Siphons

zut:

It seems to me experiments here are extremely loosely stated. You erect a weightless flag pole, and then the next thing you’re doing is climbing a ladder I hadn’t heard about, and I just have to assume the ladder is not on the flagpole (or perched on a dry cloud or a two-headed seagull).

I’m not sure how weightless things act when compressed or stretched. Is the flagpole made of photons, neutrinos or what? And leave me outta this experiment. I’ve never boned up on how to sit on even a normal flagpole, let alone a weightless, sawed-off thingy.

But I don’t understand why you’re discussing all this subtraction of compressive and tensile forces; I well understand them. I just didn’t dig what was going on with why we should worry about whether the forces, as pressures, were measured with respect to the local air pressure or wrt the zero absolute pressure. I just thought we were interested in how they subtract out, exactly as you discuss here. Jdnewmil commented on what was with the absolute measurement, but I don’t really understand it. Go ahead, kick the sawed off flagpole. . .out of “shear” joy.

I don’t know whether liquid bodies can be in tension, other than at their surfaces, or not; but I certainly don’t think gaseous materials can be in tension – at least, when you wish to think of them as perfect gases. Even when perfect gases have zero absolute pressure, it seems to me they expand infinitely. How could you stretch them with added tension? But whether or not liquids can be in tension, I certainly think they tend to hold themselves together by intermolecular forces in zero-tension, zero-compression situations. I certainly appreciate ZenBeam’s flagging somebody’s comment about the air’s being under “tension”.

R a y (to stretch a point, under tension, but not paying much at-)

Yikes, I’m replying again.

In the interest of science, I did some kitchen experimentation. For the following, I used a hose only, no reservoirs.

Fill the hose, plug both ends, invert and get ready, then release the plugs and the water will shoot out the lower end. When I put an air bubble into the hose such that it separates the liquid into two parts, then the water in the short leg will drain out that end while the water in the long end drains out its own end. Clearly, the gravity pulls down on the water in both legs. Without the air bubble, all the water exits the lower end. The water in the short leg flows up and over and down. What is driving the water up and over? I submit that you have the following forces:

weight of water in downward (long) leg
weight of water in upward (short) leg
air pressure on water at entrance (short leg)
air pressure on water at exit (long leg)

Air pressure is not great enough to balance water’s weight. Therefore, it in itself is not forcing the water to stay in the tube. The weight balance of the water drives the reaction. The longer leg of water weighs more, it falls. The water behind it has to follow. Why? What keeps it from separating at the top? That would create a vacuum, correct? But the air pressure outside the hose is greater than the vacuum, and pushes the water up over the top. Correct?

Is this what you are saying, The Brain?

Are you sure about this, Irishman? 'Cause I did the same experiment (~middle of page 1) with the result that all the water exited the lower end, just like in the no-air-bubble case. Just to make sure, I walked back out to the lab and repeated the experiment, with the same result. I have a hard time understanding how you got one result, and I got another.

[Also, as an aside, I tried unplugging both ends at the same time, unplugging the long end first, and unplugging the short end first (all with an air bubble in the tubing). All three experiments resulted in a little end-effect dribble coming out the short end, with the bulk of the water rushing out the long end, but only after both ends were uncapped. Additionally, just for fun, I tried placing both ends at the same height. I got a little dribble out of each end, until something unbalanced, and whoosh! everything came out of one end or the other.]

Can you describe you experiment/results a little more? Did you use a clear hose? Relatively long diameter? Clear? How big was the air bubble to start? Did you hold the tubes vertical? Are you sure you didn’t just see a small dribble out of the short end? Are you sure there’s no holes in your tubing?

What did the air bubble do while the water rushed out? Did it just expand, or did smaller bubbles percolate up from each end to meet it?

And, to follow up: If what you saw holds, you should be able to do the slightly-different experiments I described above in my bracketed aside [i.e., unplug the ends one at a time, and try a “same-length-on-both-ends siphon”], and get a different result than I did. Care to try some follow-up?

The only ways I see that this could be true is:

  1. You have a hole in the tubing, preventing differential air pressure.
  2. You have an extremely large air bubble, so that it must expand so much to drop pressure that a substantial amount of water drains out before the forces equalize.
  3. You have a siphon that is >32 ft high, so that there can’t be enough air pressure difference to support the water.
  4. Your tubing is so weak that the wall collapses, setting a lower limit on the pressure inside the tubing.

Otherwise, how could it not support the weight of the water? Or am I misinterpreting your point?

That’s exactly how I see it.

Hmmmm. It appeared to me to operate the way I said. However, it wasn’t that easy to see. I was using a clear tube, but it has some string reinforcing throughout so it is partially obstructed. Would have worked better with colored water for more clarity. I really didn’t see the breaks in water for the air bubble very well. It is possible I was getting dribble out, and that allowed the water to open enough to let air to the bubble, and thus prevent a vacuum from forming. I was using a pretty short hose - about 2 ft. It was longer at first, but the coiling was too difficult to handle, so I cut it.

My apparatus wasn’t great - me holding a plastic tube up in my hands in the kitchen.

I did try placing them at the same level, but had difficulty producing a stable enough equilibrium to hold for any length of time. Any slight imbalance was enough to send water spurting out the lower side.

As for air pressure holding up water, I am talking about air outside the hose. Water is heavier than air pressure. Air pressure alone can’t be holding up the water. However, what could keep the water in the tube if the ends are held at the same level would be the balance in the gravity force. Air inside the hose would be different than air outside the hose.

Ok, so I’m a little confused. Please help this misguided soul.

Why do you need an atmosphere for there to be a positive driving pressure in the liquid? Any column of fluid in a gravitational potential will create a pressure gradient. So at the top of the siphon the pressure will be lower than any other point. The upper container of liquid can then “push” the liquid up the siphon. But that’s not really a good way to describe what’s happening. You can’t push a fluid or a chain. The liquid in the lower pressure region will have fewer atoms leaving as entering. Thus the liquid has net movement towards to region of low pressure.

So now you’re saying, well why isn’t it static then? Why does it move down the longer arm of the tube? Take a look again at the pressures. The lower container has to have the same pressure gradient as the upper container. Ie the pressure at the interface between the liquid and the space above the liquid must be the same for both containers (assume that if there is air, height differences are small). So that means the pressure at the exit of the siphoning tube will be higher than the pressure of the liquid in the lower container. So the liquid has net movement out of the tube into the lower container. This causes a decrease in pressure in the tube, and so liquid from higher in the tube has net movement into that region. Follow this chain back until you get to the upper container.

So have I missed anything here?

I honestly think that that’s exactly what was happening: I can’t for the life of me figure it happening any other way, unless you’ve got a pretty substantial leak. If the tubing has a relatively large diameter compared to the height of the short end of the siphon (and I don’t know exactly how large “relatively large” is) then I can imagine this happening pretty easily.

Yeah, same here; however, if you did this with an air bubble in the center and you got water rushing only out of one end, then that’s a different result than you got originally. My thought was that trying a “same-height” siphon would maximize the height-to-diameter ratio.

Assuming my bracketed paraphrasing matches what you meant, then yes, I agree: pressure differential from inside to outside of the hose is the key.

Welcome aboard JA, and congratulations for posting at the end of one of the longest threads in Comments on Cecil’s Columns in living memory. Well, not really, but it sounds good. Anyway, to answer your question, look at the bolded phrase above. If there was no atmosphere, the pressure at the base of the siphon is zero psi absolute. As you correctly point out, the pressure at the top of the siphon must be lower, but you can’t get any lower than zero. Thus, there’s no pressure differential to drive the water up the siphon. Now, if the tensile strength of the liquid were strong enough, the liquid could be pulled up the short leg, but I’ve not seen any convincing evidence that the tensile strength is of any substantial size. In any case, that point is moot on the Earth, since we do indeed have air pressure.