How does siphon work?

russ_watters

Surface tension and gravity?

Awful. Just awful.

russ_watters

Er, well - to prevent boiling and to push the liquid up the tube! Typical liquids do not have much in the way of "tensile strength", so they can't pull themselves up a tube. As the wiki mentions, you can easily demonstrate that tensile strength is irrelevant to a normal siphon by starting a siphon with a bubble in it.

sophiecentaur

Too right.
And, if most liquids have very low tensile strength, then the tensile strength explanation is not actually relevant to most liquids. It is an interesting slant on the whole thing and adds a healthy amount of confusion to the situation.
Could turn out to be a bit of a big-endian and little-endian clash here.

OmCheeto

When I first read the "tensile strength" argument of liquids, I once again thought this was another April fools joke:(adding water to rocks makes them melt at a lower temperature.)

After all, I can reach into a sink full of water and pull the water apart very easily.

But then again, if I fill a hypodermic syringe with water, put my finger over the little end thing, and pull on that big-endian thing, I feel a tension.

So something is going on.

Anyways, my faux homework problem was based on the PDF I briefly looked at yesterday: www.phys.uhh.hawaii.edu/documents/TPT-final.pdf

sophiecentaur

What you are feeling when you do that is the effect of atmospheric pressure on the back of the piston (the bit that's in the air). You have to fight against that to pull the piston out. You can,in fact, 'beat' a piston with a very small diameter and pull it out against the vacuum which forms inside. (The pressure times the small area gives a small resulting force). Any tensile force that you may be experiencing is very small - the same sort of force that pulls the edges of a water surface up the side of a glass. These forces are sufficient to prove embarrassing for small insects who want to get out from under a water surface and useful for pond skaters, who spend their lives standing on the top of the water. It is only at that scale of weight / forces that the inter molecular forces in water become significant.
If you did the same syringe thing up in space, you could easily pull the plunger out and the water would start to bubble as the pressure inside became less than the vapour pressure.

Infinitum

This was quite helpful too, thanks.

A.T.

What I don't like about the wiki is : "atmospheric pressure is the driving mechanism".

For equal pressure siphons (be it 1bar or vacuum on both ends) gravity (acting on the fluid) is the driving mechanism. Equal pressure on both sides cannot drive anything. It just creates an offset (equal on both sides) to gravity. But it is the differential weight of the columns which causes the flow, and thus "drives" it.

sophiecentaur

It all boils down to gravity in the end, of course. No g, no AP.

A.T.

That's not what I mean. I specifically stated gravity acting on the fluid drives it. The differential weight of the fluid columns "drives" the flow, not the virtually identical weight of air columns above the vessels. Therefore I don't like invoking atmospheric pressure as the "driving mechanism".

OmCheeto

Actually, the wiki article states that that isn't true. Under the Chain Analogy:



Even though the weight of the fluid in the upper siphon section is greater than that in the lower container, the fluid still flows downwards.


I liked this line from the wiki article:

And I think it all depends on what kind of siphon you're talking about. The vacuum siphon video implies that the chain analogy is correct, in that instance.

But under atmospheric pressure, a broken siphon(one with an air bubble) works also.

But I think you can still use the chain analogy here also, as long as there are pistons at each end of lots of little chains.

Hence my new "Infinite layers of jello shots siphon" model.



Which I will explain in about 23 hours, as I'm late for work again.

russ_watters

I prefer to say it is the interaction of the two: gravity pulls down on one side while atmospheric pressure pushes up on the other.

It could be said more simply that the driving force is the difference in hydrostatic pressure, but that may be what causes people to think about tension. It works mathematically but not conceptually.

A.T.

Okay, with different cross-section areas the offset to gravity by atmospheric pressure is not equal anymore. So it is better to say the difference in weight/area "drives" it. Which of course boils down to pressure difference.

sophiecentaur

So, we've eliminated the tension effect for all but 'special' liquids. We are now just chasing our tails about what causes what. If it were not for gravity then there would be no siphon. Atmospheric pressure (only there because of gravity) is necessary to produce the condition at the top of the U where there is fluid available to flow down the other side (this downward flow also needs gravity).

A normal siphon will not work without enough atmospheric pressure. What does it matter if we say that the pressure "drives" the siphon or not? On the downward leg, the hydrostatic pressure is Higher than that on the lower surface - so liquid can flow down.

As with all hydrostatic situations, we know that the cross sectional area of tubes does not affect pressure so that is a red (or in the case of the picture, blue) herring.

If someone looks at it one way or another (yet using the right mechanisms but in a different order [Eric Morcambe rules]) do they need to be afraid of being 'WRONG"?
Is it the term "driving" that is throwing everyone into this frenzy. That word is not a front-line, defined term in Physics so stop worrying. There is no need to bicker further if we have all learned something from this thread.
I hope that all this thread has not just managed to confuse people.

vin300

Does it? The chain analogy is all about the heavier column pulling up the lighter column. The guy in the video doesn't even bother about the liquid levels in the inverted U-tube.

vin300

Whatever be the pressure of the atmosphere, or even if there is no atmosphere, the difference of hydrostatic pressure at the points B and C at the top for a given siphoning setup will be the same.

sophiecentaur

There is no mention of the actual tensile strength of that "ionic liquid". How many metres would the tension support or was the inverted U that we saw as much as the tension would support?
The video was an interesting diversion but it says very little about real siphons with real liquids. It has been shown that a siphon works even with a bubble in it so that evidence shows nothing other than how a very novel liquid behaves. Plumbers use water and barometers use mercury; real, everyday liquids and it is those that are of real interest.