The OED definition of the word erroneously states that atmospheric pressure makes siphons work, when in fact it is the force of gravity. Siphons draw fluid from a higher location to a lower one and are often used to remove liquid from containers, such as petrol tanks, that are hard to empty otherwise.
Dr Hughes said he was stunned when he realised that the dictionary had got the definition wrong. "It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm," Dr Hughes said. "We would all have an issue if the dictionary defined a koala as a species of bear, or a rose as a tulip," he said.
The error was introduced in 1911 and has gone unchallenged ever since. An OED spokesman said the definition was written in 1911 by "editors who were not scientists" and that Dr Hughes's notes would be taken into account when the entry was rewritten.
3 comments:
If it has nothing to do with pressure, why do you need to suck to get it going?
Dr. Hughes' explanation, if as short as in the brief new article, causes further confusion. Atmospheric pressure itself is the result of gravity. Another way of saying standard atmospheric pressure is 14.7 psi would be to say that the column of air above a square inch of earth's surface, all the way up to the edge of space, weighs 14.7 pounds. Without gravity there would be no atmospheric pressure.
In an operating siphon, the difference in pressure from the surface of the liquid in the vessel to the top of the siphon loop is that vertical distance times the density of the liquid. This is the static head across that elevation difference. Since the absolute pressure within the liquid cannot become less than zero, and the vapor pressure of the liquid in the vessel and in the siphon tube will be the same, there must be a force to balance the static head difference. That force is atmospheric pressure. The drop in liquid pressure in the upward leg of the siphon is the difference in static head minus the friction loss due to fluid flow within the tube. There is a maximum height of the loop above the level in the vessel, at which point the pressure of the liquid equals its vapor pressure. For water and standard atmospheric pressure, this height is around 33 feet. For mercury, with almost negligible vapor pressure but much higher density, this height is about 30 inches. Indeed, mercury barometers were the standard for measuring atmospheric pressure for a long time and remain in use.
Liquids have no real tensile strength (ok, it has surface tension). You can't fasten a handle onto a chunk of water and lift it; you still need the bucket. You can't "pull" a liquid, as you can a solid. Liquids move by being pushed. The pressure below the surface of a liquid is greater than at the surface, due to gravity. This pressure difference can push water out a hole below the surface of the vessel. But to make water move up a siphon tube you need pressure above the surface of the liquid. Thus a siphon will not work in a vacuum.
Dick Russell is basically correct except that water can have tensile strength. However, this has no effect on the operation of a siphon until the height of the siphon reaches about 33 feet.
Dr Hughes made the mistake of copying the chain analogy from http://www.exo.net/~pauld/physics/syphon/syphonphysics.htm without doing a full engineering analysis of the fluid mechanics of the problem. The pressure model of the siphon is shown further on in the same article and the last section of the article explains the problem with the chain analogy.
Pascal's Siphon, illustrated in http://mysite.du.edu/~jcalvert/tech/fluids/hydstat.htm, demonstrates that the liquid in a siphon is under pressure therefore, in a siphon less than 33 feet, atmospheric pressure causes the liquid to rise in the siphon and not the water's tensile strength. The model presented by Dr Hughes MIGHT work in a siphon greater than 33 feet if the water is pure enough not to "break" the tensile strength of the water.
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