# How does a siphon work
**URL:** https://boards.straightdope.com/t/how-does-a-siphon-work/423969
**Category:** Cecil's Columns/Staff Reports
**Created:** [October 25, 2007, 3:19pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969 "2007-10-25T15:19:34Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@LarryParker](https://boards.straightdope.com/u/LarryParker)
#### Post date: [October 25, 2007, 3:19pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/1 "2007-10-25T15:19:34Z")
Cecil - with reference to your article on how siphons work - it would seem to me that there is a setup to confirm the hypothesis that the weight of water in the length BC is greater than the weight of water in length AB, thus pulling (by vacuum abhorrence) the water from AB over point B.
Simply recreate the setup with the endpoint C above the level of the original water, but keep the hose length the same (e.g. turn it into a spiral).
In this way, the weight of the water in BC is still greater than that in AB and should siphon the water back into the original container. Simply add a turbine at the bottom and “siphon” off the energy.
This might have the appearance of a “perpetual motion machine”, but it would actually fail as soon as the Earth’s gravity disappeared. :dubious:
I am now off to develop my own siphon-based free electricity apparatus in a nearby barn and make millions from setting it up in back yards across the country. 😃
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### Author:  [@LarryParker](https://boards.straightdope.com/u/LarryParker)
#### Post date: [October 25, 2007, 3:25pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/2 "2007-10-25T15:25:17Z")
Sorry - with reference to [this](http://www.straightdope.com/columns/010105.html) article.
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### Author:  [@bibliophage](https://boards.straightdope.com/u/bibliophage)
#### Post date: [October 25, 2007, 5:05pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/3 "2007-10-25T17:05:42Z")
Welcome to the SDMB, Larry.
We turned off HTML coding here a long time ago because it was abused. I’ve fixed your link, putting it in [vBulletin code](http://boards.straightdope.com/sdmb/misc.php?do=bbcode) which still works.
bibliophage
moderator CCC
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### Author:  [@DanBlather](https://boards.straightdope.com/u/DanBlather)
#### Post date: [October 31, 2007, 6:52pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/4 "2007-10-31T18:52:29Z")
[QUOTE=LarryParker]
Cecil - with reference to your article on how siphons work - it would seem to me that there is a setup to confirm the hypothesis that the weight of water in the length BC is greater than the weight of water in length AB, thus pulling (by vacuum abhorrence) the water from AB over point B.
Simply recreate the setup with the endpoint C above the level of the original water, but keep the hose length the same (e.g. turn it into a spiral).
In this way, the weight of the water in BC is still greater than that in AB and should siphon the water back into the original container. Simply add a turbine at the bottom and “siphon” off the energy.
This might have the appearance of a “perpetual motion machine”, but it would actually fail as soon as the Earth’s gravity disappeared. :dubious:
I am now off to develop my own siphon-based free electricity apparatus in a nearby barn and make millions from setting it up in back yards across the country. 😃
[/QUOTE]
Cool thought experiment. A spiral is essentially an incline, and as such the force generated by material sliding down it is a function of the slope. I’d guess it all nets out.
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### Author:  [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)
#### Post date: [October 31, 2007, 7:23pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/5 "2007-10-31T19:23:12Z")
For “spiral”, read “helix”.
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### Author:  [@grumpy44134](https://boards.straightdope.com/u/grumpy44134)
#### Post date: [May 11, 2013, 7:13pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/6 "2013-05-11T19:13:10Z")
“The action [of a siphon] depends upon the influence of gravity …”
What about a siphon in zero gravity?
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### Author:  [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)
#### Post date: [May 12, 2013, 4:44am UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/7 "2013-05-12T04:44:10Z")
The answer, unsurprisingly, is that a siphon in zero gravity doesn’t work.
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### Author:  [@Malacandra](https://boards.straightdope.com/u/Malacandra)
#### Post date: [May 12, 2013, 8:34am UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/8 "2013-05-12T08:34:43Z")
And (since the thread’s been revived) should anyone be wondering about **Larry Parker** ’s suggestion, it’s the _height_ of the water column that matters, not the _weight_. People have been trying for centuries to devise a perpetual-motion machine based on a siphon, and it ain’t gonna work.
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### Author:  [@grumpy44134](https://boards.straightdope.com/u/grumpy44134)
#### Post date: [May 12, 2013, 6:14pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/9 "2013-05-12T18:14:51Z")
> [@John\_W.Kennedy](#):
>
> … a siphon in zero gravity doesn’t work.
That would, to me, prove that air pressure does **NOT** make a siphon work.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 12, 2013, 6:29pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/10 "2013-05-12T18:29:26Z")
In order for a siphon to work, the water has to hold together. Under ordinary Earthly conditions, air pressure is the dominant force causing that to happen, but you could still get a very low siphon to work in a vacuum, due to water’s own cohesiveness. So you don’t actually need air pressure, and it’s not the only thing you need, but it still really helps.
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [May 12, 2013, 6:29pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/11 "2013-05-12T18:29:37Z")
> [@grumpy44134](#):
>
> That would, to me, prove that air pressure does **NOT** make a siphon work.
Could you explain how siphons working in the presence of air pressure and not working in it’s absence tells you that air pressure does NOT make a siphon work?
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### Author:  [@tim314](https://boards.straightdope.com/u/tim314)
#### Post date: [May 12, 2013, 7:34pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/12 "2013-05-12T19:34:04Z")
> [@grumpy44134](#):
>
> That would, to me, prove that air pressure does **NOT** make a siphon work.
I think you misunderstand. It’s not a choice between “does a siphon work due to air pressure or due to gravity?” It definitely requires gravity to work, The fact that it doesn’t work in zero-G demonstrates this. But the fact that gravity _is_ required does not prove that air pressure is not _also_ required.
That’s the question Cecil was discussing: In _addition_ to gravity, is air pressure also required for a siphon to work?
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [May 12, 2013, 8:06pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/13 "2013-05-12T20:06:49Z")
> [@TriPolar](#):
>
> Could you explain how siphons working in the presence of air pressure and not working in it’s absence tells you that air pressure does NOT make a siphon work?
nm, totally misread the post you were responding to.
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### Author:  [@Gary\_Robson](https://boards.straightdope.com/u/Gary_Robson)
#### Post date: [May 12, 2013, 8:13pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/14 "2013-05-12T20:13:20Z")
Air pressure and gravity are not mutually exclusive. Try this thought experiment:
In zero G, fill two identical water balloons with identical amounts of water, and connect them (at the mouths) with a hose, also filled with water. No water will flow.
Now try it with a clamp in the middle of the hose to prevent any flowing of water between balloons, and each balloon in a jar, routing the hose through pressure-tight caps on the jars. Fill both jars with oxygen: one at 1 atmosphere pressure and the other at 2 atmospheres.
Remove the clamp.
Water will flow from the balloon at the higher pressure to the balloon at the lower pressure until the jars equalize (assuming infinitely stretchy balloons).
You have just “siphoned” water at zero G, using air pressure but no gravity.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 12, 2013, 10:40pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/15 "2013-05-12T22:40:47Z")
> [@](#):
>
> In zero G, fill two identical water balloons with identical amounts of water, and connect them (at the mouths) with a hose, also filled with water. No water will flow.
Actually, it will, but due to the elasticity of the balloons, not due to any siphoning effect.
> [@](#):
>
> Water will flow from the balloon at the higher pressure to the balloon at the lower pressure until the jars equalize (assuming infinitely stretchy balloons).
>
> You have just “siphoned” water at zero G, using air pressure but no gravity.
And while water will flow here, it’s not because of a siphon. That’s a different phenomenon.
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### Author:  [@njtt](https://boards.straightdope.com/u/njtt)
#### Post date: [May 13, 2013, 1:22am UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/16 "2013-05-13T01:22:41Z")
> [@grumpy44134](#):
>
> That would, to me, prove that air pressure does **NOT** make a siphon work.
Why? Air pressure (the relevant sort, i.e., in a planetary atmosphere) is an effect of gravity.
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### Author:  [@Gary\_Robson](https://boards.straightdope.com/u/Gary_Robson)
#### Post date: [May 13, 2013, 4:46am UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/17 "2013-05-13T04:46:21Z")
> [@Chronos](#):
>
> [quote=“Gary “Wombat” Robson, post:14, topic:423969”]
>
> AIn zero G, fill two identical water balloons with identical amounts of water, and connect them (at the mouths) with a hose, also filled with water. No water will flow.
Actually, it will, but due to the elasticity of the balloons, not due to any siphoning effect.
[/QUOTE]
I don’t understand. Why would water flow from one balloon to the other when they have the same elasticity, the same amount of water, and the same outside pressure (aside from Brownian motion, which I don’t consider to be flow in this context).
> [@Chronos](#):
>
> And while water will flow here, it’s not because of a siphon. That’s a different phenomenon.
That’s why I put the word “siphon” in quotes.
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### Author:  [@Irishman](https://boards.straightdope.com/u/Irishman)
#### Post date: [May 13, 2013, 6:35pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/18 "2013-05-13T18:35:58Z")
> [@LarryParker](#):
>
> Cecil - with reference to your article on how siphons work - it would seem to me that there is a setup to confirm the hypothesis that the weight of water in the length BC is greater than the weight of water in length AB, thus pulling (by vacuum abhorrence) the water from AB over point B.
> Simply recreate the setup with the endpoint C above the level of the original water, but keep the hose length the same (e.g. turn it into a spiral).
This is an interesting idea. More importantly, try it the other way.
Take a long tube and two containers. Place one container above the other, so that their surface levels are uneven. Place the filled tube such that the tube coils in cup 1 (higher cup) and runs straight to cup 2, so there is more tube above the surface on side 1 of the apex of the curve than on side 2. If the weight in the tube is the difference, then there is more water in AB than BC, so the water should flow from the bottom container 2 to the top container 1.
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### Author:  [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)
#### Post date: [May 14, 2013, 5:24pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/19 "2013-05-14T17:24:44Z")
Is the cohesive-ness of the water column geometry dependant?
Cecil’s article states that you cannot siphon higher than about 34 feet. One of his experts insists that cohesiveness (cohesion?) is the important factor (disallowing for impurities in the water- BTW-standard physicist dodge). Trees can pull up water more than one hundred feet based on cohesion (through really tiny tubes). This water is just as loaded with impurities as just about any water you might want to siphon.
I wonder if the cohesion is a lot stronger for very narrow columns of water.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 14, 2013, 6:32pm UTC](https://boards.straightdope.com/t/how-does-a-siphon-work/423969/20 "2013-05-14T18:32:39Z")
> [@](#):
>
> Quoth **Wombat** :
>
> I don’t understand. Why would water flow from one balloon to the other when they have the same elasticity, the same amount of water, and the same outside pressure (aside from Brownian motion, which I don’t consider to be flow in this context).
Because one balloon containing all of the volume has less surface area than two balloons each containing half the volume. The energy stored in an elastic membrane is proportional to the surface area.
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