Siphons Work Due To Gravity, Not Atmospheric Pressure: Now With Peer Review 360
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."
corrected link (Score:5, Informative)
This is the corrected link to the letter: http://www.theguardian.com/science/blog/2010/may/10/dictionary-definition-siphon-wrong
Re:corrected link (Score:5, Informative)
And if you look at that link, you'll see it's a letter from 2010. Which is yesterday by Slashdot definitions, of course.
By the way, the letter was nicely debunked here:
http://www.theregister.co.uk/2... [theregister.co.uk]
Gravity is responsible for lowering the pressure in the outbound leg of the siphon, but you still need atmospheric pressure at the inlet to push the water over the top of the siphon. You can't siphon up more than about 10 m under normal atmospheric pressure.
So in a way, it's not really wrong to say that atmospheric pressure is pushing the water over the top. Just like atmospheric pressure pushes liquid into a drinking straw as well. Would you say that a drinking straw has nothing to do with atmospheric pressure?
Of course the exact value of atmospheric pressure doesn't matter much as long as it's enough, the flow rate will only depend on the difference in height between the two water levels, but without enough atomospheric pressure the siphon stops working. Which was clearly shown in the experiment described in Nature as well. In fact that experiment dispoves rather than proves his point.
Gravity! (Score:5, Funny)
Not "Nature", a lesser journal of the Nature group (Score:4, Informative)
I'm Confused (Score:2)
Does it suck or blow?
Somebody thought it was atmospheric pressure? (Score:3, Interesting)
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.
Re: (Score:2)
Re: (Score:2)
If the pressure difference is great enough, then yes... it will flow upward. Have you ever used a straw? That's pulling liquid upward entirely through difference in air-pressure
However, there is still a maximum height that can achieved even if the higher end were in complete vacuum, and for something like water, that distance is not even 35 feet (it's even less for heavier liquids, like mercury). The difference in atmospheric pressure with so little difference in altitude is not gong to be sufficient
Re: (Score:2)
That's pulling liquid upward entirely through difference in air-pressure
Wrong. Thats atmospheric pressure PUSHING the liquid up through the straw due to the lower pressure in the part you're 'sucking' on.
The reason you can't 'suck' water up past 30 feet or so is because the water column becomes equal to atmospheric pressure, so even when the upper end is in a 'vacuum', the atmospheric pressure is in equilibrium with it and it stops flowing upwards.
You can not pull water up, only push.
Likewise, those people who taught you that airplanes fly because a vacuum on the top edge of t
Re: (Score:2)
Re: (Score:2)
Right, basically a siphon can only work if the pressure at the high point is above the vapor pressure of the liquid. If you pump a vacuum, then your water will vaporize which will kill your siphon. Gravity has nothing to do with it aside from being the driving force behind atmospheric pressure in the first place. As long as you have an unbroken liquid in your siphon, atmospheric pressure is what drives it. If "tension" between water bonds were what drove it, it would behave like a chain fountain and you wou
OED? (Score:2)
vac pump can't raise liquids > atmo pressure (Score:3)
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.
Re:vac pump can't raise liquids atmo pressure (Score:2)
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).
Re:vac pump can't raise liquids atmo pressure (Score:2)
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 interes
Re: (Score:2)
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. T
Re:vac pump can't raise liquids atmo pressure (Score:2)
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.
Plot twist: (Score:5, Informative)
Atmospheric pressure is actually due to gravity.
Re: (Score:2)
Re: (Score:2)
It does work, with some caveats. I was about to post that it wouldn't, and then remembered the first shuttle payload I worked on : http://istd.gsfc.nasa.gov/cryo... [nasa.gov] which transferred helium between to dewars without pumps in a microgravity enuvronment. It's been a couple of decades, and I wasn't on the principal investigators team (I was carrier support), so I don't remember the details of how the transfer worked.
Re: (Score:2)
SUPERFLUIDS DON'T COUNT.
(going back to rocking in the corner, hallucinating about my boat sinking into the ocean of superfluid helium as the stuff crawls up over the gunwales and flows down into the hold...)
Re: (Score:2)
I know you know this, but just to be sure no one else gets confused: A causes B and A causes C does not imply that B causes C. So for instance falling rocks are also caused by gravity but they don't have anything to do with siphons.
Atmospheric pressure also caused by gravity (Score:2)
Many years too late... (Score:2)
Huzzah! If only my high school physics teacher was still alive. We frequently argued this point.
Published in the journal Duh? (Score:2)
Pedantic (Score:2)
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.
Re: (Score:2)
Of course it's both forces (pressure and gravity). This is simply a pedantic attack at the way the dictionary defines the process.
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.
Gravity is acting as a suction pump, which requires the air pressure to push the water - get rid of the air pressure and you can't "suck" the stuff up in the first place or keep it flowing. The "cohesive bonds between the liquid molecules" are pretty darn weak compared to the forces involved in stealing "petrol" or siphoning most other fluids. Under normal atmospheric pressure you cannot siphon over a hump of about 10m.
An even simpler experiment (Score:2)
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 lett
Encyclopedia Brittanica Has It Right (Score:3)
http://www.britannica.com/EBch... [britannica.com]
"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."
Re: (Score:3)
Equations of motion. (Score:2)
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?
Bullshit (Score:2)
...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.
the Guardian article is wrong too (Score:2)
what's the debate? (Score:2)
Gravity pulls one side down; pressure pushes the other side up.
Air pressure is necessary, because... (Score:2)
Re:Actually it's both. (Score:5, Funny)
>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.
oh man (Score:5, Funny)
that sucks!
Re: (Score:2)
you blow! ;-)
Re: (Score:2)
Re: (Score:3, Funny)
Re: (Score:3)
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)
Re:Actually it's both. (Score:4, Informative)
Re: (Score:3, Insightful)
Re:Actually it's both. (Score:5, Informative)
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 [nature.com] 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.
Re: (Score:3)
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
Re:Actually it's both. (Score:5, Informative)
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
Re:Actually it's both. (Score:4, Interesting)
Re:No. Try it with no air pressure, like TFA did (Score:4, Informative)
It bears mentioning that atmospheric pressure is created by gravity in the first place. The wall of air above any patch of ground has a mass which gravity gives a weight. The sum of the weight of this mass creates the atmospheric pressure: without gravity, this gaseous atmosphere would spread and disperse into space until pressure was eventually that of the vacuum.
The mechanism of a siphon does rely on fluid pressure to work just not atmospheric pressure. 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.
If pressure is reduced by 80%, it stops working at all. See the article for details.
After pressure is reduced by 80%; the substance ceases to be a proper liquid -- in essence, it loses the properties of water.
You can also accomplish this by increasing temperature as well, until the liquid begins to vaporize.
This does not mean that a siphon is something caused by low temperature.
The siphon is something caused by gravity, that relies on some properties of the liquid to work that have some sensitivity to pressure/temp.
For a siphon to work; the liquid needs to have certain properties that water does have. Including surface tension/adhesion, and specifically -- capillary action.
sorry, you can siphon a gas (Score:5, Interesting)
> 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.
Re: (Score:3)
Which doesn't matter. Try the dry ice CO2 experiment to see for yourself.
You need pressure to have a body of CO2 gas form a fluid as well; the little bit of dry ice CO2 would just rapidly dissipate, without the atmospheric pressure.
Pressures of course have a role in a syphon. As liquid is displaced out of the bottom by gravity, the fluid's pressure in the tube will tend to equalize by drawing in more fluid (water or air) at the submerged source side of the tube.
Until such time as the fluid pressure co
Re:Actually it's both. (Score:4, Interesting)
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?
Re:Actually it's both. (Score:5, Informative)
Re: (Score:2)
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.
Re:Actually it's both. (Score:5, Informative)
Re: (Score:2)
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 a
Re: (Score:2)
Re: (Score:2)
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.
Re: (Score:2)
A straw only has one hole. It just happens to be a very long one.
Re: (Score:2)
It would be pedantic to accuse you of being pernickity.
Re: (Score:2)
I wasn't using the topology definition. There's no gravity or atmospheric pressure in topology.
Re: (Score:2)
Re: (Score:2)
Are you a policeman?
Re:Straws don't make good siphons anyways. (Score:4, Interesting)
I'll resist the temptation to carry on with the bad puns and innuendo, but....
u-tube plonked in water in an elevated bucket, one end outside the bucket.
1) You suck on the dry end. Water moves up to the apex of the tube.
It's atmospheric pressure pushing the water up the tube as your sucking reduces the pressure in the tube.
2) Water keeps moving around the bend, past the apex.
It's a combination of your sucking and momentum that keeps the water moving.
3) The water reaches a point lower than the surface of the water in the bucket. You stop sucking.
It's the gravity (or the water seeking a lower energy state in a gravitational field) that keeps the water moving through the tube.
So all things are having an effect, which makes sense. Atmospheric pressure doesn't magically stop happening just because gravity is having a stronger effect.
Re: (Score:2)
Re: (Score:2)
Hmm. Perhaps we need a new improved kind of atmospheric pressure that runs on bunnies.
Re: (Score:2)
Exactly. You beat me to it.
Re: (Score:2)
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.
Re:Actually it's both. (Score:4, Informative)
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.
Re: (Score:3)
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 w
Re: (Score:2)
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.
Re:Actually it's both. (Score:4, Insightful)
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".
Re: (Score:2)
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 p
Re: (Score:2)
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.
Re: (Score:2)
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.
Re: (Score:3)
Re: (Score:2)
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.
Re: (Score:2)
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.
Re:Actually it's both. (Score:4, Funny)
It is not theoretically stable, but in practice it is.
In theory, there is no difference between theory and practice.
In practice, there is.
Re:Actually it's both. (Score:5, Informative)
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!
Re: (Score:2)
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.
Re: (Score:2)
That was my thought. It sounds like the issue may be largely confined to dictionaries though - the 1911 Oxford dictionary got it wrong, and it and most every other dictionary since has continued to do so. Presumably because dictionary editors are not typically skilled in the hard sciences, and anybody that knows the science is unlikely to look up siphon in the dictionary. The resulting tiny set of people aware of the error probably just didn't include someone concerned enough to send in a convincing bug
Re: (Score:2)
Well, there are also tubes that people might loosely call "siphons" that do work by atmospheric pressure involved in some clever pumpless perpetual fountain designs, where the high air pressure in one chamber pumps water up through a pipe. But if we want to change even the non-technical definition of siphon to exclude that, that also seems reasonable. "Reverse-siphon" maybe, since it goes against gravity?
Re: (Score:2, Insightful)
AIUI a siphon needs BOTH gravity and air pressure.
Gravity makes water in the down side of the siphon move downwards. This reduces the pressure at the top of the siphon to less than atmospheric pressure. Which in turn allows atmospheric pressure to push water up the up side of the siphon.
Take away gravity and there is nothing to pull the water down the down side. Take away atmospheric pressure and there is nothing to push the water up the up side. Either way your siphon won't work.
Re: (Score:2)
I'm very sorry that your grade school taught that.
If "the whole amount is bonded together", how do drips happen?
Re:wrong (Score:4, Informative)
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.
Re: (Score:2)
Mod Anon Informative... Siphoning works with all liquids within their vapor pressure limits regardless of surface tension or cohesion properties.
Re: (Score:2)
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.
Re: (Score:2)
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
Re: (Score:2)
And what, pray tell, causes the water to go downward?
Re:wrong (Score:5, Funny)
Pessimism.
Re: (Score:2)
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 tensil
Re: (Score:2)
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.
Re: (Score:2)
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 pressu
Re: (Score:2)
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.
Re: (Score:2)
One can also make a perfectly workable siphon using two immiscible fluids - e.g., oil and water.
Re: (Score:2)
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 do
Re: (Score:2)
Re: (Score:2)
Re: (Score:2)
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.
Re: (Score:2)
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