OED “siphon” definition wrong for decades

Air pressure is not just needed to get the siphon action started, it is necessary to keep it going. The physics professor's explanation that gravity causes the action is only partially correct. If gravity were truly the only force at work here then the liquid would run down both ways from the crown of the siphon leaving a vacuum, it does not because the external air pressure makes it impossible for such a vacuum to form. Hence while the gravitational potential toward the long side is what pulls the liquid from the upper reservoir to the lower, it is only able to do so because of air pressure acting to maintain the continuity of the flow.

Siphons will not work in the International Space Station where there is air but no gravity, but neither will they work on the Moon where there is gravity but no air[*].

[* Accepting, of course, where other cohesive forces that act internally to maintain the continuity of the liquid allow for a siphon-like effect.]

I checked both the Oxford and Websters and both use, essentially, the same definition.

However, having had to deal with Oxford's English courses and tests over the decades, this is the least of their problems. (NOTE: The wife is an English teacher)

I like DF's explanation.

Nice link, as the diagramme shows the main point is to have a degree of "head" on the system (i.e height difference between input and output), but the system is only capable of raising water above the height of the channel if "primed" (i.e. filled with liquid). If the top of the crest is too high the liquid will break and the siphon will not work.

http://www.vl-irrigation.org/cms/uploads/pics/siphon_722_162_01.jpg width=66%

One way to show this is to try and siphon water using a tube without a rigid cross-section. Assuming you can somehow prime the tube at all (perhaps by filling it while upright, sealing the ends, then lying it between the upper and lower bowl), when you unseal the ends, rather than siphoning any water, the contents of the tube will flow out into both basins (unequally) and the tube itself will simply collapse flat.

So I guess what I'm saying is if your siphon's too floppy there's no point anyone sucking on it.

*sigh* Yes, I hear that a lot! Nowadays I have to pay them extra to pump it quite a bit first.

I'd certainly believe that might be true of epoxy or silly-putty, but if you consider how water coming out of a tap has a natural tendency to break into droplets I hope you can see that the water in the tube doesn't pull strongly enough to keep itself together along the whole length of the siphon.

N.B. The maximum height the siphon can rise above the source is about 34 feet, beyond that they need the assistance of pumps. Incidentally the height of a column of water that can be supported by atmospheric pressure is also about 34 feet.

[PolkaDotShorts]Coincidence? I think not![/PolkaDotShorts]

Someone should tell Mythbusters. First, because operating a siphon in a vacuum is right up their street and two because having just now thought of it I desperately want to see someone siphon silly-putty. :gulp:

The vapor pressure of water (at STP) is about 25 mbar, while atmospheric pressure is 1 bar, yet you can't siphon water over a rise greater than the height to which atmospheric pressure can lift a column of water. If a vapor pressure of 1/40th of atmosphere isn't small what is?

Let's skip water for a moment and think about mercury. If you consider the standard mercury column experiment, atmospheric pressure will support a column of about 760mm of mercury.
http://i155.photobucket.com/albums/s308/CrustyBear/Siphon-1.jpg
Now imagine you could attach a primed siphon at either point A or B. If you attach it at A, nothing will happen except the level of mercury in the siphon will fall to be 760mm above the reservoir at the other end of the tube, but if you attach it at B, mercury will flow into the tube from the column causing more mercury to flow out the other end, also as mercury flows into the tube, more mercury will rise up the tube to replace it driven by the atmospheric pressure on the top reservoir. Note that the vapor pressure of mercury (about 0.0025 mbar) is not important here, the space above the mercury level in the column is essentially a vacuum, only the pressure on the reservoir at the base matters.

In a vaccuum, the height of mercury in the column would be 0, and whether a single column or an inverted U (i.e. siphon tube), the mercury will just fall back into the reservoirs and siphoning will not occur.

Yes, mercury for one, yet as demonstrated above in a vaccuum mercury will not siphon since it will not support a column of liquid in the siphon tube.

In a vacuum, a mercury barometer reads 0 mmHg.

If you think atmospheric pressure doesn't play a role, try this simple test. siphon water from the seat of a chair, over the back, to a bucket in the floor. Now cut a hole in the siphon tube where it goes over the back of the chair.

To be honest, I really didn't think this was so difficult to understand. Let's start with a barometer filled with mercury (which is now by some miracle golden), it's U-shaped rather than a straight tube but it's a barometer for all that. We fill the tube with mercury, invert it in a bowl of the stuff, unplug the ends and the mercury falls to a height above the level in the basin determined by atmospheric pressure, leaving a vacuum above. Big deal.

http://i155.photobucket.com/albums/s308/CrustyBear/Siphon-2.jpg

Now we take a tube, fill it and invert it into two basins, side-by-side. Well I hope everyone can see that as far as the tube is concerned there is no difference, each end is in a bowl of mercury that's the same level and under the same pressure, that they're not the same bowl isn't an issue. Two barometers alongside each other will read the same, that they are "joined" at the top by a vacuum is irrelevant.

http://i155.photobucket.com/albums/s308/CrustyBear/Siphon-3.jpg

But what if they are not at the same height? So we dutifully repeat the last experiment with a slightly modified tube and one basin a foot lower than the other, and this time the mercury on the side of the lower basin falls further. This is not because the atmospheric pressure is significantly lower, but because the pressure is exerted on the open basin, so the column height is always relative to opening of the tube (i.e. it's bottom).

http://i155.photobucket.com/albums/s308/CrustyBear/Siphon-4.jpg

But wait, this is a siphon, shouldn't the mercury have flowed over the top of the tube and into the lower basin? Why would it, there is no extra force at play to lift the mercury against the force of gravity higher that in any of the other experiments.

If the top of the tube were lower than the height of mercury supported by atmospheric pressure then yes, gravity would cause the column on the right to fall further, and liquid on the left to flow to the right, but it isn't gravity that raises more mercury up the tube to replace the loss to the lower basin. Gravity causes a siphon to flow, atmospheric pressure keeps it full to do so. The lower the air pressure you operate one in, the lower the maximum "lift" a siphon will allow.

Except maybe for particularly cohesive substances like silly-putty, siphons do not work in vaccuums.

Even if you used a liquid you could cool enough for its intermolecular forces to prevent boiling, a siphon would still not work in a vacuum.

And it's certainly not a pressure drop or "vacuum" (*shudder*) pulling the liquid up the tube, as this simple demonstration by Blaise Pascal in around 1650 showed.

http://i155.photobucket.com/albums/s308/CrustyBear/Siphon-5.jpg

The apparatus, consisting of two vessels of mercury at different heights, into which a three-ended tube is placed, is slowly submerged in water. As the water pressure on the mercury increases, some is forced up the tube displacing the air. When sufficient mercury has been displaced in the raised vessel to reach the bridge of the connecting tube it flows, by gravity, into the lower vessel. At no time is a vacuum formed in the tube or does the pressure drop.

dumbfounded is always right you know. sorry brian and chemist but he always is.

When Brian said "gravity is the functional element, without which a siphon will never work" he is quite correct, but it is only one part of the siphoning process.

As the Harvard link says, the pressure at the top of a siphon is p = P - ρgh, where P is atmospheric pressure and h is the height above the basin, so as long as h is less than the barometric height (given by P/ρg) liquid will reach the top of the siphon. If the value of p for each arm of the siphon is different, because h is different, there is a net force in action on the liquid and it will flow.

But P needn't be atmospheric pressure. We could for instance place the whole kit and caboodle in a hyperbaric chamber and raise the pressure inside to 10 bar. If we did we would see the siphon work at heights far in excess of the usual barometric height limit. Of course traditionally this would still be treated as a local "atmospheric" pressure of 10 bar, so the essential argument is unchanged. So what if 'P' wasn't derived from the atmosphere at all?

http://i155.photobucket.com/albums/s308/CrustyBear/Siphon-6.jpg

In the above apparatus, the local atmosphere has been reduced to a minimum (just enough to prevent boiling, in deference to A Chemist) and exerts far too little pressure to raise any liquid to the top of the siphon. Fortunately this time we have a couple of very heavy tight fitting gaskets with us which are just the right size to go round the mouths of both the siphon and the basins. So now we still have a large weight pressing down on the liquid, but it isn't derived from any atmosphere. This time the pressure due to the weight is Pw = mg/A where m is the mass of the weight and A is its cross sectional area. Hence the pressure at the top of the siphon is p = Pw - ρgh, so if similar weights were placed on each reservoir there is again a net force at the top of the siphon causing the fluid to flow.

Now this siphon will work in a vacuum.

are the ph.ds odd at yale too? :cheese:

:lol: Very much so!
http://thefaust.files.wordpress.com/2008/08/hyde-pierce.jpg height=300 http://www.go386.com/fanboy/images/202461-tony_shalhoub_large.jpg height=300

generally speaking (no hyperbaric chambers and vacuums) in normal circumstances i'll make an analogy and you let me know if i'm close to getting this. a car cannot run without an alternator, but it must have a battery to start. atmospheric pressure starts the syphon and gravity keeps it going, as long as it remains primed. close?

that idea involves all kinds of diffing atmospheric pressures and drawing a liquid denser than water a very long way up. i know we'll get an answer martel, i just don't know if we will understand it. 😊

Very close. 😊 AP keeps the siphon full, G keeps it flowing.

differing :red: