A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How does a siphon work?

January 5, 2001

Dear Cecil:

I've asked every physics professor I've ever known but have never found a satisfactory answer to the question: How does a siphon work? I understand that it allows liquid to be moved to a lower altitude, so there's no problem in terms of potential energy. But how do the individual molecules know that they're going to end up at a lower altitude? Does air pressure have anything to do with it? Would a siphon work on the moon?

Cecil replies:

The Teeming Millions are thinking: Why are we bothering with this? Everybody knows how a siphon works. All I can say is: Hah.

My initial idea about siphons, which I worked out at age eight, was as follows. Put a water-filled hose in a tub, with end A submerged below water surface D and end C hanging free over the side, below level E. The water in the hose, for reasons we shall debate directly, moves as a unit toward either A or C. Since the weight of the water in hose segment BC is greater than in segment AB, the water flows out of end C and more water is drawn into end A. If the reason for this isn't obvious, imagine that the tub and hose contain not water but a long length of chain. The greater weight of chain segment BC pulls segment AB over hump B, and in a short time the entire chain snakes out of the tub and onto the floor.

Up to this point everyone's in agreement. But why does the water (we're back to water) move as a unit? My deduction at age eight was, when the water in BC flowed out hose end C, the water in AB had to follow immediately behind, or else a partial vacuum would be created at point B. Since nature abhors a vacuum, even a virtual one, atmospheric pressure on the tub surface D pushed more water into hose end A until the tub was completely drained.

Makes perfect sense, right? Imagine my surprise on consulting the standard references to learn that this is not the accepted view. I quote from the Encyclopedia Britannica: "The action [of a siphon] 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." In other words, the water isn't being pushed over the hump by atmospheric pressure behind it, it's being pulled by the water ahead, as though it were (excuse me, but this is how I conceived of it) a giant stringy booger.

I didn't buy it. True, there are "self-siphoning" liquids containing very-long-chain polymers. Tip a beaker of such a liquid so it begins to pour out and the thing will continue to drain — a siphon without benefit of a hose. But water isn't like that. What's more, the claim that a siphon will work in a vacuum seemed questionable, since given a strong enough vacuum, any liquid will eventually evaporate away. I consulted with physicists far and wide. After arduous discussion virtually everyone came round to my view that atmospheric pressure, not the water's cohesiveness, was the operative principle in a siphon. Except for "Uncle Al" Schwartz, the Usenet physics adept, who reminded me of one thing: cavitation.

Cavitation, I thought. Damn.

Cavitation is the formation of bubbles (voids, really — they're basically tiny vacuum pockets) in fluid under tension. Example: the bubbles that form in the synovial fluid around your knuckles when you pull on them, the collapse of which creates the infamous cracking noise. Tugging on your knuckles to create these voids requires considerable force, evidence of the tensile strength (cohesiveness) of fluids. The tensile strength of water enables the transpiration at the leaf surfaces to pull sap up hundreds of feet to the top of a tree, Al said, and it makes a siphon work too.

I pondered this bleakly for a while. Then a thought occurred to me. Al, I said, the highest you can raise water in a siphon is around 34 feet. By curious coincidence, the maximum height that water can be drawn in a tube sealed at the top (a water barometer) is also around 34 feet. A barometer depends on air pressure. Doesn't this suggest that air pressure also plays a role in the operation of a siphon? Al agreed that impurities, dissolved gases, and such reduced the tensile strength of water in a siphon to "the atmospheric case." In other words, I concluded triumphantly, while the cohesiveness of water explains the operation of a siphon in theory, in practice it depends on air pressure!

Al insisted that water's tensile strength remained the conservative explanation. Fine. But I figure I'm entitled to call this argument a draw.

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