The great siphon definition debate - Experiment

This is a video related to recent media reports about the definition of the 'siphon' being incorrect in most dictionaries. Dr Stephen Hughes at Queensland University of Technology proposes that the fact that gravity is involved is a common misconception and that a siphon works because "The column of water acts like a chain with the water molecules pulling on each other via hydrogen bonds".

In this video, I show two simple experiments that prove that the siphon doesn't need hydrogen bonding to operate and that atmospheric pressure (or more precisely, the gas pressure above the upper reservoir of the siphon) is crucial to operation.

Together, these show that a mixture of gravity and atmospheric pressure drive the siphon, and that the tensile strength of water due to hydrogen bonding between water molecules doesn't.

For details on the media story, see http://www.physorg.com/news192688538.html or http://www.abc.net.au/science/articles/2010/05/11/2896189.htm

Category:

Science & Technology

Tags:

• Siphon definition
• oxford english dictionary
• fluid dynamics.

[theSolarisDragon]

What happens if this experiment is conducted in a vacuum?

[micolich]

@theSolarisDragon That's not 100% clear as I haven't seen the experiment done. It was done a long time ago and reported that it worked (there are several old papers in the literature). I'm rather keen to try it, I just need to find the time. If I do, I will definitely catch it on video.

[theSolarisDragon]

@micolich If it does work in a vacuum it would indicate that Dr Hughes is in part right, but only under those conditions. It would be interesting to see the outcome.

[micolich]

@theSolarisDragon Steady on a second, its not that simple. What if there's a limit to the height that the siphon can run to, one that then ends up being dependent on atmospheric pressure, and I don't mean the Earth's atmosphere, I mean the pressure of the atmosphere above the liquid in the reservoirs. A vacuum is not perfect, nor is it an all or nothing thing. Science isn't a yes or no, right or wrong game, there's much more to it.

[theSolarisDragon]

@micolich Yes I agree, it would not be possible to conclusively determine it, unless the experiment could be done in a perfect vacuum, but what I was saying was that if the it was possible to siphon in a very low pressure vacuum it would give an indication of what drives the siphoning - pressure or hydrogen-bond chain. I may not have done a comparable amount of science as you, but I think you would agree that it is not unreasonable to form a hypothesis prior to an experiment .

[micolich]

@theSolarisDragon No problems with forming a hypothesis, that's good science. Conclusions are another story. Personally, I don't believe this hydrogen bond chain hypothesis, because if it was true, it would work if you just had a little semicircular channel to have the water flow through. You need a closed sealed tube for a siphon to work and even a small hole in it is enough to make it not work. Leaks being a problem are a classic hallmark of pressure playing a vital role.

[nelsongast1]

Excellent

[kimikozembower]

excellent videos.

[ariejudy]

An excellent demonstration. Thank you.