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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Wobble,
“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.”
What did your father tell you about siphons that are maintained when the water on both sides of the tube are at the same height. The siphon will be maintained in this situation even though there is no difference in height. In fact this is the whole mystery of the siphon. It seems to defy gravity. It works on the same principle that keeps water in an inverted bottle in your automatic pet waterer. It’s the atmospheric pressure that keeps it up.
The definitive experiment to show that Dr. Hughes’s hydrogen bond or “molecular chain” explanation is false is the siphon fountain. A siphon fountain is designed to maintain an air gap at the top of the siphon. There is no continous chain of water molecules between the two water reservours in that case. Heck you can prove this with a simple tube. Start with two vessels of water at the same height and a six foot long flexible tube raised into a three foot high inverted U shape. Have the tube filled with water except have an air gap at the top. Now lower one side and raise the other till the water starts to siphon. It will siphon even though there was not a continous chain of water from one vessel to the other.
REPLY: Sorry, no. Actually the definitive experiment to show that siphoning works by gravity alone has been done, see this siphon made with bead chain. Gravity and inertia is all it needs, and it wouldn’t work if the beads weren’t connected, just like water molecules are with bonds. – Anthony
Jorge, thanks, I understand that the boiling begins immediately. I was getting at the notion that not all of the water would instantaneously turn to vapor, and enough liquid water would remain in the bucket to move through the siphon tube (if you can hypothetically keep the bubbles out) to prove that atmospheric pressure is not the pressure pushing liquid water up into the tube.
Mike M. accuses the video vacuum experiment of sophistry. How so? Are you saying that what happens in the video is NOT a siphon? The video demonstration works exactly like a siphon in an atmosphere, but without any atmospheric pressure on either side of the hose. That’s what I thought I was arguing about.
The siphon fountain also shows that the chap in the ionic fluid video is wrong. A water siphon is not maintained by cohesion, nor by gravity as he summarized at the end. I also think he is wrong in the case of the ionic fluid. Mere cohesion to itself would not keep it from flowing out of the tube in a vacuum. There would also have to be a strong cohesive force with the wall of the tube, or some other factor he did not control for like high viscosity. It might be that his liquid is more like a gel. I haven’t worked with the stuff so I can’t tell you. I’d like to see him repeat the experiment with a valve at the top of the tube which he could open, or with a tube of a substance to which the ionic fluid is phobic.
If it doesn’t involve the evacuation of a gas from a tube, the replacement of the gas with a liquid, the “raising” of the liquid in the tube to a height greater than the higher reservoir, and then the flowing of the liquid of the liquid into the lower reservoir, then it is not a siphon, by definition. If you disagree, then you and I have a disagreement about the definition of the word “siphon,” not a disagreement about physics.
Why do we assume that the video at the top of this article and the one referred to by Mindbuilder (water “raised” to a height of 24M) are examples of siphons, just because the creators of those videos say so? In both videos the tubes are pre-filled with a liquid *before* the tubes are raised above the height of the higher reservoir. That is not a siphon, as we commonly refer to one in everyday life. Put a mouse in the vacuum chamber (with an O2 tank, of course) shown in the video and have the mouse try to “suck” the liquid from the higher reservoir to the lower reservoir through the *evacuated* tube that rises above the higher reservoir then down into the lower reservoir. Will the mouse be able to siphon (as classically defined) the liquid to the lower reservoir?
I prefer Merriam-Webster’s definition of a siphon: “a tube bent to form two legs of unequal length by which a liquid can be transferred to a lower level over an intermediate elevation by the pressure of the atmosphere in forcing the liquid up the shorter branch of the tube immersed in it while the excess of weight of the liquid in the longer branch when once filled causes a continuous flow.”
If we accept the new definition in the Oxford English Dictionary, bedazzled upon its editors by a respected PhD, then the water system at my home could be termed a “siphon.” I do not accept such a *change* in definition.
This post sucks.
😉
Mickey Reno, “Mike M. accuses the video vacuum experiment of sophistry. How so? Are you saying that what happens in the video is NOT a siphon? The video demonstration works exactly like a siphon in an atmosphere, but without any atmospheric pressure on either side of the hose.”
We just don’t have enough information. The guy in the video did not understand the proper explanation for how a siphon works in the first place, so he did not eliminate all the possible differences about an ionic fluid that would not make it a proper model of a siphon. Capillary wicking can be used to create something similar to a siphon which is not actually a siphon. How do we know this ionic fluid doesn’t strongly attract to the sides of the tube he used.
Try wetting a dishtowel in a almost full bucket of water, then hang it over the side of the bucket so that it is in the water but also lower than the water level. It will wick all the water out of the bucket down to the whichever end of the rag is high. The water drips off the end of the rag. There is no tube involved, and it is not a siphon although it acts like one.
Seems like the ionic fluid does in the experiment may not be acting like a siphon or a wick. We can’t tell because he had no ability to try other things. Like what if he were to open a hole in the top of the tube? A small diameter hole might not cause the siphon to break as it would with a normal siphon. A large diameter hole might cause the attraction to the side of the vessel to collapse. I’d also like to see an ionic liquid in a inverted jar under vacuum.
I’d like to see this experiment instead: Have a vacuum chamber. Inside a deep pool of ionic liquid and above that an inverted jar. The inverted jar is lowered till it is half submerged. Since there is no gas inside it should fill half way up with liquid so that it is level with the outside liquid. Now raise it. The liquid should not be pulled upwards. Now lower the jar all the way down into the liquid until it is completely full. Since the liquid is not touching the inside bottom of the jar there are one of two options when you pull it out. a) It has no attraction to the jar surface and just falls out. b) it is strongly attracted to the jar and as you pull it out the jar remains full. Of course even in case b) there will be a limit to how high you can pull (and how long the jar is) before the weight of the liquid overcomes the attraction of those molecules clinging to bottom of the jar from inside.
Mindbuilder says:
April 25, 2014 at 2:03 pm
Umm, can you explain that a bit more clearly? Exactly what do you mean by negative pressure? I can understand relative negative pressure up to -1 atm, but something several times that, e.g. -5 atm seems as problematic as reaching Kelvin temperatures that are several times ambient, e.g. -1000K. That would either be really cold or negatively hot.
Back when I was in high school, our biology teacher talked about root pressure and transpiration pull for getting water high into trees. I also considered that redwoods grow in foggy environments a rather important point….
Anthony, Did you even bother to look up what a siphon fountain is. Even with an air gap in the bottle the water will remain very high in the tubes on both sides. As I said you can have a plain old straight tube with a air gap at the top and the water will remain very high in the tube. As high as you want it to. Try doing that with your bead chain. Cut it at the tapex where it hangs over the side of the cup. It will NOT remain suspended vertically. Both sides will fall down.
REPLY: Yes I did, and that’s the point, if you cut either the water connectivity or the bead chain it won’t work. Connectivity, be it water or chain has everything to do with it (maintaining momentum), air pressure has zero role.
If you break connectivity of the water in the hose at the top, no siphon.
– Anthony
The bead chain rises above the edge only because of the fact it has a minimum bend radius. I played with those things as a kid. If you think this is at all analogous then how come when I spill water over the side of a glass it doesn’t fly up in the air like this draining the entire cup. This has just about zero in common with a siphon.
With your bead chain example. Did you even bother to look up what a siphon fountain is. Even with an air gap in the bottle the water will remain very high in the tubes on both sides. As I said you can have a plain old straight tube with a air gap at the top and the water will remain very high in the tube. As high as you want it to. Try doing that with your bead chain. Cut it at the tapex where it hangs over the side of the cup. It will NOT remain suspended vertically. Both sides will fall down.
You can in fact start a siphon where there is no water whatsoever in the tube which is above the water level. Start with two deep 5 gallons buckets of water on a table next to each other. Place a long tube from one to the other, and make it rise say four inches above both. Have a t joint at the top with a valve so you can let out all the air that is below the water surface on both sides. Now lower one of the buckets over the side of the table to the floor. The water will start flowing up out of the top bucket even though there was no connection between the water in the buckets.
REPLY: Sure, but again, it is about connectivity of the fluid. The siphon won’t start unless the water is connected. Your valve trick just connects the water so it is a single column, and when it is a single column, then it can act as a siphon. – Anthony
Hey, it looks like the commenters didn’t read the Wiki page all the way to the bottom where it says:
@ur momisugly Ric Werme
“settled science” then. LOL!
“Your valve trick just connects the water so it is a single column”
No it doesn’t. The tube is four inches above the bucket and that is all air. Eight full inches of air in the tube with no connection. You then close the valve and lower the one bucket, leaving the apex of the tube four inches above the top bucket and the siphon will still start. I’ve worked with siphons quite a bit. I know way more about them then any of these scientists. This nature article needed some serious peer review by someone who understood the subject.
REPLY: OK, thank you for the clarification. Your previous explanation sans diagram wasn’t clear to me. In that case air is connecting the two water columns in the tube, and because the air is resisting stretching/decompression it acts to connect the two as a fluid. Atmospheric air pressure, as a force of action, has absolutely nothing to do with that. It’s all still gravity, inertia and connectivity that is moving the fluid. Any fluid between the two water columns would act the same.
You can believe whatever you want, but I’m perfectly fine with the explanation given in the article. – Anthony
I just build the apparatus I described except instead of having a valve I just put the air bubble in manually. If I start with both buckets on my deck and raise one up to the railing the water rises in the tube of the bucket I’m lifting in what looks like a contrary to gravity fashion. The siphon starts with water pouring over the apex in this “waterfall effect” that he describes. There is nothing mysterious about it. It happens at 1 atmosphere if there is already air in the tube. Plus it should have been a gigantic hint that the water wasn’t being pulled by some chain effect. It’s dark out and I have plans to go to a gardening show tomorrow morning, but I’ll post a video of it if I get a chance.
BTW, my position is that Oxford English Dictionary is wrong, that Dr Stephen Hughes is wrong, and the summary by Brady Haran in his video is wrong.
I remember an experiment I did in fluid dynamics class. It was a syphon and we measured the flow rate as the long part of the outflow arm was increased in length. Conclusion; the flow output increased as the length of the outflow pipe increased up to a certain length then did not increase any further even as the length of the output arm increased and I think even decreased due to the friction increase in the extra pipe. would a syphon work in zero gravity of the space station.
@ur momisugly Paul
Question
Kurt Larson asked:
Would a siphon work in space?
Answer
No it wouldn’t, because you need gravity to make that siphon work. If you imagine the fluid at different points along the length of that siphon, at some points the siphon will be going uphill, and at other points it will be going downhill, and there’ll be gravitational force on the fluid inside your siphon. And where the siphon is going uphill, the force is pulling the fluid backwards along the pipe. When it’s going downhill, that force is pulling the fluid forwards along the pipe and what you need is more bits of length of your siphon going downhill with those forward forces than you have going uphill. And that means that because of the water pressure inside that siphon, the places where the force is going downhill is dragging fluid along the whole siphon from the beginning to the end. On the International space station, you have no gravitational force so there’d be nothing to drive the fluid along the siphon.
http://www.thenakedscientists.com/HTML/questions/question/3177/
@ur momisugly Rick Werme – Pressure is a push, negative pressure is a pull. The ability to resist negative pressure is like tensile strength. The purpose of a rope is to resist negative pressure. Water can resist negative pressures about as well as a rope made of rubber. The ability of liquids to resist negative pressures is not quite like tensile strength though because liquids need tube walls to adhere to or else they will not exhibit any tensile strength. A rope needs no tubes.
@ur momisugly Anthony – An air bubble trapped at the top of a siphon with no water from each side touching, does not “connect” the two sides of the siphon. Said air is pressurized(though less pressure than atmospheric) and wants to expand and therefore is pushing DOWN on the water on BOTH sides. The air at top cannot pull the water up. Gasses cannot pull. The water goes up for the same reason the liquid goes up in a drinking straw – because atmospheric pressure pushes it up when the pressure at top is lowered (by gravity in the siphon, and by your lungs in a drinking straw).
REPLY: Your description and my visualization of the description don’t match. Somebody should draw a diagram. But it doesn’t matter, a siphon can’t work without gravity. A drinking straw is NOT the same setup, and is not self sustaining like a siphon. – Anthony
Take a look at the diagram and description of figure 4 on the Wikipedia siphon page. And yes, you’re right that gravity is the energy source that lowers the pressure at top to enable atmospheric pressure to take over. It’s gravity AND atmospheric pressure that makes a typical siphon work.
Aren’t there materials that allow air to pass through but not water. If so, then another experiment could be done that would minimize the influence of atmospheric pressure by making a siphon tube out of such a material. If air can pass freely through the walls of the tube, a vacuum would not be able to form anywhere in the tube. It would then be interesting to see if the siphon still worked using gravity and molecular cohesion alone. My suspicion is that it might work for small distances but not when the bend in the tube is several feet higher than the source.
The idea that it is gravity rather than atmospheric pressure that makes a siphon work seems a bit of a red herring to me. Isn’t it gravity that creates atmospheric pressure in the first place, at least in normal earthly circumstances? Without gravity we would have no atmosphere. So atmospheric pressure in an open system is just another manifestation of the force of gravity.
It seems to me that without gravity you have no atmospheric pressure. Sure you could have a pump that creates artificial pressure, but that isn’t a real world condition, so it seems to me that the people who say you have to have gravity and atmospheric pressure are correct.
Gads you would think this would be settled science.
Mickey Reno says: April 25, 2014 at 6:14 pm Mike M. accuses the video vacuum experiment of sophistry. How so? Are you saying that what happens in the video is NOT a siphon? The video demonstration works exactly like a siphon in an atmosphere, but without any atmospheric pressure on either side of the hose. That’s what I thought I was arguing about.
*********************
The fluid in the video is transferring like a chain over a pulley and that is not how a siphon works. Ordinary water in an ordinary garden hose does NOT have the cohesive property to be “pulled” through the hose. When you suck on a soda straw it is NOT being pulled up the straw it is being PUSHED up the soda straw by the pressure differential you created by lowering the pressure at the top.
Now take two soda straws, one twice as long as the other, each with the bottom end slightly immersed in water and the tops of the straws level. Suck the water to the top of each one and seal it. Atmospheric pressure is pushing the water up both of them. The pressure at the top of either straw must be less than atmospheric but, additionally, the pressure at the top of the longer straw will be LESS than the pressure at the top of the shorter straw by virtue of the difference of their respective water column heights.
Because the pressure is lower at the top of the longer one than the top of the shorter one, if you then connect their tops together the water will flow from the higher pressure to the lower pressure creating a siphon.
Jim says:
April 24, 2014 at 10:50 pm
“Diagrams Showing what happens here would be useful.”
Agreed. The paucity of pictures made the article difficult for me to follow. However it is intuitively obvious that Dr Hughs’ claim is correct for well-behaved liquids.
The hypobaric stuff appears to be overkill. On the other hand, a good way to put a false scientific meme to rest is with an experiment.
Feynman notwithstanding . . . Better still if the experiment involves expensive bells and whistles. /sarc
The paper states “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.” In fact the siphon had completely stopped. Elsewhere in the paper they note that they had to switch-off the return pump or it would have run dry and burned-out. This indicates that atmospheric pressure IS requisite for the operation of a siphon.
They also state, “flow remained more or less constant during ascension indicating that siphon flow is independent of ambient barometric pressure,” (I love the words ‘more-or-less’!) This key statement should have been substantiated with a graph of Flow vs Ambient Barometric Pressure. But this was not done. Why not? The only graph in the paper is Flow versus Time. There is no graph of Pressure vs Time.
In other gems they state “In the second run, the ascent was slowed to allow outgassed CO2 to dissipate.” What the …? Were they using soda water? Did they think to take samples of the outgassed susbstance?
This paper is a travesty! And the journal ‘Nature’ published it – amazing! They have already got a poor reputation and this won’t help. It will be interesting to see what response it gets when serious physicists read it.
Why all this talk about connectivity? I reiterate that getting the liquid to the crest of the tube is the key requirement in starting the flow from the higher container to the lower. About the only connectivity that doesn’t work is vacuum, and then only if you ignore atypical effects like capillary tubes and ionic fluid.