Siphons

Cecil's Columns/Staff Reports
41

Way back up top, CalMeacham said:

I don’t think anyone is arguing that gravity is not the main driver of the effect. It is the net height difference between the input and the output (top of water level in the reservoirs) that define the potential energy, and that is what causes the water to flow. That is the direct analogy of the beaded chain system. The question under debate is the secondary effect - what is it that makes the next bit of water follow the first bit of water? In the chain analogy, it is the tension of the first part of the chain pulling the next link and the next link and so on. That is obvious in a chain. But is it the same mechanism - tension - that is working in the fluid model? Or is it, as most of us think, pressure difference that is driving the fluid to fill the siphon behind the water leaving?

I am reminded of a similar effect used as a carnival trick. You have a meter for “measuring grip strength”, that consists of a tube of fluid and a rubber ball with a hose. The patron squeezes the rubber ball, and the fluid goes up to some level on the scale with funny headings to tell if he’s a he-man or a wimpasoid. (Pick your own funny labels.) The catch - the rubber ball is not related to the effect at all. Rather, the effect is generated by a stooge hidden behind the mechanism, who raises and lowers a cup. The fluid that fills the tube resides in the cup when the cup is low. As the cup is raised, the fluid fills the tube to the same height. That is the gravity potential of the fluid - it seeks its own level. So the ploy is to get some muscular stud to register a measly little grip, and let some delicate girl blow the top off. But the mechanism is the gravity potential, same as the driving force of the siphon.

What starts the flow is the height differential. What keeps filling the tube with water is the pressure difference.

Zut said:

Want to try that again?

Mangeorge, you’ve got pictures of your girlfriend in a bathtub you want to share? I don’t think the Straightdope allows that sort of thing. But you can email them. :wink:

weapons grade bullonium said:

Thinking hard on this. I think the first hurdle is filling the tube that goes up and over the trough. The tube will not be self-filling, in that you put one end in the high water source, another at the low end. You have to expend energy to fill the tube first before the siphon will start. That requires something to pump water up and over inside the tube. I think you will find it takes more energy to repetitively fill the hose than to just pump the water to the new level. But would it work? I don’t think so, but I can’t seem to put it in words right now.

42

Sheesh. No wonder no one complemented me on my brilliantly-thought-out and superbly-executed (but poorly-reported) experiment. Make the above quote,

weapons grade bullonium: I’m thinkin’ that your experiment will not work. jdnewmil correctly points out in his last post that “the pressure in the siphon above D [i.e., the water level of the stream] is below atmospheric pressure (though not negative).” Thus, if the siphon springs a leak, air will go into the siphon, rather than water coming out.

43

IF ANYONE STILL THINKS WATER MOLECULES PULL EACH OTHER OVER THE TOP IN THE SIPHON PROBLEM READ THIS. DO NOT BE FOOLED! THE TRUTH IS OUT THERE. ACTUALLY IT IS RIGHT HERE.

One last time I will try and explain this for all the people who still have a problem on this subject.

First to ZenBeam: Newton’s First law says that things at rest tend to stay at rest. Therefore, the only way to get things to move is to apply an OUTSIDE force to the mass. Molecular cohesion is absolutely completely internal and can therefore NOT be the force that moves the water up that darn tube OR PULLS IT OVER THE TOP. No matter how good Mr. Hussman can spell or speak he can not change that behavior of matter. So by simple deduction I assumed that the only outside forces that could be acting on the siphon are either gravity or Force = Pressure*Area. If you can think of any others please be my guest.

Second To Jdnewmil: I have no idea how someone can obviously have physics schooling but have such a small understanding of physics. So for you I am going to break this down to its very smallest parts so that you can hopefully understand. Before I do that, negative pressure is a widely used term to refer to a pressure that is lower than the pressure surrounding the system being examined. Of course there can never be a negative absolute pressure. Finally you need to take some fluid dynamics classes before you talk about fluid properties. Fluid separation happens all the time. It is called cavitation and is a huge engineering problem. Since absolute zero pressure by definition can not exist and separation does occur then by simple reasoning it doesn’t take zero pressure to cause fluid separation.

I will not go into the explanation of the siphon again. I will try another example which will more clearly illustrate the area of interest in this string. Is the water being pushed or pulled.

Many of us have done this experiment before so bear with me. Take a glass. Fill it completely with water. Cover the opening with a poker card so there are no gaps. Hold the card to opening and turn over. Remove your hand and you will see that if the card is air tight the water will not fall out. Now then is the water pulling itself up into the cup or is being pushed up by air pressure.

Let us look at the pulling up theory. Supposedly, the molecule above the lower molecule of water is pulling the lower molecule up using the cohesive property of fluid molecules. At the opening this looks really good. They look like little monkeys going all the way to the top of the glass when wait a second something is terribly wrong. What is the top monkey(water molecule) holding onto to pull up all the rest of the monkeys. Water and glass do not have a cohesive strength anywhere near the amount needed to get 33 feet of height. Infact there is your test to see the differences in cohesive strength and air pressure strength. Jdnewmill examines the capiullary rising of water. Does this capillary rising get to 33 feet, jdnewmil? So if it can’t get 33 feet out of a microliter of water how can it get it out of virtually any volume of water? It can’t because it ain’t strong enough. So magically, your monkeys just suspend themselves in the air and la dee da. This is fiction. It does not work this way.

What is going on is this. Water has weight. This weight, when not moving, has a static force. At the top of the up turned glass there is some sort of area. Using vector math (don’t get scared) and applying a pressure equation you get a vector pressure. This means there is a magnitude and direction for the pressure. At the bottom of the up turned glass there is a pressure due to the atmosphere. There is also the area of the glass opening. If you apply vector math to the atmospheric pressure and the opening’s area you can obtain a vector force. If this force is greater than the force created by the weight of the water then the water will stay right where it is. If the force is lower than the weight like when the water is greater than 33 feet in height the water will fall down.

Now then you say. That is not exactly what is happening in the siphon. The only difference is that the pressure difference is being caused by the difference in fluid velocities in the tube. The detailed explanation for this is very long and very mathematical. If you really have interest in this then take some FLUID DYNAMICS classes. I have answered it using words though and if you still don’t believe that air pressure is pushing it up then go to school and keep on trying. You might get it someday. Really, though you don’t need to because apparently you can become a professor of a physical science and not understand this.

44

TheBrain, you have completely missed the point of my last post. Calm down, and read it again. And again.

45

Question from a non-physicist:
I’ve noticed that when I change the water in my fish tank, the water drains out through the siphon quickly when the tank is nearly full. But as the level drops, the water drains more slowly. When there is less than three inches of water left in the tank, the water barely drains out. Since the top surface area of the water in the tank remains the same throughout, how can atmospheric pressure cannot account for the changing flow rate? And, since I have to empty the receiving bucket repeatedly, how can cavitation account for it?

46

Read your post again!

Why don’t you read my posts for a change. In my very first post I stated that the cohesive property of water does play a role in this problem. Your reply to my remarks were so unspecific that I mistakeingly applied your comment to something else because I had assumed you had read my earlier posts saying cohesion plays a role. Next time why don’t you try being more specific so people can understand you. And, I will try to talk down to your level so you can understand me. Deal?

47

While I can’t disagree with your statements that the remarks by “The Brain” are gratuitous they are not really that far from your own use of the name “spewing pompous ass.” Not that I disagree with that either, but simply that you don’t seem aware of the similarity. You could have conveyed the same sentiment that he/she/it seems too impressed with him/her/itself, without using the word “ass.” Even if we ASSume that you meant it in the “jackass” sense, it’s still simply an insult. Since “ass” wasn’t enough, “pompous” was added for effect, and still lacking the right jab, “spewing” finally topped it off.

Don’t get me wrong… I won’t flame you for flaming him for flaming them. Not that I’m so noble, but rather that you, like he, first make a point, then “detract from your credibility” by “sniping.” If anything, you take away from your point more than he does, because his point was that he disagreed with their scientific theories, while your point was that you sound more believable if you don’t call people names. If anyone has done a good job of responding without an edge to his remarks, it’s been Hussman. I live in TX these days, and have to say he doesn’t fit all the Okie jokes they tell around here. Of course, who believes Aggies?

On that point, I’d just like to say “All generalizations are false.”

As for the statement “…there’s no need for your sniping.” I have to disagree. Mr. Winkelried suggested “primal scream therapy” for compulsive flamers, but one could argue that there’s no need for screaming. If “The Brain” gets some sense of fulfillment or increased self-worth, then there apparently is a need for his remarks. The real issue is, “Is this the place for it?” The clear answer, since I haven’t received my bill for using the forum, is that if the moderators don’t want it here, take it outside.

Finally, I’d like to close with a quote from Rodney King, who said, “Can’t we all just get along?”

48

I realize you are referring to a lack of influence by atmosphere INSIDE the bladders, because it is a sealed system, but should you write off the effect of OUTSIDE pressure so easily? Take the same two bladders, but place one inside a pressure chamber with a reinforced hose passing through the wall, and assuming that the amount of water present was only sufficient to fill the hose and one bladder (or a certain amount of elasticity to the bladders), when pressure inside the chamber is increased or decreased the water will eventually flow to the bladder surrounded by the least air pressure, won’t it? I realize this stretches the whole useful siphon concept, but the idea came to me while thinking about external pressure models. I was thinking about atmospheric pressure decreasing as altitude increases. Granted, by the time you got bladder “A” high enough to make a significant difference in outside pressure, you’d have more water in the hose than in either bladder, but we’re talking about the principle, not “how practical is my example.”

Assuming both bladders could be immersed in a sufficiently dense media, where pressure change with depth would be more rapid, and that the hose was as flexible/collapsible as the bladders wouldn’t you eventually hit a point where outside pressure on flexible containers would be sufficient to collapse the lower container and hold the water at a point of balance between the containers, in apparent defiance of gravity? Of course at that point, we’d just say it reached neutral bouyancy. I’m leaving out the effect of the outside pressure on the water itself, though. Shoot! What am I missing here? You see where I’m going… Where does this model fall apart?

I hate it when my brain wakes up and says, “Hey, what’s going on, man?” :smiley:

49

Folks, this is my conjecture, based on what I remember from fluid mechanics. Flow is, indeed based on a difference in pressure, but the key pressure is not necessarily the “thermodynamic” pressure (which, among other things, is the what you have to consider when you want to know if water will cavitate), but rather by the “dynamic” pressure. If you dust off your fluid mech book, you might see something that looks like a script P for dynamic pressure, defined as:

grad(dynamic pressure) = grad(pressure) - rho*g

where “pressure” here is the thermodynamic pressure, rho is fluid density, and g is the gravitational vector (on earth, this is just 9.8 m/s/s “downward”).

For the case of a constant viscosity, incompressible, Newtonian fluid (a good approximation for water) in steady, fully developed flow, you can use the Navier-Stokes equation to give something like:

0 = -grad(dynamic pressure) + viscosity*grad^2(velocity)

In other words, the velocity (i.e. fluid flow) is driven by the gradient in dynamic pressure, and not just the “normal” pressure. I’m pretty sure that, regardless of the twists and turns and rises and falls that the tube may take, you will end up seeing that, as you move along the length of the tube, a constant gradient in dynamic pressure. This will be approximately rhog(elevation difference between tanks)/(tube length). Even as the tube is coming up out of the tank, and water is flowing uphill, you see the DYNAMIC pressure (but not the thermodynamic pressure) continuously decreasing. You will find that if you go up too far, the thermodynamic pressure will have dropped, and water will cavitate.

So anyway, that’s my explanation for why water is willing to flow uphill in the case of the siphon. Any of you other engineers out there care to comment?

Freightliner
(a chemical engineer)

50

All righty then! Time to flex my fingers and do some serious typing.

Here’s my view of the Comments on Cecil’s Columns forum - its primary purpose is to discuss current and past Straight Dope columns, books, slug signorino illustrations, etc… It is not meant to include a debate on “what is appropriate for this forum?” or “who called who an idiot first?” or “does arnold winkelried really wear swiss army boots?” As a moderator, I have my bias. I’m probably more tolerant of unrelated humour or witty asides than I might be of other posts not related to a Straight Dope column. One thing I really don’t like, however, is personal insults and direct criticism of another poster. There is another forum for that, or you can take it to e-mail. I hate to be the heavy-handed moderator, but that is the sort of nonsense up with which I will not put.

If someone calls you in this forum a #@##@ son of a ##@#@ (insert any egregious insult here), do not respond in kind in this forum! A moderator will come by eventually and explain our forum policy, and/or severely chastise the offending board member if the situation warrants it. If you are in doubt as to how to respond, feel free to e-mail a moderator or administrator. In most cases you will probably get an answer by the next day at the latest.

Finally, my personal advice: if you think someone has insulted you, ignore it. That’s the best response you can give that person. If they are trying to provoke you, why give them what they want?

moderator, «Comments on Cecil’s Columns» **

51

Err, you do realize that he was substantially agreeing with you, don’t you?

A couple other things here, Brain:

  1. Coming up with a convincing explanation is one thing. Phrasing the explanation clearly and succinctly is another. If you feel that your explanation was not duly accoladed, some of the onus is on you to find a way to describe your position more clearly. That’s what this message board is for.

  2. What Arnold said.

  3. I also note the following:

I’m thinkin’ one of these is a typo, yes?

52 
 The pressure simply keeps the water in the hose and able to flow. It's gravity that actually drives the flow. As the difference between the level in the tank and the level in the bucket lessens, the driving force goes down. The resistance of the tube remains the same, though--thus the flow slows down.
53

Hello and How are you?

I have started out very badly in this forum. I am not pleased with my earlier remarks. It is hard to be as witty and lucid as Cecil. With that in mind I am sorry if I offended anyone.

To truly answer the siphon problem looking at every force and reaction that takes place at every moment would take about four years of physics. This problem incorporates all the forces, it is an escellent example of the complexity of the conservation of energy, and it is very good because it is extremely counter intuitive. It is not surprising that we are having a hard time wrapping our heads around this one.

I made a mistake in my first post by being negative. In it if you think about what I say you can devine what my answer is to this problem. Am I right? Probably not. Am I close. Yes.

My only real goal is to try and get people to stop using terms they may not fully understand and stick with stuff you do know. With a little bit of knowledge about material properties, conservation of energy, pressure dynamics and gravity we can all answer this problem. And intuitively we all have this knowledge already. However, when you get stuck in equations and misused termanology the true beauty of physics gets lost. I will not name names any more but the tunnel thinking caused by equations and the like are very dangerous especially when such apparent lack of understanding of the math and termanology is there. Physics is not math or termanology. Matter will continue long after our paltry discription of it is gone. The DISCRIPTION is not physics. That is why the siphon works even though we can’t describe it, just like the atom, gravity and love.

We know the water flows. We know it isn’t magic; that it is a cause and effect like everything in physics. Think about it and it will come to you.

To all my critics. Read my posts and think about them before you critisize. I read yours. I say this because everything that people have said about my analysis had already been covered in previous posts. I have only been clarifying my first.

See you all at the next post. :slight_smile:

54

  1. A times like this I wish I had accss to our old teaching lab. If you set up a siphon in a vacuum chamber you can quickly find out whether its air pressure or inter-atomic forces.

  2. I flipped through my old Intro. to Fluid Dynamics text. There’ only one reference to siphons, a throwaway question a the end o a chapter. But it’s pretty clear from the way they worded it that they think the limiting factor is the foce holding the water together.

  3. Jus to muddy the waters a bit, I thought I’d bring up the “handkerchief” siphon. You can empty a basin by saturating a handkerchief and hanging it over the edge, with the inner end in the water and the outer end lower than the inner end. In this case it’s clearly not atmospheric pressure pushing the water anywhere. It all has to be forces between the molecules.

55

So, now that everything is theoretically clear, can you have a siphon of:

  1. A gas through a solid tube in an atmosphere of a less dense gas.

  2. The same with the densities of the gases reversed.

  3. Two liquids as in ‘1.’.

  4. Two liquids as in ‘2.’.

  5. A gas through a solid tube in an liquid “atmosphere”.

  6. Yeah, play with the state and density of the tube also.

  7. Would a vortex help?

  8. Since computers have done nothing for weather prediction where I live, maybe the existing weather models could use introduction of siphoning in them somewhere.

  9. A spewing ass may be on topic if a pump is one.

  10. Out of gas. Siphon me some more. . .with this thread that sucks.

Ray (Sigh, fun?)

56

I have to say, as a long time reader of The Straight Dope, that Cecil has disappointed me on this one. The posters on the board I expect to spout ignorance, but I expect better of Cecil.

First, let me state that I know a little something about this subject. I don’t just remember a physics class or once took a class in fluids. I am a professional, (i.e. paid) fluid mechanic, working for the Navy in propeller research. Cavitation is not a vague concept here. It’s something we work with every day.

Cecil states that “while the cohesiveness of water explains the operation of a siphon in theory, in practice it depends on air pressure!” Well, duh. Of course it depends on air pressure. But that’s not the point. It doesn’t REQUIRE air pressure, or even air or water for that matter, or if you really want to get esoteric, gravity. It is simply a matter of the balance of forces on the liquid, and the forces come from both gravity and air pressure. The bubble at the top of the tube you get when you try to siphon over too high an obsticle does not need to contain water vapor. It can contain nothing, a vacuum.

Has anyone here ever heard of a barometer? You know, the kind with mercury in a column. It’s the same principle. At the top of the column is essentially a vacuum (the vapor pressure of mercury being very very small).

Lets do a thought experiment. Imagine two tubs of mercury sitting on the Moon, connected by a mercury-filled tube. No air, but we have gravity. If we raise one tub, the mercury will flow from the upper tub to the lower tub through the tube as long as no portion of the tube goes above the upper surface of the upper tub. What happens if the tube is raised higher? A vacuum forms in the tube above the upper level of the tub, and nothing flows. “Aha!” you say. “That’s really just a drain, not a siphon, since the connecting tube never goes higher than the upper liquid surface. For the siphon to work requires air!” No, it doesn’t.

Let’s now modify our thought experiment. Place on top of the upper tub a platform sealed around the edges with an o-ring (I love those things). Now put a weight on the platform. Voila! Now the mercury will flow through the tube, above the surface of the top tub, and to the lower tub. What have we added? Not air, but a force.

“No fair!” you cry. “That’s not a siphon! That’s just creating a plunger to push the mercury through the tube!”

Now pay attention closely here. IT’S EXACTLY THE SAME THING. Air pushes on things. Your senses are not normally aware of this, but it does. Roll it over in your mind. Let the concept soak in. Accept it. Revel in it. Maybe you too can now become one of the few, the elite, the Fluid Mechanics.

And one last note to Cecil. Would you go to a biologist to remove your appendix because he studies organisms? No, you’d go to a surgeon. If you’ve got a question about aerodynamics or fluids, don’t go to a Physicist. In general, they don’t deal with this stuff enough to have a feel for it. Go to an aerodynamicist or a fluid mechanic.

Chris Chesnakas
Propulsors and Fluid Systems Department
Naval Surface Warfare Center, Carderock Division

57

TheBrain:
Fair enough, and welcome aboard.

Cal:
I’m thinkin’ that the “handkerchief siphon” muddies the water a little too much. Handkerchiefs or paper towels depend on capillary action to “suck” the water up. Capillary action depends on surface tension forces, and these forces are most effective in small spaces (like the interstices in a handkerchief). So the capillary action effect you see in a handkerchief does not carry over to a large-diameter tube siphon.

Let me sum up what I think is the concensus: A “real” siphon requires gravity to accelerate the fluid, and air pressure (or, in general, any force, as Chesnakas points out) to keep the fluid cohesive. It does not require inter-molecular forces; although these exist, their effect is limited.

58

OK, I’ll go out on a limb…

  1. A gas through a solid tube in an atmosphere of a less dense gas.
    I think theoretically, yes, although the entire apparatus might need to be much taller to counteract the lower density (and thus lower gravitational forces).

  2. The same with the densities of the gases reversed.
    But then the less-dense gas would float upwards out of the container. I think this would work if you built the siphon upside-down, and siphoned the lighter gas to a higher location.

  3. Two liquids as in ‘1.’.
    Ought to work fine.

  4. Two liquids as in ‘2.’.
    Ought to work fine, upside-down.

  5. A gas through a solid tube in an liquid “atmosphere”.
    Hard to find a gas lighter than a liquid, but if you could, yeah, it ought to work (although the siphon might need to be tall, as in 1).

  6. Yeah, play with the state and density of the tube also.
    Shouldn’t matter.

  7. Would a vortex help?
    That would just make the problem a lot more complicated.

59

Chesnakas, I don’t think it’s the same thing at all unless you put a weighted platform on the lower tub as well. There is air pressure on both tubs when the siphon is running on Earth.

If you put a weight on both tubs, I’ll agree that it’s the same thing. Is your point that it doesn’t have to be air pressure, it can be pressure from any source? I didn’t think that was in dispute here (but maybe I’m wrong).

60

Zut writes

Ummmm, you’re going to make a “tube” out of gas and then siphon through it? I’d like to see that! :slight_smile: