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?
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.
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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.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.
