Posted on 05/11/2010 9:03:35 AM PDT by Gomez
An Australian physics prof has discovered a 99-year-old error in the Oxford English Dictionary - repeated in most dictionaries worldwide - and is having it corrected.
The error is in the definition of the noun "siphon", a tube used to draw fluid from a higher location to a lower one - as when emptying a vehicle fuel tank, an aquarium or other vessel difficult to empty by other means.
Liquid is, of course, drawn up the shorter limb of the siphon by the weight of that in the longer downward one: thus the operating force is gravity. However most dictionaries follow the OED in stating that atmospheric pressure drives the process.*
Dr Stephen Hughes of Queensland University of Technology noted the error after visiting a massive siphon project in South Australia which was being used to transfer gigalitres of water into a depleted lake.
On returning, the prof decided to write an article about the siphon for use by school science teachers, and discovered to his dismay that most dictionaries described the process wrongly.
"An extensive check of online and offline dictionaries did not reveal a single dictionary that correctly referred to gravity being the operative force in a siphon," grumbled the physicist.
The OED currently says:
A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.
"The OED entry for siphon dates from 1911 and was written by editors who were not scientists," explained Margot Charlton of the Dictionary's staff. Amazingly, it seems that in 99 years nobody had ever queried the definition.
The next edition of the OED will be corrected.
According to Hughes some encyclopaedias - though not the Encyclopaedia Britannica - repeat the error. The doc has written a paper with more detail on siphons which the interested can read here.
*This may be true during the process of starting the siphon off, which is usually done by creating a temporary suction on the outflow end of the pipe so as to draw fluid up and over the hump. This works by the action of atmospheric pressure on the surface in the to-be-emptied vessel: but once the siphon is flowing this force is countered by atmospheric pressure at the other end of the pipe.
A siphon is driven by atmospheric pressure on the upside unless the liquid adheres to the tube. Then it can be raised higher than atmospheric pressure or in a vacuum if you start with a full tube and not suction. Suction will raise it over a hump no higher than atmospheric pressure, i.e. 33 feet of water or 30 inches of mercury.
It is explained in detail in Wikipedia.
Oh, I don't know. The government sucks, does it not? Thus pressure might be the driving force in that case.
“Are you sure siphons don’t work in a vacuum?”
I question this too. The atmospheric pressure is exactly equal at both ends of the tube. The only reason it might not work in a “perfect” vacuum is that many liquids boil in vacuum.
The scientific method doesn't work by hypothesis and experimental proof. It proceeds by consensus and by considerations of social justice. Oh, and the assertions of C-list celebrities.
The debate is over. Siphons used to work by air pressure 99 years ago, but man's destruction of air pressure's native habitat has changed the way they work, for ever.
P.S. I have Siphon Credits to sell.
Back in high school physics, we had a kid in the class that insisted atmospheric pressure was part of the siphon. The teacher set up an experiment: He started a siphon and plugged it with a stopper that could be released from outside a bell jar through a stopper in the top. He then evacuated the bell jar and released the plug in the siphon. The siphon flowed (despite the fact the water was boiling at room temperature), but there was no pressure.
Therefore, the siphon is all gravity, pressure has nothing to do with it.
Of course it would, other than the little problem of the liquid boiling off. You could fill the long leg in the liquid resevoir, cap the end, and lift it up over the lip of the container. It would work just fine via gravity alone.
Yes, but not in zero G.
Not exactly. See 41.
See Wikipedia re trees higher than 33 ft.
Doesn't need to be atmospheric pressure, the weight, and thus pressure of the liquid in the tank would be enough.
How useful the principle is to you depends on the opportunities you have to apply it.
“Besides, just by an energy equivalence analysis, atmospheric pressure cannot drive a work process in that manner. Gravity-based potential energy by way of the relative difference in liquid levels, is the main source of energy causing the flow.”
The problem with that is the reason you are “Siphoning” is because in order to get the liquid from the higher spot to the lower spot, you need to get the liquid from the higher spot a little bit higher first.
In order to do that you are decreasing the pressure in the tube (relative to the pressure pushing down on the top of the liquid) in order to bring liquid to the high point in the tube, where after the force of gravity will keep the pressure in the tube at its lesser state by itself (which continues to draw more liquid uphill.
If it was in a vacuum, there would be no way to force the liquid to defy gravity and go up, either to start the process, or after the process got started. You are relying on differential pressure between the atmosphere and the tube to make the liquid go “uphill” first.
Excellent point El Gato.
The higher end of the siphon would have to be very close to the top of the liquid in a vacuum bell-jar to remove that possibility.
Mainly because, by definition, there's nothing to siphon in a vacuum.
Water goes higher than 33 feet in trees because of capillary action. An entirely different phenomena called “surface tension.”
You are both not quite right. See 41
Water has tensile strength and viscosity depending on the tube material that can overcome negative pressure.
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