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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Greg, maybe this will help.
Imagine you get a siphon going between two buckets, then you raise the lower bucket until the water levels are at equal elevation and the flow stops. Even though the flow has stopped, obviously water remains in the tube, stationary above both buckets.
The water in the tube is at a higher level than the water in either bucket, and there are 2 open paths through the tube for it to flow down. What’s keeping it from doing that — flowing back down, half of into one bucket, half of it into the other? Gravity is still operating, so there must be some force stronger than gravity keeping the water up there.
Of course it’s because in order to flow down, the water would have to “break into two columns” and pull a vacuum in the tube, and yes, that potential negative pressure is stronger than gravity.
So a siphon and a drinking straw do work on very similar principles. In a straw the water flows up because your mouth creates a partial vacuum. In a siphon the water flows up because of the vacuum that would be formed if it didn’t.
But keep in mind, really the mechanism keeping the vacuum bubble from forming at the top of the tube is the atmospheric pressure on the water in both buckets. If you were to put both buckets in a vacuum, there would be no problem for the water in the tube to form a vacuum as well, and then it could yield to gravity and flow down both paths.
Because we feel atmospheric pressure all the time, sometimes we don’t realize how strong the air around us actually is. For example, in an atmosphere, people could theoretically use suction cups to climb a wall, but a similar idea couldn’t be used to help astronauts stay attached to the outside of a space station. The force that sicks suction cups to things is atmospheric pressure. It’s also atmospheric pressure that sticks the water together at the top of the tube in a siphon.
Ultimately, it’s gravity that makes water flow from one bucket to another, but air pressure is absolutely required as well. If it’s absolutely required, I think it has a bit more than “sod all” to do with it.
Both air pressure and gravity are required for a siphon to work. A siphon will not work in zero gravity. A siphon will not work with zero air pressure.
The “lift” section requires air pressure. The “fall” section must be longer than the “lift” section so that gravity provides the net energy to maintain the flow.
The higher the “lift” the more air pressure required. At sea level a siphon can theoretically lift water 10 meters, so long as the “fall” section is more than 10 meters. The size of the opening at the bottom end of the siphon must be reduced as the “lift” approaches the height of the “fall”, otherwise the water column in the fall may separate due to air entering via the bottom of the fall.
Nancy C says:
April 25, 2014 at 6:52 am
Ultimately, it’s gravity that makes water flow from one bucket to another, but air pressure is absolutely required as well. If it’s absolutely required, I think it has a bit more than “sod all” to do with it.
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correct.
Mickey Reno says:
April 25, 2014 at 6:49 am
Do people arguing for atmospheric pressure as the driver of siphons, really think this siphon apparatus wouldn’t work in a vacuum?
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a siphon will not work in a vacuum. the “lift” section cannot operate without air pressure.
You can however run a hose between two buckets in a vacuum and drain one into the other, so long as you attach the hose to the bottom of the buckets (and drill holes, etc). However you cannot siphon water (which involves lifting it over the edge of the bucket) in a vacuum.
of course the talk about siphoning water in a vacuum is hypothetical. In a vacuum water boils at room temperature, so your siphon quickly fills with steam, not water. If you sealed the containers and the siphon, in theory this steam would provide the pressure required to drive the siphon. Allowing you to perhaps build a siphon that could operate in a vacuum, so long as the apparatus itself was not open to the vacuum.
@ur momisugly Nancy C “air pressure is absolutely required as well” I agree if you are only talking about water, but the (virtually) equal air pressure at each end isn’t providing energy for the flow, it is only pushing equally on both ends to keep the water ‘joined together’.
@ur momisugly Fredberple “A siphon will not work with zero air pressure” ?
Have you seen the very clear experiment at http://www.youtube.com/watch?v=8F4i9M3y0ew which I have referred to several times? How do you explain it?
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.
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.
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about what one would expect. 1.5 meters is 15% of the 10 meters air pressure will raise water at sea level. it makes sense that the column would split at about 15% of sea level air pressure.
If you heat a bucket of water with a heat gun….
;-}
One last thought. This argument started because of a definition. What differentiates a siphon from any regular tube going from a high to low reservoir where water flows due to gravity (see, we don’t even have a special name for this) is the part where the water flows up due to pressure differences before it goes down the other side. The definition focuses on what is special in this case rather than the fact that gravity causes water to flow downhill (as it does in the longer leg of the tube)
Réaumur says:
April 25, 2014 at 7:21 am
Have you seen the very clear experiment at http://www.youtube.com/watch?v=8F4i9M3y0ew which I have referred to several times? How do you explain it?
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the liquid is not water. you can build a device that will siphon in a vacuum so long as the molecular bonds of the liquid are stronger than the lift height. This would require a viscous liquid that does not boil in a vacuum.
So the science is not settled then?
Lots of long descriptions.
I’d shorten it all to…
The gravity driven falling water in the lower tube creates a vacuum in the upper tube sucking water into the other higher end producing a steady stream of falling water, vacuum & replenishment.
The real mystery is gravity itself.
http://chapelboro.com/columns/common-science/gravity-still-a-mystery/
“We are so accustomed to the effects of gravity that we fail to remember that although we can predict its effects, we don’t really know how it works. Really, we don’t.”
Compare this to alarmists pretending AGW scientists know how the climate works. I’ve heard some even suggest that denying AGW is like denying gravity?
It’s funny that such a comparison would be made because scientists do not really know how our climate or gravity works. Yet alarmists pretend that 97% of scientists know how out climate works?
Yeah sure, just like gravity?
I recently watched the series Wonders of the Universe S1 E3 Falling
It explored gravity from here through the universe.
I wouldn’t give these guy many marks at all for their work. Not impressed. They only experimented with a 1.5 m siphon. To be meaningful they should have experimented with a full range of siphon heights up to 10m.
Réaumur says:
I think you’re right that I shouldn’t have said “absolutely” required. But the video doesn’t demonstrate that a siphon “does” work in vacuum, it demonstrates that a siphon “can” work in a vacuum in very special circumstances.
Also, the youtube experiment would fail if they used a long enough, steep enough tube connecting the two containers. Eventually the weight of the liquid in the tube would overcome the ionic attraction between the molecules in the liquid, the liquid would break, and it flow down both sides of the tube defeating the siphon. So maybe atmospheric pressure isn’t absolutely required, but *some* second force beside gravity (and stronger than gravity) is absolutely required.
You could say that the difference in water pressure (not air pressure) causes the siphon. Gravity creates the pressure difference.
Yes gravity drives it but no, you still need atmospheric pressure to push the fluid up the inlet!
Consider that the elevation of a water siphon’s height above the inlet water surface must be less than ~32 feet, (@sea level). (A waste of hypobaric chamber time when all he needed was 75 foot garden hose and pulley mounted to some high rafter to get the same result.)
Réaumur says:
April 25, 2014 at 7:21 am
I know Fred has already answered this. I don’t think you understand what an ionic fluid is. That’s okay, I never heard of it either until that video. How do you explain what an ionic fluid is? How do you explain the decision to use an ionic fluid in the YouTube video? Are you aware that fluid does not behave at all like water? I strongly suggest we point out second order effects in future comments here and focus on the primary effects of fluids like water, tubing big enough to make surface tension and capillary effects insignificant, and concentrate on just air pressure and gravity.
> “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.
I take back any positive comments I may have uttered above or thought to myself. I hereby claim Dr. Hughes is the second coming Bill Nye.
Fred, perhaps I should not have used water as my example. Water will tend to form bubbles in a vacuum, and that could interfere with the pressure equalization within a siphon tube. You can siphon in a vacuum, as long as the fluid involved doesn’t form bubbles that break the siphon.
The point being, that air pressure is irrelevant as a force in the operation of a siphon.
By the way, for those people who suck on the siphon tube to siphon gasoline, that’s really dumb. To safely siphon gasoline, put the siphon tube into the gas tank. Blow into the tube and listen for bubbles, to make sure your tube is under the fluid surface. After you hear bubbles, use a rag to create an airtight seal around the siphon tube at fill tube (gas cap). Now BLOW! All you need is enough pressurized air in the gas tank to push enough gasoline into the tube to fill it. In this way, there’s no inhalation of gas fumes, no gasoline ever has a chance to enter your mouth.
Well the lower bucket is superfluous, unless you want to collect the water. To put it another way, the low end of the tube, only needs to be lower than the water surface in the upper bucket; it does not need to be under water in another bucket.
But try making the loop height higher than 34 feet above the upper water surface, and then ‘splain me how atmospheric pressure is not involved !!
Having siphoned plenty of gasoline out of my boat sitting on its trailer (emptying it for winterizing), I can attest to the fact, that the output end can be open to the air. It is better if it’s not, so that any flow interruption doesn’t let air in the tube.
I’m thinking OED and the physicist blew it again.
I don’t believe there can be many engineers that didn’t know a siphon works by gravity – the long column pulling water up the short column. To stop a siphon but maintain the tube filled, one only has to raise the lower bucket up to level with the upper. This in itself cries out gravity. Surely they weren’t thinking that the upper liquid surface, being higher had a different atmospheric pressure – it does, but the upper level has (infinitesimally) LOWER atmospheric pressure, and the siphon would then work the other way! Perpetual motion machine anyone? Probably the mistake was perpetuated by the fact of the prestige enjoyed by Pascal if it were he who did indeed (shame, shame) state this.
Despite correcting this, Dr. Hughes shouldn’t come off totally unscathed in all this! He didn’t seem to understand that although having nothing to do with air pressure per se, a very low pressure causes ‘boiling’ of water and this will break the siphon in the tube when the bubbles become numerous enough.
Have the people thinking the atmospheric pressure will drive the liquid above the short end of the tube ever used a siphon? If that was the case, one would only need to put the empty tube in the liquid and the liquid would spill automagically over. The air pressure pushing the liquid is also pushing inside the tube, annihilating the force. The 10 m water column of a barometer happens because the atmospheric pressure in the tube doesn’t exist (vacuum in the column).
Having never read the definition of a siphon, I was never confused by the reference to atmospheric pressure. It is pretty obvious that a siphon would not work in the space station with atmospheric pressure but not gravity, but would work on the moon (in a closed system to prevent the liquid from vaporizing) with gravity but now atmospheric pressure.
This is a bit ridiculous. Water flows are well explained by the Bernoulli equation: p/w+V^2/2g+z. By inspection, it is a simple matter to determine that pressure, fluid density, and gravity all play a role. To the extent what forces are more dominant is determined by the situation (boundary conditions). In a classic siphon, gravity is indeed the dominant force. However, it is NOT the only force.
Janice Moore says:
April 25, 2014 at 12:36 am
“Because of the small atmospheric pressure on Mars (less than one hundredth that on Earth), the spectral absorption lines of carbon dioxide on Mars are very narrow, and therefore act like a picket fence that lets most of the thermal radiation emitted by the Martian ground surface to escape directly out to space. This does not happen on Earth because of the atmospheric pressure that is exerted by the radiatively inactive nitrogen and oxygen, causing the spectral absorption lines of carbon dioxide and water vapor to be greatly broadened, making them more effective absorbers of thermal radiation. ”
Ah. Recently somebody wanted to tell me that H2O and CO2 do not cannibalize each other’s IR photons because their lines are not at the exact same places. So that guy was wrong.