presumably
in a gravity-less environment, you could substitute a ferrous liquid and a magnetic field.
Physicist unmasks 99-year-old mistake in English dictionaries
An Australian physics prof has discovered a 99-year-old error in the Oxford English Dictionary - repeated in most dictionaries worldwide - and is having it corrected. The error is in the definition of the noun "siphon", a tube used to draw fluid from a higher location to a lower one - as when emptying a vehicle fuel tank, an …
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Monday 10th May 2010 14:15 GMT Paul Bruneau
Side effects of gravity
The earth, the sun, the earth's magnetic field, are all side effects of gravity. So should we say that gravity causes everything? I think not.
Yeah, OK, gravity is creating the partial vacuum but by far the most interesting and noteworthy aspect of a siphon is the idea that it is atmospheric pressure pushing the liquid up the short leg. I'm fine with the so-called error in the dictionary.
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Monday 10th May 2010 14:15 GMT John Robson
Thought experiment
Only 'cos I can't go there...
Two open bowls on the surface of the moon. One (filled with mercury) on a table 1m above the other.
Coil a hose in the top bowl, put a bung in the end and lift that end out
- On earth it would stay full of mercury, even when put upright
- On the moon I'm not so sure. I think it would stay level.
On earth the height difference is the basis of a barometer / manometer, with zero pressure acting on the main surface of the liquid why would it object to zero pressure inside the tube?
If you pumped the mercury up the tube and put the blocked end in the lower bowl...
Then I expect that it would split at the top and each half would fall in the obvious direction. Liquids are not known for their tensile strength, and that is all (I can see) that would prevent this.
Can we please ask the Myth Busters to visit NASA and have a play in one of their really big vacuum chambers?
Variables:
- Atmospheric pressure
- Fluid
- Tube diameter (capillary effect?)
I think that height difference is not relevant to the discussion.
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Monday 10th May 2010 23:17 GMT Nexox Enigma
Nope...
Your hypothetical tube of mercury on the moon, if it didn't freeze, would indeed fall out of the sealed tube, because it would boil off under it's own weight and fill the void above the liquid with gas. This is the same effect that limits a terrestrial siphon to 10m height differential.
And yes, that limit is derived from the atmospheric pressure, but saying that a condition limits a phenomenon is entirely different from saying that it drives said phenomenon.
If you look at where the force to accelerate the fluid originates, it's always gravity.
"""I think that height difference is not relevant to the discussion."""
And centuries of fluid statics would call you a dimwit. Until you hit some limiting condition, height differential is /all/ that matters (in a first order model, anyway - for more accuracy you'd need to look at density, viscosity, siphon geometry, siphon surface roughness, etc. You'd probably want to go full CFD at that point.)
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Tuesday 11th May 2010 09:45 GMT MarkBrown
no need to go to the moon
You don't need to go to the moon to test whether atmospheric pressure is responsible for siphoning. Just put a hole at the highest part of the hose and see if the siphon keeps working. It is atmospheric pressure (and a little surface tension, I guess) that stops the fluid in each side of the hose from separating and flowing back into its respective container.
Bootnote: The fact that the pressure is (roughly) balanced at each end of the hose is not the whole point. The important part is that the pressure is not enough to counter gravity on the long side, so the fluid flows down, whereas it is more than enough to counter the gravity on the short side (which weighs less) so the fluid flows up.
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Monday 10th May 2010 14:17 GMT Robert Carnegie
No atmosphere, no liquid.
No liquid, no siphon!
However...at sea level, immerse the whole of a hose in a tank of water so that the hose itself is filled; stopper one end of the hose; pull that end up out of the tank, drop it into the sea (I mentioned we are at sea level), and unstopper it. Water drains from the tank without anything being sucked or blown. It's gravity.
Elsewhere, it may not be gasoline: http://www.snopes.com/autos/theft/siphon.asp
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Monday 10th May 2010 23:13 GMT Charles 9
A centrifuge would provide the gravity...
...nullifying the presumption that you can't run a siphon without gravity. I strongly suspect it doesn't work without gravity because gravity is a factor in producing atmospheric pressure (as in, the force of air pressure is generated by the acceleration due to weight of the atmosphere on the planet). So, for the siphon effect to work, you need atmospheric pressure, which requires both atmosphere (to provide the substance) and gravity (to allow the substance to exert a force).
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Monday 10th May 2010 16:13 GMT Anonymous Coward
There's no kind of atmosphere?
Take two cylinders partly filled with fluid, and with pistons of negligible mass that move with minimal friction.
Connect the fluid reservoirs in the two cylinders with a long tube.
Fiddle with the apparatus until you remove any air or other gas by bleeding it off through a suitable valve.
Push one piston so that almost all the fluid is in one cylinder and arrange the apparatus so that the tube goes up and then down (just like a siphon!)
Raise the full cylinder above level of the other (even more like a siphon!).
Question 1: will the fluid drain from the high reservoir to the lower? (ie in the manner of a siphon)
Question 2: does "atmospheric pressure" play any role in any siphon-like action that might occur in this apparatus?
Question 3: How might the fluid properties affect any siphon-like action?
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Monday 10th May 2010 23:17 GMT Nexox Enigma
technically...
"""Question 2: does "atmospheric pressure" play any role in any siphon-like action that might occur in this apparatus?"""
Technically atmospheric pressure would be exerted on the back side of each piston, but as they'd be close together, it would be the same pressure, and would cancel out.
If you stipulated that the whole contraption was enclosed in a vacuum, then there would be no atmospheric effect, and it would still work just fine.
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Tuesday 11th May 2010 08:22 GMT Rattus Rattus
If I understand your setup correctly,
ie, the fluid in the upper cylinder has to go a short way up the tube, above the level of the cylinder, before going down the rest of the tube to the lower cylinder, then no the fluid will not drain in the manner of a siphon. Whatever fluid is in the longer portion of the tube will fall into the lower cylinder, and any fluid in the shorter leg of the tube will fall back into the upper cylinder.
Atmospheric pressure plays a role in that it's absence means there is nothing to push the fluid up the shorter leg and over the bend.
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Tuesday 11th May 2010 19:21 GMT Rattus Rattus
Oh, I see
I see now, he wasn't talking about being in a vacuum. In that case, yes, the fluid will move as for a siphon. The fluid will fall in the longer part of the tube, reducing the pressure inside the tube. The atmosphere will push on the outside of the piston of the upper cylinder, thereby equalising the pressure and pushing the fluid up the shorter leg.
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Monday 10th May 2010 16:13 GMT gdeinsta
Proven false by Hero of Alexandria in the 1st century AD
"Liquid is, of course, drawn up the shorter limb of the siphon by the weight of that in the longer downward one:..."
Hero of Alexandria disproved this in the first century AD. In the book "Pneumatics" he discusses an experiment using a siphon with a large diameter for the short leg and a small diameter for the long leg. Result, a greater weight of water in the short leg. Did the water travel up into the higher container (short leg), thereby generating free energy and saving us all from having to dig coal for a living? No it did not.
It's not the weight, it's the pressure. Pressure does not act at all like a chain. You can't pull on pressure. And the tension in a chain only pulls in one direction, whereas the pressure in a liquid is the same in all directions - turn the pressure sensor in any direction and it registers the same pressure.
Fail is for the grade he would get if he was a student in my Ancient Science and Technology course at Carleton University.
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Monday 10th May 2010 23:13 GMT Pablo
Experiment
I just did an experiment, to put this to the test.
Materials: A plastic jug of water with two holes in it (not counting the top, wish is capped). Two lengths of tubing which fit through the holes tightly. Each tube extends below the water level.
Result: By blowing air into either tube, I could make water come out the other. And it continued flowing as long as the exit of the water tube was lower than the bottle. The relative position of the air tube made no visible difference (i.e. it could be lower than the water tube).
Interpretation: Though pressure is necessary, and there is an interplay of effects, gravity IS the driving force.
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Monday 10th May 2010 23:15 GMT The Cube
Erm..... double entendres away
The experiment you cite has water moving from the leg with the smaller weight to the leg with the larger weight, this would seem to vote in favour of gravity over pressure not against.
The issue is in the wording, it is assumed in the basic phrase "Liquid is, of course, drawn up the shorter limb of the siphon by the weight of that in the longer downward one:..." that the siphon tube is of constant cross sectional area.
The experiment you cite has a tube of varying cross sectional area and is the equivalent of putting a tall thin girl on one side of a see-saw and then putting a short fat munter (of greater overall mass) on the other side. Clearly the fat munter is going down (assuming there is a local gravity or simulated gravity through other acceleration). One would not expect the skinny girl to go down simply because she is taller.
I leave it as an exercise for the student to determine the nature, position and net effect on the "siphon tube" between the tall pretty girl and the short munter under the relative "suction" applied by each.
</Double Entendres>
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Monday 10th May 2010 23:17 GMT Nexox Enigma
Ancient
Not really sure what experiment you're talking about, but you're pretty much just wrong.
"""It's not the weight, it's the pressure."""
pressure = force * area
weight = force
So under the influence of gravity, pressure and weight are linearly related, given a constant cross-section pipe. So your different leg diameters actually create different pressure, which, really, is all that matters in determining which direction water will flow in your hypothetical ancient siphon.
"""Pressure does not act at all like a chain. You can't pull on pressure."""
Actually... Yes you can. I mean, that statement is really, extra wrong. You're clearly a history major, and not an engineer. You can 'pull' on a fluid in a rigid container until you lower it's pressure enough to cause it to boil. All sorts of hydraulic systems depend on this phenomenon.
"""turn the pressure sensor in any direction and it registers the same pressure."""
For a static fluid, sure. If you measure pressure in a moving fluid, it does indeed depend on the direction (and the geometry of your measurement device.) That's still not really relevant to your chain theory, since in a chain you'd measure force, and in a liquid you'd measure pressure. In order to determine force in a liquid, you'd have to measure the pressure at multiple points, not in multiple orientations. And you can bet that for any static fluid pressure will vary linearly with height.
"""Fail is for the grade he would get if he was a student in my Ancient Science and Technology course at Carleton University."""
And fail is the grade you'd get in any sort of fluid statics class.
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Tuesday 11th May 2010 08:19 GMT fred #257
Right but irrelevant
Respectfully, I think you're hairsplitting. Most commentards here seem to be thinking of a siphon as a hosepipe, in which case the diameters of both legs are the same and hence pressure is proportional to weight and the quote you object to is actually correct. It's certainly the weight of liquid in the longer leg (all right, manifested as a reduction of pressure at the top of the siphon, if you insist) that does the work. But in the argument of gravity-vs-atmospheric pressure as a power source, your point is kinda irrelevant.
And it IS the 'weight' (i.e. gravitational force on the liquid) that 'powers' the siphon. You need external pressure (e.g. atmospheric) to keep it operating, but since that's equal at both ends of the tube then obviously it can't be providing the energy. In the case of 'suck-it-to-start' it's atmospheric pressure starts forcing the liquid up the tube, but even then the motive power is being supplied by the sucker, sucking. (And strictly speaking at that point you don't yet have a siphon operating).
The maximum height of a siphon is determined by (external pressure minus vapour pressure of the liquid)**. Which is why the height gets less as the siphon liquid gets hotter (you can't siphon boiling water), just as it gets less as atmospheric pressure decreases. The Victorians had all this sort of thing worked out to a high degree of precision, since they used steam power.
** And by the density of the liquid, before someone goes all pedantic on me.
(Beer, cos it's an interesting subject for experiments...)
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Tuesday 11th May 2010 09:40 GMT Ken Hagan
Re: Proven
I can't believe that an experimental test gets down-voted.
I can believe that someone in the 1st century was smart enough to realise there is a question worth asking here and smart enough to figure out a way of answering it. 'Tis a pity that the median intelligence of the human race hasn't changed much since then.
Thanks to gdeinsta of Carleton and two hero points to Hero of Alexandria
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Monday 10th May 2010 23:13 GMT PassingStrange
He's wrong, pure and simple.
"Liquid is, of course, drawn up the shorter limb of the siphon by the weight of that in the longer downward one: thus the operating force is gravity."
That's an even bigger howler than the stock explanation, and the sort of thing you get if you only look at the maths, without thinking about the physics. Liquid isn't "drawn up" (as can be proved, as lots of people have pointed out, by raising the shorter limb above the column height that can be supported by atmospheric pressure - the flow stops, and a vacuum forms above the two limbs). For any flow to occur, liquid has to be *pushed* to the top of the shorter limb by atmospheric pressure. No pressure, no column of liquid; no column, no flow.
So. Air pressure is pushing the liquid up in both limbs; the weight of the column of liquid in each limb is opposing that pressure. The column of liquid in the longer limb is greater than that in the shorter limb, so its weight cancels out more of the force due to air pressure at that limb, resulting in a net force (and resultant flow) from short limb to long.
Atmospheric pressure at both ends is the same, so the *size* of the resultant force is, indeed, directly down to the difference in weight of the two columns of water. But. Without atmospheric pressure at the shorter limb, no column of water - and hence no siphon effect - is possible. Gravity does not *cause* the flow - it works on both sides to *oppose* it (and fails more badly at the shorter limb). The operating *force* is the unbalanced component of atmospheric pressure at the short limb.
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Monday 10th May 2010 23:15 GMT John Savard
If Push and Not Pull
As noted, "you can't pull on pressure". Hence, the weight of the fluid in the longer arm of the siphon does not pull the fluid up in the shorter arm. What, then, does cause the fluid to ascend in the shorter arm?
It is air pressure after all; while the air pressure is higher at the lower level, the weight of the fluid in the longer arm more than counterbalances the difference in air pressures. So we have:
push of air pressure at the top overpowers push of air pressure at the bottom minus the weight of the extra fluid in the longer arm.
So instead of "air pressure" being wrong and "weight of fluid" being right, the right answer is both of them.
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Tuesday 11th May 2010 13:42 GMT Charles 9
But what about the Hero experiment...
...in which the short leg is much larger and thus actually heavier than the longer but thinner long leg? The siphon still works normally usually, but the restriction could cause the siphon to stop if the difference is too great and the flow restriction causes the liquid to cavitate ("boil") in the siphon. From what I've read, gravity and fluid density are the only things related to hydrostatic pressure. Still trying to see why siphons typically don't work in a vacuum in spite of this fact.
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Monday 10th May 2010 23:17 GMT Steven Jones
The answer is...
Whether atmospheric pressure is required or not depends on if this is an inverted siphon (like a trap on a sink) or a normal siphon. An inverted siphon doesn't require any atmospheric pressure. Of course water would boil in a vacuum, but and inverted siphon of mercury would work quite happily in such conditions.
In the case of a normal siphon, there is a height limit (of about 33 feet for water) as it is necessary for the lower leg of the siphon to "suck" the water over the high point. Of course there is no such thing as "suck" - it's pushed by atmospheric pressure when a partial vacuum is at the upper end of the tube. However, once the column height reaches about 33 feet, that's it - normal atmospheric pressure won't push it any further, even with a perfect vacuum at the other end (and it can never be quite perfect due to the vapour pressure of the evaporating liquid).
Of course, ultimately, it is the pressure difference between the upper and lower legs caused by the action of gravity on the water that provides the force to move the water. However, it simply would not work without enough atmospheric pressure to push the water over the "hump".
Now this is not to say that it might not be possible to get a siphon flowing with a "hump" of more than 33 feet. Perhaps if it could be started by some other means (such as excess pressure on the higher side) it might be started and continue under momentum once flow started and the pressure was returned to atmospheric pressure, but I doubt it would give you much extra height and I'm feeling too tired to work out the physics of this at the moment.
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Monday 10th May 2010 23:22 GMT Jolyon Smith
As ever, isn't the truth somewhere in the middle
Gravity acts on the liquid in the long leg creating a force that acts in *conjunction* with the atmospheric pressure on the liquid in the vessel into which the short leg is immersed. This surely creates the imbalance in forces required to cause the liquid to flow. Gravity isn't pulling the water on it's own... gravity creates the partial vacuum which atmospheric pressure then fills by pushing liquid to fill that vacuum.
This experiment should be recreated with beer. In a pub. All good theoretical science is done in pubs with beer.
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Tuesday 11th May 2010 09:47 GMT Vladimir Plouzhnikov
C'mon people
It's hydrostatic pressure (gravity related, of course, but nevertheless pressure) that explains the communicating vessels.
in each vessel p = \rho g h,
if h1 > h2 then p1 > p2 and the water flows from vessel 1 to vessel 2
The atmospheric pressure does play a role but because the \rho of air is much lower than the \rho of water, the extra pressure differential created by the air will always be less than the differential between the water columns (the height of both water columns is measured against a reference point below the surface of the lowest column).
In other words, it's the difference in potential energy that forces the water flow (just like the difference in potentials causes the electrical current flow).
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Tuesday 11th May 2010 10:11 GMT Robert Carnegie
...no, I'll get this...
Yes, there's no such thing as suction really, just pressure without equal pressure on the other side. And yes, air pressure - in the typical example - pushes water up the siphon tube. But it does so only because with unequal amounts of water in the two arms of an inverted and asymmetric U - more like a J - gravity pulls unequally, more strongly, on water in the longer arm, and cancels the pressure on that side.
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Wednesday 12th May 2010 00:48 GMT JC 2
Not Atmospheric Pressure Needed
By writing "atmospheric pressure" you would have to imply it is necessarily pressure from the atmosphere, which is obviously not needed! Instead, it only needs to be a sufficient pressure differential to overcome gravity, regardless of the cause of that pressure difference. For example, a vacuum container which within holds a sealed system with two cannisters at dissimilar pressure and a liquid in a tube between them.
There is such thing as suction, writing out a definition as a way to explain it away is ignoring the whole POINT of words in a language, to have a term that eliminates the need to constantly write out a much longer definition using other words.
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