What Forces Must a Siphon Overcome to Work?

[gsal]

Here is a "siphon" that does not use hydrostatic pressure nor atmospheric pressure to work...it only uses gravity

 

[McQueen]

Gsal,
Brilliant! Loved the description and the explanation and I think that by now we are aware that a lot of sources are active in the working of a siphon. BUT I think that you have missed the main gist of my argument, which is to try to determine what forces are at work against the working of a siphon or to put it more succintly, what forces does a siphon have to overcome before it begins to work. THINK if it were just gravity, then when he holds up the pipe with his thumb over the end, water would begin to flow out of the pipe, sure molecular attraction is ONE of the factors. For instance if the scientist in your video put his chain in the tube placed his finger over it and inverted the tube, the chain would flow out! Remember that solids are even more closely linked than liquid molecules. Now look closely again at the video I had posted in my link, and I think you will see that hydrostatic pressure plays an even more important role than either gravity or atomospheric pressure. Again what is not shown in the experiment where both vessels are placed on a level is that as soon as the water in the second beaker reaches the level of the water in the first beaker the flow would stop. And this ofcourse is due to hydrostatic pressure. Calculate: The hydrostatic pressure in the tube he he is holding up with the water inside would be at most a few grams of pressure per centimetre, atmopsheric pressure would be 1 Kg per centimetre, no contest, water remains in the tube. So if you cross those two forces ( and gravity) out of the equation what you are left with is that water will not flow until there is a difference in hydrostatic pressure.