Physicist demonstrates dictionary definition was dodgy
It is the defining moment that demonstrates a QUT physicist was correct in pointing out a 99-year-old mistake to one of the world’s most authoritative dictionaries.
QUT Senior Lecturer in Physics, Dr Stephen Hughes, sparked controversy over how a humble siphon worked when he noticed an incorrect definition in the prestigious Oxford English Dictionary.
In 2010, eagle-eyed Dr Hughes spotted the mistake, which went unnoticed for 99 years, which incorrectly described atmospheric pressure, rather than gravity, as the operating force in a siphon.
Dr Hughes demonstrated the science of siphons in a paper published yesterday in Nature Publishing Group journal Scientific Reports.
For Exploring the boundary between the siphon and barometer in a hypobaric chamber, Dr Hughes conducted an experiment in a hypobaric chamber, which simulates the effects of high altitude, at the Institute of Aviation Medicine at the Royal Australian Air Force’s Base Edinburgh in South Australia.
A siphon 1.5 metres high was set up in the chamber and when the pressure was reduced to an altitude of 40,000 feet a waterfall appeared at the top, but the water flow remained nearly constant.
At 41,000 feet, the siphon broke into two columns of water and, when returned to 40,000 feet, it reconnected as if nothing had happened.
Atmospheric pressure at 40,000 feet, which is more than 10,000 feet higher than Mount Everest, is about 18 per cent of the sea level value.
For the experiment, two buckets, one higher than the other and connected by tubing, were set up and a pool pump returned water from the lower bucket to the higher bucket.
“The fact that the water level in the upper and lower buckets is constant indicates that atmospheric pressure is not pushing water into the siphon,” Dr Hughes said.
“The stable water surfaces act like energy barriers between the atmosphere and siphon. For energy to be transferred from the atmosphere to the water the water level would have to go down, since the amount of energy transferred is equal to force times distance.
“If the water level is constant the distance is zero and therefore no energy can be transferred.”
Dr Hughes, whose previous research has taken him to Bhutan to examine how siphoning could prevent inland tsunamis, said siphons had been used since ancient times but how they work was still debated.
“If you think of a car, atmospheric pressure is like the wheels, it enables it to work. But gravity is the engine,” he said.
“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.”
The Oxford English Dictionary corrected the error and removed the reference to atmospheric pressure after Dr Hughes pointed it out. However, he said the new entry “unfortunately remains ambiguous”.
“This definition still leaves the question open as to how a siphon actually works,” Dr Hughes said.
“But at least the reference to atmospheric pressure has been removed. The vast majority of dictionaries of all languages still incorrectly assert that siphons work through atmospheric pressure and not gravity.
“I hope these findings are a useful contribution to the debate about how siphons work and will enable people to make more effective use of them.”
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Charlies Nelson, “If you reduce the atmospheric pressure further the water will eventually boil and stop the syphon!”
Precisely correct which is exactly what has happened in his “waterfall effect”. He essentially has a barometer on the down tube while the up tube still is short enough that atmospheric pressure is strong enough to keep the water height above the point where it goes over the apex. The atmospheric pressure is not what is making the water move (that’s caused by gravity) but it is what allows the siphon to be maintained.
george e. smith says (April 25, 2014 at 12:41 pm): “If I’m not mistaken (I was once), SF6 is used to insulate UHV (oltage) transmission components; transformers and the like.”
Will a taser spark in a SF6 atmosphere? See for yourself (at about 1 min):
The first figure and caption mention “the hydrogen bond”. What would that have to do with a siphon? Wouldn’t, say, carbon tetrachloride much the same way?
@ur momisugly Brian Macker – How ironic that you claim to be a siphon expert and complain that other siphon specialists don’t understand their own field, yet you don’t realize that water siphons have been demonstrated to 24 meters and mercury siphons have been demonstrated to more than 15cm above the barometric height of mercury(and that in glass tubes to which mercury adheres poorly – much higher is likely possible in say a copper tube). The Xylem of tall trees is another example where water resists vaporization at negative pressures several times atmospheric pressure.
Furthermore, while everyone agrees that gravity is the ultimate energy source of a siphon, it is nonetheless ALSO true that it IS atmospheric pressure that pushes the liquid up in typical siphons after gravity has lowered the pressure at the top. Atmospheric pressure doesn’t just prevent vaporization. See my links and comments above. Be extra careful when you call people idiots.
Brian Macker is exactly correct. Every engineering hydraulics book for well over 100 years has correctly analyzed the siphon via Bernouli’s energy equation, and the driving force is gravity.
The only role of pressure is to prevent dissolved gases from coming out of solution at the top of the siphon and to prevent water boiling, which also breaks the siphon by putting vapor at its top.
Prof. Hughes could have found the answer in any freshman physics book or any engineering hydraulics book, but it would not have been as much fun.
@ur momisugly bob skyes – If the only role of pressure is to prevent bubbles, then how do you explain Figure 4 in the Wikipedia siphon article, where the siphon starts out with only air at the top? Is not pressure needed to push the liquid up?
Brian Macker says:
April 25, 2014 at 1:11 pm
Amen! to that.
Reference number 2 in the Wikipedia siphon article (Would a siphon flow in a vacuum – Minor 1914) reports siphoning mercury 30cm above the barometric height.
It is hoped that this paper may assist in correcting the common misconception that the operation of a siphon is dependent on atmospheric pressure. In view of the extensive search made of online and offline dictionaries, it is possible that every English dictionary in the entire world needs to be corrected.
.Talk about delusions of grandeur!
Let’s face it Dr. Hughes, you are simply wrong. If ordinary water needed the cohesive properties you require for the function of an ordinary siphon you would never be able to get out of a swimming pool because you’d be trying to pull out all the water behind you. That your feeble idea passes as science is frightening.
If you do not believe that atmospheric pressure is required at the inlet Dr. Hughes then simply just try taking it away and see what happens.
==============================================================
Mr. Layman here.
It would seem to me that gravity pulls the liquid (and the air) down then the liquid rises to fill the vacuum.
What would happen if there was also a vacuum above the upper liquid….? No siphon?
@ur momisugly Mike M wrote:
“If you do not believe that atmospheric pressure is required at the inlet Dr. Hughes then simply just try taking it away and see what happens.”
It’s been done. It worked. The atmospheric pressure was not required. There is a Youtube video of it linked above. Not all siphons require atmospheric pressure. Some do require it. Some don’t require it but use it anyway.
=====================================
DUH!
If the vacuum was only over the upper liquid the “siphon” would reverse … but then it wouldn’t be a siphon.
“Giant sucking sound” indeed.
Gary Hladik says: April 25, 2014 at 1:24 pm
Exactly right, thanks for the links:
http://en.wikipedia.org/wiki/Siphon (which I found to be well written) and
http://www.phys.uhh.hawaii.edu/documents/TPT-final.pdf
.. both of which annihilate Dr. Hughes’s hydrogen bond or “molecular chain” explanation of a typical water siphon.
The latter stating:
The chain model predicts that, as the gravitational driving force has not been altered, the siphon will continue to operate. Bernoulli’s law predicts that, once enough fluid is drawn out of the top bottle, pressure of the gas trapped inside it will lessen until at some point the siphon stops operating. We observed that the walls of the plastic bottle caved in as the pressure within the bottle decreased, as seen in Fig. 3(b); once the pressure inside the top bottle was low enough, as shown in Fig. 3(c), siphon operation stopped.
Stop the presses Dr. Hughes.
Réaumur, There is still a height above which even the ionic fluid will not siphon. Guess what. Adding atmospheric pressure would make it work again. That tiny chamber in the video cannot test the heights required.
The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber. Once the flow is established it will keep flowing. Air pressure will actually tend to counter this as the lower chamber has a higher surface pressure. I am a simple Engineer am I missing something?
Mindbuilder says: April 25, 2014 at 2:38 pm ” Not all siphons require atmospheric pressure. ”
Yes they do or they are not “siphons”. The chain over a pulley model is NOT how a siphon works. The youtube video is a chain over a pulley – actually pulling the mass behind it purely by gravity.
Reduce the pressure at the inlet of a REAL siphon, (i.e. garden hose and tap water), and the siphon will stop. In the wiki entry they correctly point out that in an ordinary siphon the fluid is in compression, (pressurized) at all points along the way. That is NOT true with a highly cohesive fluid which is in tension – the exact opposite.
Mr. Layman wrote:
“It would seem to me that gravity pulls the liquid (and the air) down then the liquid rises to fill the vacuum.
What would happen if there was also a vacuum above the upper liquid….? No siphon?”
Right. The question is where does the force come from to make the liquid molecules rise to fill the partial vacuum. Since the partial vacuum of air at the top of the siphon is pushing DOWN on BOTH sides of the siphon, and gravity is pulling DOWN on all the liquid, the only force left to make the liquid molecues rise is atmospheric pressure. Same as in a drinking straw. If there was vacuum above the upper liquid, there would be no siphon effect in a siphon starting out with air at the top.
Fascinating, just fascinating. Thank you Anthony, not for the post itself but for all the discussion it has generated. This has been a real learning experience in quite unexpected ways.
For your next trick, try posting something about how wings generate lift!
BeauGatun,
“The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber. ”
No, the pressure at the surface of the higher chamber is lower than the pressure at the surface of the lower chamber. You don’t even need any chambers for a siphon to work as long as you have a thin enough tube, or a movable plug in the uphill end of the tube. All the water will flow out the lower end. The pressure differential is in the tube, not the chambers.
If the pressure is the same at both ends of the hose, then it can’t be atmospheric pressure. My father taught me that it was gravity when I was a kid.
“””””…..Gary Hladik says:
April 25, 2014 at 1:28 pm
george e. smith says (April 25, 2014 at 12:41 pm): “If I’m not mistaken (I was once), SF6 is used to insulate UHV (oltage) transmission components; transformers and the like.”
Will a taser spark in a SF6 atmosphere? See for yourself (at about 1 min):…..”””””
You’d never guess in a million years, where I first learned that.
It was in my first year in high school, about 1948-9, and I was a “lab assistant” in the “elec and mag” lab.
Which meant basically that I came in before class or labs, an put out the equipment that we were going to be using to do whatever experiments we were assigned, or what the teacher wanted for class. That day, we had been introduced to the Whimshurst machine.
After class, I had to return the machine to the teacher’s office, where it belonged on top od some high book cases, alongside: believe it or not, a mercury filled barometer.
Naturally, I fooled around with things, including the barometer, and ended up spilling a lot of the mercury on the wooden floor. Swept it up with a dustpan and brush, and got most of it back in the tube.
While I was in there I was looking at his tech journals, which he received regularly. One of those was the Brown-Boveri Technical Journal. I think, that’s some giant European Electric company (maybe Swedish ??).
And I read an article in there about using SF6 to encapsulate > 100 KV transformers, and in testing those giant long ceramic insulators, on transmission lines.
I have no earthly idea why I remembered that event.
Well actually, I spilled the mercury while standing on a stool playing with the Whimshirst machine, and I accidently touched one of the balls, while furiously cranking on the handle.
The resulting blast knocked me A over kite off the stool, and a flailing arm knocked over the mercury barometer. I landed on my back on the floor, with all that glass in my lap, so it never broke; but the mercury all went flying.
Yes I did catch hell from the teacher, and he made me clean up all that mercury in alcohol or something, to get the dust and crud out of it. I even kept my lab job.
So many of these explanations are vitiated by considerations that water might boil at low pressures etc. Explanations that a syphon are like a chain and rely on cohesive forces are rubbish. All this is ‘noise’ that gets in the way of thinking about what is going on. Why so?
Consider a thought experiment: we have a syphon with both ends immersed in their respective reservoirs filled with a hypothetical liquid that has the same density as water but is incompressible and has zero tensile/cohesive strength and infinite boiling point/zero vapour pressure. No longer can the argument be used that liquid gets ‘pulled’ uphill since there is zero tensile strength. And remember, vacuums don’t suck, they don’t do anything.
So, will this syphon work? Yes, like an ordinary water syphon if there is atmospheric pressure. No, if there is no atmospheric pressure. How does the liquid get up the tube from the upper reservoir? By air pressure. Gravity is simply not going to make the liquid ascend from the upper reservoir, neither can it be ‘pulled up’ by any putative vacuum, nor by liquid cohesion (which in our thought experiment is zero).
Extend the thought experiment further: would the syphon work if the reservoirs were filled with an ideal gas that was very dense (compared to air) rather than a liquid? Here there is no possibility of cohesion/tensile strength. It will still work as a syphon if provided there is atmospheric pressure above the reservoirs. What makes the gas ascend (ignore diffusion) from the higher reservoir? The atmospheric pressure.
Simply stated you can start a siphon with properly differentiated air pressure but it maintains flow due to gravity. You shouldn’t need a paper to establish that.
Water will flow along a pipe from a reservoir with an elevated phreatic surface to a reservoir (or outfall) where the phreatic surface is lower. If the full length pipe alignment is below the hydraulic grade line* there is no siphon involved – it’s ‘just a pipe’! And (if the atmospheric pressure is the same at both upstream and downstream ends) water will flow down the pipe by gravity – as ball-bearings would (if friction was negligible).
But a siphon is not ‘just a pipe’. A siphon is where the pipe rises above the hydraulic grade line. If you were to allow a ball-bearing to roll along the pipe it would not get over a siphon. Gravity alone may explain normal pipe flow (also open-channel flow) but it does not explain a siphon.
At sea-level a siphon will not flow if it rises more than 10m above the hydraulic grade line. Both gravity and atmospheric pressure are needed. But the role of atmospheric pressure is the defining difference between a siphon and other gravity-driven liquid conduits.
Also, water has no cohesion or tensile strength. (It does have viscosity which is related to velocity but is not relevant to a basic siphon.) So the chain over a pulley analogy is flawed.
* Hydraulic grade line is a line running downhill from the upstream phreatic surface to the downstream phreatic surface level. It must always run downhill – the only exception is pump locations.
Overlooked constituative properties.