"Thanks for thinking of me, especially on a thread about siphoning."
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.
Wow. I never realized how much people use syphons. I thought kids just used them to steal gas.
I nice trick I learned is to place two tubes into the tank to be siphoned and wrapping a cloth in a figure eight around the tubes. Below the cloth one tube should be long enough to reach as much liquid in the tank as needs siphoned while the other should be just long enough to enter the tank. Then push the rag into the tank opening trying to get the best seal that you can. Then blow into the “short” tube. I was amazed how well it worked.
LBFM ?
Nokes M. C. (1948), “Vacuum siphons”, Am. J. Phys. 16: 254
Simple: reduce the atmospheric pressure around the shorter leg and see if any flow occurs. No mistake has been found, just someone trying for a “gotcha”. Ho Hum.
Who gives a crap and who uses a syphon?
Just using increase in pressure to get the flow going, then gravity does the rest, so long as you equalize the pressure inside and out of the tank, not allowing the pressure to become greater outside than in. Thanks for the word picture. Well presented
The Australian Physics Professor is wrong.
The only reason water flows UP the pipe is because atmospheric pressure PUSHES it up.
If there was no atmospheric pressure, nothing would happen.
Of course, atmospheric pressure is caused by gravity, and gravity is what pulls the water down on the other side.
But if you had a vacuum on both sides, the water would run down both sides of the pipe, because the “suction” created by the water on the longer side of the pipe wouldn’t overcome the “suction” of the vacuum.
And a siphon wouldn’t NEED gravity, although by definition I guess it does; you could siphon with a pump, or any other mechanism that caused a lowered atmospheric pressure.
I guess in the end, you could argue that “siphoning” is the special case of suction where the suction is created by gravity pulling down on the fluid.
Oops, sorry, wrong article!
Think about it; the receiving end is lower, so it is under greater ambient pressure than the higher, sending end.
Assuming that both the ambient pressure and experienced gravity are nonzero, the gradient in ambient pressure works to impede the flow, but only very slightly compared to the flow-creating differential due to the weight difference in the two halves of the siphon.
The difference in the magnitude of the two effects is proportional to (1) the strength of the gravity, and (2) the difference in mass density between the ambient air (or other gas) and the liquid, and (3) the net length of the drop.
The siphon would still break. It is the release of the water itself into gaseous form that will form the gap.
This is like a barometer. I used to wonder, “how can a space open up at the closed top of a column of mercury, so that the column will reach about 760 mm and no higher?” The answer, of course, is that the space is a near vacuum, with a small amount of mercury vapor.
No. The energy is provided by gravity. The pressure is the difference in height of the 2 surfaces. The amounts of water in the 2 reseviors and their shapes do not matter, only the head does. The head is the driving pressure.
p = mgΔh, where Δh = the difference in height of the 2 surfaces. g is the acceleration of gravity and m is the mass/unit area.
The mass/unit_area of air is too small to have a significant effect on the system. Air is essentially equivalent to a vacuum in this system. The cohesive energy density of the liquid is what keeps the water together.
"Thanks for thinking of me, especially on a thread about siphoning."
Siphons also work in zero gravity, because of inertial. Once started, they will continue until the frictional losses diminish the momentum to zero.
What a brilliant way to agree with my position!
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