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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Hmm, an incorrect definition that made no difference to those not susceptible to the dictionary fallacy. Or that actually used manometers.
A large practical volume was stabilized, slightly pressurized (health physics considerations moot) and rate of depressurization carefully recorded. Then the test was repeated with various orifices installed, and the results compared to estimate the volume’s leak rate. In this case, conceptually radioactive air-borne contamination.
“When you start sucking on the straw no part of your body is touching the liquid. Your body does not exert any forces on the liquid molecules. Even after you have lowered the air pressure in the straw, the remaining air pressure is still pushing DOWN on the liquid, yet the liquid rises because the atmospheric pressure pushes up harder. Isn’t this true?”
Make your straw a 40 ft vertical pipe. 1 atm is about 32 ft head of water.
If you put a vacuum pump at the top you will be able to suck the water up to almost 32ft, no higher. It will be the pump which is excreting an upward force on the liquid via the air. It is you pump that is doing the work not the atmosphere. At some point before 32ft the water will be boiling at the surface and from then on you’ll be madly pumping out water vapour and the level will get not higher.
If there was not atmosphere this all falls apart because there’s no liquid water either.
In the upper U of the syphon there is a hydrostatic pressure less than surrounding atm pressure because of the weight of water in the descending column in the syphon tube. While this is greater than the vapour pressure of water at the ambient temperature there is no need to go any further. It’s the cohesion of the liquid (the force that binds the molecules into a liquid state rather than a free gas) that transmits this hydrostatic pressure to the opening of the tube, where the hydrostatic pressure is 1 atm.
The pressure in the lower basin is also 1 atm. Therefore the rate of flow of the water (or whether it flows at all) is not a function of the surrounding atmospheric pressure. It is NOT the air pressure making the syphon work.
Now if you want to ask why does the water continue to flow upwards at 42000ft when the water column breaks due to evaporation, we’re back to the 40ft pipe. It’s the pressure _difference_ that causes the water to rise , not simply the atm pressure at the bottom. And what causes the pressure difference is the pump, or your mouth , or the depression caused by the weight of the column of water …. which is caused by gravity.
So whichever way you play it, it’s gravity not atmospheric pressure that drives a syphon. Even one with a vapour lock.
Surface tension is capillary action and the force calculation trivial.
I was going to post my understanding, but Mindbuilder pretty much hit the high points. I see gravity, air pressure, and molecular cohesion working in concert.
Can cohesion work without air pressure? Using a special liquid that holds together ‘like magnets’ in the video you can siphon in low pressure… sure, same as throwing a chain over a pulley.
Can air pressure work without cohesion? I haven’t done the experiment but I don’t see why a siphon with a small air bubble at the top wouldn’t work, can’t explain that with cohesion.
All in all, I wouldn’t rate earlier explanations as wrong so much as incomplete.
Unbelivably funny. It’s the pressure gradient, caused by the height h of the fluid column (dp = ρgh). Again, this is basic fluid statics.
Jeez! I sussed this better when I was eight!
A is height of the the siphon rise above the liquid level each side of the “U” So is in balance.
B is the drop to the end of the pipe or the lower liquid level, whichever comes soonest.
A – (A + B) = B
Therefore it drops.
The only mitigating exceptions for engineers are:-
Gas pressure differences on the liquid at both ends. If the upper bowl was *very* high above the other, it experiences a lower pressure which reduces the siphon effect slightly. But in practice, ignore.
The other as, as told before on here is the pressure-temperature of the liquid at the top of the siphon tube. Too low a pressure-temperature will boil the liquid which might stop or stymie the siphon action.
“If you put a vacuum pump at the top you will be able to suck the water up to almost 32ft, no higher. It will be the pump which is excreting an upward force on the liquid via the air. It is you pump that is doing the work not the atmosphere. ”
Simple experiment then. Try your same setup in a vacuum and see how much upward force ‘work’ your pump does.
This is ridiculous!
Both pressure and gravity are vital components of a siphon. Cohesion (Réaumur) and surface tension (Mindbuilder, though there’s more to it than just surface tension) are secondary effects.
Here’s a better sort of a experiment:
Two buckets, one higher with water, one lower, empty.
Add tubing or piping from high bucket, up and down to the low bucket.
Vapor lock means nothing flows.
Add a tap at the top, connect that to a vacuum pump, start pumping out air.
Water levels will rise in each tube, and the distance between the level in one side of the tube and that bucket will match the same measurement on the other side.
Keep pumping air out, eventually the water in the high side will reach the crest of the tube.
At this point, gravity is pulling water back on each side, atmospheric pressure is pushing water up on each side. This happens during the entire experiment.
Suck more air out, now water can spill from the high side to the low side and the siphon operates.
Sucking more air out will increase the flow rate.
Now gravity is pulling back on water on both sides of the tubing, but on the high side it’s also pulling water down the low side.
This is a process called spilling well know to any baby since the development of drinking utensils.
Augh!
Note that normal atomospheric pressure can only push water up about 10 meters, so don’t make the tubing too high….
Formation of bubbles. Cavitation anyone?
The vapor pressure of water at room temp atmospheric pressure is around 2 kPa.
Pressure is about 18 kPa at the water surface. At the top of the siphon it will
Be 1 kPa lower for every 10 cm of height above water surface. There will some
Other reductions in pressure due to the bend and due to the inlet of the tube.
Therre will be a further reduction due to dynamic pressure of the water as a result
Of it’s moving. Static pressure decreases as fluid velocity increases. If the end
Of the u tube is discharging into air, reduction in pressure (for 1.5 m height difference) be
About 15 kPa due to velocity of water in the tube. The fluid velocity might be slightlyDifferent
If the end of the tube is in the bucket.
One needs numbers like water temperature, tube diameters, height of ubend above water
Surface, and radius of bends in tube to do a more detailed calculation. But the first rough
Estimates of the numbers are suggestive that the water is simply boiling due to a low
Pressure at the top of the siphon.
Note, I have not read the paper, just going by wot is here.
Ps, questions about cavitation at the top of siphons are a staple
In many fluid mechanics courses for engineering students.
Criminy, folks, don’t make me explain siphonage to some of you.
Reminds me of something I used to ask the other pilots back when I was a contributing member of society. We’d be cruising along and I’d ask what was keeping the passengers in their seats after we turned off the seat belt sign, Gravity, or the Upward Acceleration Generated by the Wings?
They’d inevitably say Gravity, so I’d propose that we had two switches in the cockpit, one that turned off Gravity worldwide (for fuel savings), and the other that Jettisoned the Wings (in case of emergency).
If I jettisoned the wings, they’d go flying off and the rest of us would start falling and everyone would float out of their seats, thus proving it was the upward acceleration of the wings that kept coffee in our cups and our bottoms in the seats.
I’d get a lot of skepticism, but no one could prove me wrong.
“At this point, gravity is pulling water back on each side, atmospheric pressure is pushing water up on each side. This happens during the entire experiment.”!
It the pressure _difference_ that produces a net force that causes movement and does work. It’s like electric potential (voltage) . It’s no good having an earth potential by sticking a rod in the earth, you need a potential difference to make a current flow.
With pressure you need to induce a pressure difference. and the static atm pressure is not going to do this for you. That’s why you need your pump.
It’s the pump which is doing the work to create a pressure difference that will make the water rise and eventually start the syphon.
Yes it is ridiculous. Atm pressure does not make a syphon work any more than the earth wire in your house makes the lights come on.
“Formation of bubbles. Cavitation anyone?”
That is the term use in the paper.
“Atm pressure does not make a syphon work any more than the earth wire in your house makes the lights come on.”
So you’re saying you don’t need a ground/neutral for your house wiring to work…. interesting take.
RyanP “Simple experiment then. Try your same setup in a vacuum and see how much upward force ‘work’ your pump does.”
Well I already said you can’t because the water would boil and there’d be no air to pump out either, by definition, in a vacuum. So you need to define what you mean.
But whatever way you define it , if you want to raise a mass of fluid by a certain height you will have to expend h.g.mass joules of energy to do it and the answer is exactly the same whether you are in the Netherlands or on the top of Everest (using local g) or inside a hypobaric chamber.
It has sod all to do with air pressure.
“So you’re saying you don’t need a ground/neutral for your house wiring to work…. interesting take.”
No I’m saying you need more that one wire. And it does not matter what the ground potential is, it’s the potential difference that makes things happen. That’s why we need a power company.
Like you need a pump ( or a mount or gravity feed ) to make a syphon work. The ambient pressure does not do any work, is not supplying energy and is not making anything happen.
Seems too many here have been reading OED to learn physics.
Enough for me today.
Greg has it almost right. It is the pressure differences caused by gravity that drives a syphon. Saying it is pressure or gravity is misleading, both are needed. The pressure at the top of the U-tube of the syphon is the surrounding gas pressure minus the hydrostatic head (weight per area due to height of the liquid column above the reservoir) of the long side of the syphon. Since this is a lower pressure than the hydrostatic head of the short side, the gas pressure pressure in the short side reservoir pushes the replacement liquid in to replace liquid exiting the long side. This of course ignores other effects such as capillary effects or strong bonding forces, which may allow special exceptions.
I didn’t notice any discussion of a barometer. It is purely atmospheric pressure driven and the liquid would not go up the center tube at all if the barometer were in a vacuum.
http://hydraulicspneumatics.com/site-files/hydraulicspneumatics.com/files/uploads/2012/10/Vacuum%20Figure%201.png
For most practical siphons atmospheric pressure has to be the main operative force and explains why a water based siphon at sea level can only go up 10 meters before it comes back down.
At least for normal water the intermolecular binding power would be much weaker. The video experiment shows a very special liquid with a very strong intermolecular binding strength.
I wonder with that liquid how strong the surface tension is? Ie. how far above the top of a breaker could you go before the liquid overflowed? I’m betting a lot higher than with normal water.
Janice Moore says:
April 25, 2014 at 12:36 am
First, don’t take a physics class from Dr. Hughes. He may understand what’s going on, but he’ll spin it so fast you head will unscrew.
Second, the greenhouse effect doesn’t involve siphons. Well, if I spin it hard enough I could show pressure differentials pushing air against gravity and letting it fall again, but thunderstorms do that every day and I’ve never heard anyone call that a siphon. Dr. Hughes probably would.
As for the issue in the piece you quoted, I don’t have much to quibble with with Lacis’ comments except this one. This goes back to James Hansen’s early days, I think he was one of the original proponents of a runaway greenhouse effect on Venus.
The actual reason the surface of Venus is hot is much simpler and has to do with “adiabatic expansion” which show up any time you climb a mountain or a diesel fuel burns in an engine.
Venus’s atmosphere is so dense, pretty much no IR makes it off the surface, at least not at wavelengths CO2 blocks and clouds reflect. We know some sunlight does make it to the surface, thanks to the Venera spacecraft photos of the surface. That heats up the ground, that heats the air, and the most efficient way to get that heat away from the surface is via convection, think thunderstorms or “fair weather cumulus”. As the air rises, it expand and cools, eventually it gets high enough for IR to escape Venus. When it reaches typical Earth pressure, it has cooled so much that it’s comparable to Earth’s surface temperatures, after taking into account the increase sunlight due to being closer to the Sun and increased reflect due to all those clouds.
There are a couple WUWT pages about this, see http://wattsupwiththat.com/2010/05/06/hyperventilating-on-venus/ for one. (Warning – it’s a Steven Goddard post. those always became flame filled.) Most of my comments there are mild.
http://theendofthemystery.blogspot.com/2010/11/venus-no-greenhouse-effect.html is also good, the author, Harry Dale Huffman, sometimes posts here.
@ur momisugly Greg – If you attach a vacuum pump to the top of a 40ft pipe and suck water up say about 20ft, the air pressure in the top of the tube will be about 1/3 of an atmosphere. Air at any positive pressure above pure vacuum will be trying to expand, thus the air at 1/3 atmosphere will be trying to expand both upward and downward and therefore exerting a downward force on the liquid in the tube. The vacuum pump doesn’t exert any force on the liquid through the air, it only takes away a force that was pushing the air and the liquid down. It’s like lifting a weight off the top of a spring. You’re not exerting an upward force on the spring causing the spring to move up, you’re just removing a downward force. If you were asked whether it was the steel of the spring coil pushing the spring up or is it your arm lifting the weight off the spring that is pulling the spring up, what would you answer? If you remove the cork from a champangne bottle or compressed air bottle, is the stuff inside pushing its own way out or are you pulling stuff out or just providing the energy to remove an impediment?
Would you also agree that in a typical, say 1m tall, practical siphon, at sea level, that all of the molecules are at positive absolute pressure, and therefore they are all being squezed together and repelling each other, and therefore no pull can be transferred over the top of the siphon? Isn’t it true that the liquid or gas at the top of the siphon is pushing down on both sides, just pushing down a little less on the up side?
Are you aware that the siphoning of carbon dioxide gas has been demonstrated? No cohesion necessary there.
Are you aware that small medium or even large bubbles so big that they completely disconnect the liquids on each side of a siphon, can pass through the siphon without hardly slowing it down? Any cohesion to pull is defeated by bubbles. Since those bubbles contain pressurized gas, they would be happy to expand very large if there was any pull at the top of the siphon. But they don’t expand much. They stay squeezed small.
If cohesion is the explanation, how would cohesion take part(other than to prevent boiling) in raising the liquid in the siphon of figure 4 in the Wikipedia siphon article, where it starts out with only air at the top?
A pressure difference is basically two opposing unequal forces. Imagine two children in a supermarket aplying pressure to a shopping cart in equal opposition to each other. Then imagine one child steps out of the way and takes his hands off the cart. Wouldn’t you say that the cart moves because the remaing child is pushing the cart rather than saying that the cart moves because the child that stepped aside with his hands not even touching, is pulling? A vacuum pump doesn’t pull on anything, it just clears out some space so that the remaining molecues have somwhere to push and spread on out to unopposed.
The vacuum doesn’t draw the fluid up the tube. A vacuum is not a force and as such cannot do any work. A vacuum is a lack of force. The weight of the air column above the top container generated by gravity provides the force. As for the experiment, since we cannot generate a complete vacuum the force of gravity will still generate a pressure differential as it draws the fluid down the long tube. The fluid will still flow with a bubble in the tube as long as the flow is fast ,enough to keep the bubble in the down leg of the siphon. The bubble is still a partial vacuum caused by the force of gravity acting on the fluid below the bubble.
“No I’m saying you need more that one wire. And it does not matter what the ground potential is, it’s the potential difference that makes things happen.”
Of course, and that is how gravity works in this case as well, you need to different heights in your reservoirs to get the gravitational potential. So it’s not really gravity that drives siphons then, (gravity and pressure and electricity just do what they do), it’s the person/scientist/force who lifts and holds one reservoir above the other, lol.
This is an interesting quote with regards to AGW.
“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.”
How on earth is the heat hiding in the oceans then ?
Has he inadvertently proved the hypothesis that the heat is hiding in the oceans theory false ?
Do people arguing for atmospheric pressure as the driver of siphons, really think this siphon apparatus wouldn’t work in a vacuum? Imagine this siphon apparatus inside a vacuum tube (at the Earth’s surface). You wouldn’t claim the water in the upper bucket would fly out of the bucket to fill the vacuum because the pushing force of the atmosphere is removed, would you? No, of course not. Because gravity would still be there, pulling the water downward in the bucket. Gravity gives the water in the bucket its internal pressure. The internal pressure within the water itself pushes some of the water up into the siphon tube. I won’t dispute that atmospheric pressure adds its force to the water surface, but that explains only a tiny fraction of the internal water pressure in the bucket. It is not the driving force of the siphon.
I posted this on my e-mail list.
Interesting discovery: especially for a physicist. Nearly every dictionary printed in the past, until very recently, has miss-defined the word siphon.
The action of a siphon was attributed to atmospheric pressure rather than gravity. Of course A.P. does have a secondary effect. If you try to lift water more than 10 meters above the U a vacuum will form and the flow will stop; forming two separate columns, provided the second column is immersed in the second fluid container. Therefore the maximum amount of lift before the U is dependent on the S.G. of the fluid. As A.P. decreases the maximum lift also decreases before a vacuum forms.
The flow rate is controlled by the differential fluid level of the first and second container.
I would assume that at some maximum differential, greater than 10 meters for water, the flow rate would pulse do to intermittent vacuums forming in the flow line, provided the up flow is less than ten meters
A similar phenomenon takes place with cavitations of a propeller in fluid.