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.
The test: Will a siphon work in a vacuum?
Would a siphon work in zero G conditions? Has this ever been tested in space? If the atmospheric pressure theory is correct the siphon should work in the zero g artificial atmosphere of a spaceship. If not, then one can only conclude that gravity is the culprit.
Gravity?
Not sure about that.
If I take hose from one elevation say a pond and run the hose over a higher 8’ fence and then to a lower elevation and begin the siphoning, That is all gravity?
If it were, I could just leave the hose at the lower elevation and it should automatically drain and yet without pressure to encourage flow, there is no siphon.
It might be a combination of atmosphere and gravity but not both.
Guess I wansn’t the only only to note both requirements.
This physisict needs to go back to screwel annd I am not going to use the spell chequire on this post, purposelful.
Professor Hughe' definition can be found here and he claims that the siphon action has been demonstrated in a vacuum (footnote 3.)
His explaination relies solely on gravity and the molecular attraction of individual water molecules to form a "chain" that is pulled up and over the hump, in the same way that a chain and pulley work.
It would be simple to test which theory (atmospheric pressure or gravity alone) is correct by building two sealed boxes connected by a flexible hose. Each box would be halfway filled with liquid, then the atmospheric pressure drawn out of the empty space above the liquid in both containers. Once the hose is filled with water, lowering one container below the other container should begin liquid transfer. Switching the positions of the two boxes should reverse flow. If water flows, then gravity is the root cause of the siphon action.
Alternately, one Atmosphere is roughly equal to 34 feet of water. Try to create a siphon that can lift water higher than 34 feet before it makes it's turn downwards. If the siphon action stops when the lifted column of water exceeds 34 feet, then atmospheric pressure is the root cause of the siphon action.
For a short time, I was a sixth grade school teacher in Fairfax County, Virginia, decades ago. And my son always had our house full of his little friends, so any activities I did in the garage or yard had ‘close scrutiny’ from a little army of inquisitive minds who could not get explanations at home but could get it from ‘the teacher’ in the neighborhood. I love kids.
No, I don't believe anybody would say that a siphon would work in zero G, because you do need gravity to lower the water in the long leg of the siphon to create the lowered pressure above the short leg of water.
What the good Professor is claiming is that a siphon would work in the presence of gravity, but in the absence of any atmospheric pressure (vacuum.)
“Who gives a crap and who uses a syphon?”
I do not use a siphon as frequently as I used to on desert trips since I now have 2 gas tanks, but some people do not have 2 gas tanks on their 4 wheel drives thus must carry jerry cans. (we go on trips in the desert that require more than one tank of gas, where gas is unavailable in Nevada). It is very nice to not have to remove the jerry cans, screw on a spout, and manhandle them to refill the tank. With the jiggle siphon, you do not need to do anything, but remove the top of the jerry can, and jiggle the hose a couple times.
So thats who uses a siphon.
But a siphon has both ends open to the atmosphere. The barometer doesn’t.
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.
Professor Hughe' definition can be found here and he claims that the siphon action has been demonstrated in a vacuum (footnote 3 2.)
Any diameter.
Thank God we have a couple of smarties to help guide the FR discussion on this and straighten us all out. :-). You make excellent points and I am sure you are both correct.
So what happens in a vacuum? Gravity pulls the water out of the longer leg, then nothing replaces it so the process stops? Or nothing moves at all and it all holds together via surface tension? What if the long tube is so long or the tube such a wide diameter that the force of gravity overcomes the surface tension...
So would siphons work in a vacuum?
Young boys, like cub scouts, are fascinated by science. I would put on a “Mr. Science” show at Pack meetings. One of my favorites is to demonstrate atmospheric pressure.
I attach an 8 foot hose to a hot water bottle, with a funnel on the other end. I put a board over the HW bottle. I get a Scout to stand on the board. On a ladder, I hold the funnel high and pour water in. The pressure on the water fills the bottle and lifts the Scout standing on board. Cool stuff.
I guess I'm easily amused.
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