Archimedes approves. ;)
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.
Nope. Another way of looking at it is this: If pressure has anything to do with the siphon, why does the siphon work with the same (or negligibly different) pressure at both ends of the tube?
I think the problem with the water “boiling” (changes from liquid to gas) at 0 atmospheres. The expanding gas allows the water to fall back out of the tube.
I thought it was caused by the person sucking on one end.
Exactly. And using that, you can construct a siphon higher than 33 feet that is not driven by atmospheric pressure but by tensile strength and gravity.
Sucks, don’t it.
If you could not pre-fill the "down" tube, getting it started would be a problem. Remember the atmosphere is pushing up on the open end of the down tube (or down on the lower reservoir) just as much as it's pushing down on the upper resovoir. (Yes there is a tiny fractional difference, but it's in the wrong direction, the lower reservoir or the open end of the down tube experiences slightly higher atmospheric pressure, because there is more air above it.)
Now if the upper reservoir were sealed the at the top, then the pressure above it would decrease, compared to the pressure pushing up on the open end of the down tube, and that differential would tend to counter the weight of the water in the down tube, and the siphon would at some point stop.
A siphon won’t work in a vacuum for the most basic reason that liquid’s don’t exist in a vacuum. But if you were to have a liquid it wouldn’t flow anyway. The upper loop of the siphon represents an insurmountable energy barrier, this isn’t quantum mechanics. Liquid’s don’t magically flow to lower areas if they don’t have a path to get there.
Archimedes approves. ;)
Here is a good way to explain it.
You have a glass of iced tea. If you drink it without a straw you need to have the glass higher than your mouth and tip it and its all gravity.
If you use a straw, you can drink it with your mouth higher than the liquid. You use suction from your mouth to lower the pressure between the straw and the atmosphere pressing down on the top of the tea, which allows the tea to “defy gravity”. If you tried to drink the tea in a vacuum, you could not do it with a straw because you could not suck greater than vacuum.
So a siphon requires both gravity and atmosphere. Remember you need to suck on the siphon to start it (ie raise the height of the column of liquid)...once the liquid passes the high spot, it starts falling down the other side, which continues the sucking by itself.
Usually what you will get is not boiling gas, but gas going around the liquid on the down side if the low end is not submerged and the liquid does not “stick” to the tube all the way around. If it does stick to the tube, the tensile strength of the liquid will allow it to be raised higher because the viscosity will hold it up.
I hope this physicist is not under the impression that todays kids ever pick up the dictionary.
I knew a chick who could siphon a golf ball through a garden hose. Not sure if she was a physicist.
Ah, you say,the tube must be bent. Fine, run the tube around the earth until the sky end is below the equator.
courtesy of Fractured Flickers, science division.
Wrong on all counts. see 41
The pressure cannot be responsible for pushing the water up the small leg because the same pressure is applied to the water up the large leg. It cancels out in equilibrium. The siphon is 100% gravity.
The tube would have to be very small to use tensile strength. A way to illustrate this would be to make a siphon out of cloth. It takes a while, but it will eventually suck the water out of the reservoir, provided the free end of the cloth is lower than the surface level of the reservoir.
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