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.”
@ur momisugly Greg – After the siphon broke and he raised the atmospheric pressure and the water started going back up to the top, from where was the force comming from to push the water up. The water vapor at the top would have been pushing down a little. Gravity would have been pulling down. Wasn’t the water being pushed up by atmospheric pressure, same as in a barometer and a drinking straw?
Greg says: April 25, 2014 at 1:42 am
No, some atmospheric pressure is needed to prevent the water evaporating. Nothing to do with the syphon. Same would apply to mercury (except that the vapour is highly toxic)
Exactly. See my previous post (link to YouTube video) above.
@ur momisugly Reaumur – Siphons in vacuum do rely on liquid cohesion. But what about a siphon at sea level with the taller discharge side filled with liquid, but the top and entire short up side empty? If the liquid is allowed to fall from the discharge tube, and the up tube isn’t too tall, liquid will rise out of the upper reservoir despite there being no liquid cohesion between the sides of the siphon. Doesn’t this demonstrate that siphons at sea level use atmospheric pressure to push the liquid up, as in a barometer or drinking straw, unlike siphons in vacuum?
Actually the atmospheric pressure at the lower bucket is higher than that acting on the upper one so if air pressure was the driving force then the water would flow from lower bucket to the upper one. It flows from upper to lower so gravity must be the defining agent acting.
If atmospheric pressure is not pushing the liquid, then explain the operation of the common drinking straw.
@ur momisugly johnmarshall – It’s not one explanation or the other, it’s both. Gravity supplies the energy to lower the pressure at the top and atmospheric pressure pushes the liquid up into the low pressure zone, as in a drinking straw. The almost identical atmospheric pressure at the entrance and exit don’t get a chance to cancel because of the greater weight of liquid above the exit, defeating more of the atmospheric pressure at the exit than at the entrance.
John S. says: If atmospheric pressure is not pushing the liquid, then explain the operation of the common drinking straw.
Where the ‘if’ ? A drinking straw is not a syphon. Incredible.
@Greg – A drinking straw is not a siphon but they share a major mechanism of operation. Understanding the drinking straw helps to understand the siphon. Do you agree that atmospheric pressure pushes the liquid up a drinking straw?
Mindbuilder: “What pushes the liquid up in a barometer? What pushes the liquid up in a drinking straw? In none of these cases can liquid cohesion explain the rise of the liquid. ”
OMG, another one.
OK in a drinking straw, the tongue applies a force causing a depression. The tongue exerts this force while moving a distance. Energy = force x distance . The muscles of the tongue convert chemical energy into kinetic energy which displaces the fluid against gravity thus increases the gravitational potential energy to the liquid being drawn into the mouth.
All this is relative to the background ambient atmospheric pressure, but it is not the atmosphere that provides the energy thus it is not the cause of the movement.
A syphon is just a kind of gravitational flow, it just passes through a tube. It is not air pressure that causes water to flow downhill, whether in a tube or not.
Consider an inverse syphon, like an irrigation canal that passes under a road. It is not the atmospheric pressure sucking it up the other side, it’s simply gravitational flow.
It is my understanding that both gravity and atmospheric pressure are needed to create a siphon. The fluid in the long tube creates a partial vacuum at the bend it the top of the tube from gravity trying to pull the fluid down. then the higher pressure on the fluid surface will push the fluid up the short tube to cancel out the partial vacuum. That seems pretty straight forward. What I find more interesting is how a rope siphon works.
@ur momisugly Greg – It’s true that your body provides the energy to lift the liquid in a drinking straw and it’s true that gravity provides the energy to lower the pressure at the top of a siphon. But it is also true that the atmospheric pressure pushes the liquid up. It is not a choice between atmospheric pressure or gravity. BOTH work together to make it happen.
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?
I sometimes wonder if these ‘contrarian physics’ posts are put up as some sort of test of WUWT readers. Who will applaud reflexively? Who will declare it all a plot by the physics establishment?
Water motion is dictated by pressure, and the pressure of a body of water under gravity is atmospheric at the surface and increases linearly with depth in proportion to depth, as it is the weight of the material above an area pushing down on it. This means water surfaces at different levels is at different pressures, and if connected by a channel full of water at positive pressure the pressure difference will drive water from the higher to the lower to even out the level.
This works even if the channel is a narrow tube, and even if the water rises above the surface of both exposed surfaces, so long as the pressure remains positive – in fact, it must remain above the vapour pressure of the water at it’s current temperature, or the water will boil and produce bubbles of vapour, or if things move fast enough even vacuum. The relevance of air pressure to the siphon is that it is only due to air pressure that the pressure can remain positive when above both the exposed surfaces. The pressure in the siphon is above zero but below atmospheric.
In vacuum, the pressure at the surface of a liquid would be zero, increasing linearly with depth. So long as the connecting tube stays below the uppermost of the two surfaces, it will have positive pressure extending from this surface and the ‘siphon’ will work. As soon as the tube rises above the surface, the fluid in the tube breaks. (There are complexities associated with vapour pressure and surface tension here that I am ignoring.) Water flows from the higher level in the tube to the lower level in the two exposed reservoirs.
So “air pressure” is *not* the answer to the question: “What drives water from one bucket to the other?” So long as the tube remains below the level of the water in both buckets, people find that intuitively obvious: the weight of water at a higher level pushes it through the channel. It is instead the answer to the question about the thing people really find surprising about siphons: “What force lifts water into the tube above the surface level in either bucket?”
The question “How do siphons work?” is vague and ambiguous – at the least it requires more than simple one or two word answers like “gravity” or “atmospheric pressure”. The classic answer is attempting to explain the most surprising feature of it, and the likely reason somebody has asked the question. But what people find most surprising depends on how people think things work in the first place.
@Greg From the article:
“If you think of a car, atmospheric pressure is like the wheels, it enables it to work. But gravity is the engine,” he said.
Agreed.
“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.”
Disagreed. Just as in a straw, it is atmospheric pressure moving water up the shorter arm, but it is gravity providing the “sucking” by falling down the longer downward arm. The water in the downward arm isn’t “pulling” it is “sucking” just as the mouth “sucks” on a straw.
Nullius in Verba says: April 25, 2014 at 3:29 am
As soon as the tube rises above the surface, the fluid in the tube breaks.
Do I really need to invoke the great god Feynman? As has often been quoted, he sayeth “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”
Please just watch the short video I linked to above – it shows a liquid siphoning in vacuum without the fluid ‘breaking’.
@ur momisugly Reaumur – I agree that siphons in vacuum rely on liquid cohesion. Would you agree that the siphon in figure 4 of the Wikipedia siphon article relys on atmospheric pressure to push the liquid up, at least while it is starting?
@Reaumur Ah, the chain over the pully analogy of how a siphon works. Is the chain over the pully a “siphon?” I think not. Is the very carefully set up circumstances of “siphons in a vacuum” really a siphon? I think not.
Look again at your own video. At 1:17, as his initial demonstration depletes it’s source of water, behold the movement of liquid with bubbles in it. No direct contact of water from end to end, yet the bubbles and fluid continue to move until the lower column no longer maintains an airtight seal around the circumference of the tube.
Two similar looking setups, two completely different mechanisms.
“Please just watch the short video I linked to above – it shows a liquid siphoning in vacuum without the fluid ‘breaking’.”
Yes, it uses a special sort of liquid that is able to sustain negative pressure (i.e. can be put under tension). Most liquids aren’t like that, and for liquids that aren’t, atmospheric pressure *is* the explanation.
Feynman is absolutely right, the problem is usually in figuring out what the theory predicts the experiment should show. If you come up with a theory but then apply it wrongly, you can wind up rejecting the theory when you ought to be rejecting your own misapplication of it.
My theory predicts that fluids that cannot sustain negative pressures would break, but that hypothetical fluids able to sustain a negative pressure would still work. (As you’ll note, I did mention above that I was skipping over complexities due to surface tension.) So I’m still quite happy with it, thanks.
“Yes, it uses a special sort of liquid that is able to sustain negative pressure ”
There’s not such thing as negative pressure. Because a pressure is less than some arbitrary std like sea level pressure at 20 deg C does not make it “negative”. It is still finite and positive.
Funny indeed. This is basic Hydrostatics or Fluid Statics.
Surprisingly, water can, under some circumstances, sustain incredibly high negative pressures without vaporizing. See the very clever spinning Z-tube on Wikipedia for how this is measured. Negative pressures up to 280 atmospheres have been measured. Thus water can be comparable with rubber in tensile strength. There is a video on Youtube of a siphon operating up to 24 meters. Even more surprising is that the 24 meter siphon used nylon berage tubing, to which water doesn’t even adhere very well. If the siphon had used glass tubing it may have operated up to 2800 meters or more. To achieve such negative pressures, the water must be degassed and the tubing must be smooth and clean. Normal siphons of water at sea level break at 10 meters because the gas disolved in them promotes vaporization.
I was happy to leave my last comment to stand, as I just wanted people to see the experimental evidence – and the discussion seemed to have the makings of a flame war! – however, I have been asked two questions:
@ur momisugly Mindbuilder: I haven’t seen the experiment in the Wikipedia diagram, but if it works as shown, then in that special case the descending water presumably causes a pressure drop in the air gap and allows atmospheric pressure on the top reservoir to force the liquid up. It is a different case and doesn’t affect my argument in any way. All I’m saying is that experiment shows that atmospheric pressure is not necessary to make the fluid move in a siphon, only a potential energy difference due to the different elevations of the two reservoirs.
@ur momisuglyJohn S: If a “siphon in a vacuum” isn’t a siphon, then we are descending towards a semantic argument about the definition of “siphon”. If you discount the earlier mistaken Oxford Dictionary entry, most say something like “A bent pipe or tube with one end lower than the other, in which hydrostatic pressure exerted due to the force of gravity moves liquid from one reservoir to another.”
“Look again at your own video” – it isn’t my video, I just linked to it!
Again the behaviour of the liquid in air at 1:17 doesn’t affect my argument that experiment shows that atmospheric pressure is not necessary to make the fluid move in a siphon.
A common real world example of very high negative pressures in water is in the xylem of trees where pressures reach negative nine or ten atmospheres relative to pure vacuum.
There is a typo in my post above that should have read beverage tubing instead of berage tubing.
@ur momisugly Réaumur – I agree that atomspheric pressure isn’t necessary for all siphons. However I would go farther and say that the liquid cohesion demonstrated in the vacuum siphon is not only not necessary, but that practical siphons under normal atmospheric conditions that are not too tall, always have atmospheric pressure pushing the liquid up, because all the fluids in the siphon are under positive pressure and therefore the molecules are all repelling each other and therefore they are not pulling over the top. Of course it is gravity lowering the pressure at the top.
“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.”
Surface tension? Capillary action? Pulling water? Pushing a rope?
BS meter is pegged on this one.