Siphons -- not a draw, an experiment!

(1) Nature does not abhor a vacuum. This is an Aristotelian notion, and it is unbecoming of Cecil to slouch towards Aristotelianism. In fact, it appears that nature loves a vacuum since most of the universe is made out of one.

(2) I didn’t buy the air pressure explanation when you were explaining it and I don’t buy it now. I have always been of the opinion that water is drawn along through the siphon. My qualification for having an opinion at all: Ph. D. in theoretical physics. The key fact is that water moelcules tend to bind together into temporary complexes because water is a polar molecule. This is the same property that is responsible for surface tension. Water molecules have a positive end and a negative end and that means that they tend to line up and stick together as the oppositely charged ends attract. Make one of them move and it will tend to drag the next one along. This means that all you need are local interactions along the length of the siphon to explain its action. You do not need external influences at all.

(3) This is not an issue that should be subject to opinion. As a matter of science, it can be decided by experimentation. If Cecil is right, all fluids should siphon equally easily. If the Brittanica (and me) is right, then nonpolar fluids should be significantly more difficult to siphon than polar ones as the only intermolecular interaction is the much weaker van der Waals force. Unfortunately, the only nonpolar fluid I can dredge up at the moment is liquified natural gas and I have no real desire to suck a hose immersed in that. I will leave the test to the SD staff, who have sucked on worse things in the name of science.

(4) You forgot to mention superfluids, which are also self siphoning but for wildly different reasons.

(5) You can prove that something other than air pressure must be going on here. From Bernoulli’s equation:

p1 + rhogh1 = p2rhog*h2

where the p’s are pressure, rho is the density of the fluid, and h’s are the heights of the two fluid columns. Anywhere near the surface of the Earth (such as siphoning beer in your friendly kitchen), p1=p2=1 atmosphere. Consequently, either h1=h2 as well (i.e. there is no siphon) or something else is going on, or you are attributing the entire function of a siphon to the miniscule difference in pressure between two fluids whose altitudes may differ only by centimeters, no matter what the viscous drag may be in the tube (or rag) that connects them. That doesn’t seem very plausible. Do the experiment.


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Cecil’s column can be found on-line at this link:
How does a siphon work? (05-Jan-2001)

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There is also an on-going (and unfortunately, testy at times) discussion going on here.

pjcamp, there are two ways pressure can come into play in a siphon. The first way, which you described, is through the pressure difference between the inlet and outlet ends. As you point out, this is completely negligable. This could easily be shown through measurements by putting the inlet and outlet ends in separate containers, and compensating for the pressure difference due to the height difference.

The second way pressure can be a factor is that the atmospheric pressure serves to hold the liquid together. For a siphon up to 34 feet, the pressure on the water is positive. Up to this height, any tensile strength the water has doesn’t come into play, because the water is being pushed together, not pulled apart (neglicting the tendancy for the water to boil at low enough pressure). The longer column of water in the outlet end is being pulled more strongly by gravity than the shorter column of water on the inlet end, so it drops, and the air pressure at the inlet end pushes the water up the inlet tube, and over the hump.

If you had paid attention to the column, you would have realized that this statement was attributed to an eight-year-old Cecil. While Cecil’s current vast store of knowledge is undeniable, I hardly think it reasonable to chastise Cecil for once not possesing this knowledge.

To pjcamp:

(1) “Nature does not abhor a vacuum” is a figure of speech, but important here on Earth, where atmospheric air pressure rules.

(2) I do buy the air pressure explanation. See the other thread that ZenBeam references for more detail. My qualification for having an opinion at all: Ph. D. in mechanical engineering.

(3) Another good experiment (if I do say so myself) is to try a siphon containing an air bubble big enough to seperate the water into two discrete parts (no intermolecular forces across a gas). See the other thread for the results of that experiment.

(4) Superfluids. Cool.

(5) You’re missing the velocity term in your Bernoulli’s equation. It should be:

p[sub]1[/sub] + rhogh[sub]1[/sub] + rhoV[sub]1[/sub][sup]2[/sup]/2 = p[sub]2[/sub] + rhogh[sub]2[/sub] + rhoV[sub]2[/sub][sup]2[/sup]/2

From this, you can get the velocity of the siphon:

V[sub]2[/sub] = SQRT(2g(h[sub]1[/sub] - h[sub]2[/sub]))

Also, realize that the pressure within the siphon is not atmospheric. As a matter of fact, you can find the pressure at any point (3) from Bernoulli’s also:

p[sub]3[/sub] = rhog(h[sub]2[/sub] - h[sub]3[/sub])

which will be negative (atmospheric). Since it only takes a couple feet of siphon to produce a psi of pressure differential, I’d disagree with the characterization “miniscule.”