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.”
Yes I did follow your original argument and I disagree that atmosphere plays a significant role in the siphon once it is running. (Obviously initiating the filling of a siphon by sucking one end does require barometric pressure).
The criticism of the chain analogy is false however, as I tried to explain earlier. Putting aside the beads (which in fact if you had followed the explanation video and paper does not rely on centrifugal force alone) the fat leg siphon does not lead to the collapse of the chain model, as you suggest. Rather than taking the case of a 9m long 2 m diameter tube which is an extreme example, try considering a more plausible case.
Imagine a 10 cm high siphon in a vacuum using the ionic liquid as the fluid. In this case the tensile strength is not at issue as it will not break. Now construct this siphon with one fat leg, say twice the diameter of the other e.g. one 10mm diameter and one 5mm diameter. This fat leg siphon should still work as a siphon, i.e. the liquid will still flow without breaking as it is easily with the tensile strength of the liquid to support this difference. However, as I’m sure you will agree, one side is heavier than the other.
Now set the reservoirs of this siphon at the same height. In this case there is no atmosphere and the tension in the liquid is able to support the difference in weight. By YOUR analogy the liquid in the fat leg will pull down on the other thin leg thus causing the liquid to flow from the thin side to the fat, creating an imbalance in the level in the reservoirs.
My contention is that that cannot happen. If it were the case then putting this siphon in the same reservoir would lead to liquid flowing up the thin leg and down the fat. (This is the same as connecting the reservoirs via a second direct tube). If the fat leg worked as YOU suggest there would be a continual flow of liquid around the tube i.e. a physical impossibility.
@ur momisugly Adrian
Re your comment: “…….and I disagree that atmosphere plays a significant role in the siphon once it is running”, I never ever claimed this so you must be disagreeing with others.
I viewed the chain bead video but never followed the explanation further because it is not a siphon, does utilise liquid and does not contain a tube. I didn’t claim it works by centrifugal force alone, just suggested it might have been one of the issues involved.
The chain analogy is put forward by others, not by me. With the chain analogy, it is others who are claiming the heavier weight of the down side is pulling the up leg over the siphon crest. The fat leg siphon rules out the heavier weight winning out on a siphon. This means that the chain analogy is not a good one to use.
Re: “Rather than taking the case of a 9m long 2 m diameter tube which is an extreme example, try considering a more plausible case.
The 9m diameter tube is realistic, it can be built. You can buy 2m diameter pipes. There is no need to change to something more plausible.
Re: “By YOUR analogy the liquid in the fat leg will pull down on the other thin leg thus causing the liquid to flow from the thin side to the fat, creating an imbalance in the level in the reservoirs.
I am not claiming the fat leg will pull down the other leg of a siphon. And it is not MY analogy. If one built a chain with a heavier short side compared to the longer side, the shorter side would win out. But in a siphon, the diameter and weight are not relevant to the working of a siphon. And I am NOT suggesting there would be a continually flow of liquid.