon' flows a little slower compared to a liquid.(less mass=less gravity=less flow when talking siphons)
Those 'WTF??!!??' moments in the 'research labs' are the golden moments of Good Science...A 'true scientist's' Wet Dream!
'Eureka!' is over-rated due to 'gaming the system' to produce results to support your 'discovery'.
(Score: 2) by unitron on Friday April 25 2014, @09:39AM
...even from yourself, air pressure is what you have to overcome or eliminate before gravity can make the siphoning work.
(which is really zen since gravity is the reason for the air pressure in the first place).
something something Slashcott something something Beta something something
(Score: 1) by broken on Friday April 25 2014, @07:21PM