This is an archive of past discussions about Siphon. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.
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Do you realize when you talk about potential energy that energy is defined basically as the ability to do work and work is defined basically as a force over a distance, so that "potential energy" means the ability to exert a force over a distance? That's what pressure energy IS, the ability to exert a force over a distance from a higher pressure zone, like the bottom of the siphon, to a lower pressure zone, like the top. And don't you realize that when pressure energy is traded into gravitational potential energy, that that is how the trade happens, by a force, i.e. the higher pressure fluid exerts the pressure force to raise the object up the gravitational field? Thus the gravitational potential energy is increased because a force from the higher pressure energy zone pushed it up. In a falling object, the force of gravity converts gravitational potential energy into kinetic energy. If an object with kinetic energy hits a spring, it exerts a force to trasform the kinetic energy into elastic potential energy. When an object is accelerated by a spring, the elastic potential energy is transformed into kinetic energy by a force on the object being accelerated.
And do you realize that the forces change in a flexible siphon tube when you move it up and down, even if the flow rate doesn't? For example, if you put the tube up high like a siphon, the pressure forces in the tube will be lower at the top than they will be at at that point of the siphon when you let the tube go low like an inverted siphon. If your model doesn't incorporate that different pressure in the two situations then your model is incomplete. As it happens, even an incomplete model can give a correct and equal flow rate in the two situations, but different things are going on inside. It's kind of like a pendulum, where you can calculate the period without knowing the amplitude, but that doesn't mean all the forces in the swingarm and the velocities are the same just because the period is the same. The same end result can be gotten to by different internal forces. Mindbuilder (talk) 00:27, 9 November 2014 (UTC)
You don't understand the difference between force and energy. Release a ball down a hill to a level plane. Then roll the same ball down the same hill and at the bottom put a small hill in the way. Gravity is the only force. The ball ends with the same velocity in both cases. The ball went up the second hill and down with any need for treating gravity differently. Kinetic energy was exchanged for gravitational potential energy. Not a force was added or needed to explain climbing the second small hill. The same is true for siphons exvept the exchange is pressure energy instead of kinetic energy. Pressure energy is exchanged for gravitational potential energy. Pressure energy is not a force any more than velocity is a force. In a fluid, whether in a siphon or filling a tub, energy is exchanged. Don't confuse forces with energy. This is clear in Bernoulli's equation. The force is gravity over a distance (height from upper surface to lower oulet). The intermediate distribution of different types of energy (pressure, height, velocity) vary. No magic forces are required to describe intermediate states. --DHeyward (talk) 03:45, 9 November 2014 (UTC)
Ah, now maybe I'm starting to see what you've been getting at from the beginning. You seem to think that pressure energy can cause the liquid to go up the siphon without exerting a force pushing the liquid up, in a similar way to how an object moving upward with kinetic energy can move upward against gravity without any other force to push it up. Is that sort of what you're getting at?
You need to realize that the word pressure is defined as force per unit area (Quoting the MIT courseware site http://ocw.mit.edu/courses/mechanical-engineering/2-00aj-exploring-sea-space-earth-fundamentals-of-engineering-design-spring-2009/study-materials/MIT2_00AJs09_lec04.pdf page 5, "Pressure is a Force per Area (P = F/A)" or Young's University Physics, the number one or number two most popular calculus based physics textbooks, "Pressure is force per unit area."). e.g. Newtons per square meter. That is to say, at some point somebody said, "what are we going to call this force per unit area instead of the excessively long 'force per unit area'", and somebody said, "how about we call this force exerted by fluids on surfaces or other bodies of fluids, 'pressure'". The "pressure energy" of a fluid is the potential of the fluid to exert a force over an area, and exert that force across a distance. That's the definition of "pressure energy". The very name "pressure energy" implies that if that pressure energy is expended, (e.g. in increasing the gravitational potential energy of some liquid going up a siphon) then there will be a force exerted from the source of the pressure energy. Of course pressure energy is not a force, it's the potential to exert a force over a distance. It does imply by definition that a force will be involved as the pressure energy is transformed into other forms of energy like gravitational potential energy. If the shorthand of using the word pressure is confusing things, you can just throw away the word pressure and substitue "force per unit area". So for example you could just remove the pressure variable from Bernoulli's equation and replace it with a "force per unit area" variable. That would not be changing the meaning of the equation at all. It would just be using different names or symbols, like writing it in French or something. Same meaning, different style.
So maybe instead of saying that atmospheric pressure is exerting a force to push the liquid up the siphon, it would be clearer for me to say that the atmospheric force per unit area is exerting the force to push the liquid up the siphon.
Also notice that I'm not equating energy with force. Notice I defined energy above as basically the ability to exert a force over a distance. The "over a distance" part is an important part of the distinction between the two. Mindbuilder (talk) 08:16, 9 November 2014 (UTC)
Once again, NO. You have no concept in fluid statics or dynamics. Read our articles on pressure. Learn about the concept of "head" as it related to describing fluids. We are also discussing the steady state, not whatever creates the initial condition. Start the flow from the ground tube and lift it above the source. Or start it by filling from the top or start it from a vacuum. same as a pendulum -> start it by a horizontal push while at the bottom or start it by pulling it up to a height and releasing (the only force acting on it is gravity even if the first movement is "up" or "down". Gravity is the only force. No additional forces are required to explain it. Same with a siphon. Starting the mercury siphon in water should be a really big clue. The tube is open to air. Add water which raises the energy equally on both the source and drain (pressure is higher on drain). Mercury rises from both the top and bottom reservoir (the tubes have level amounts out mercury and reach the cross tube at the same time) until they meet and siphon flow starts. As soon as the flluid system is complete, flow starts. How it got to that fluid condition is irrelevant and the only force is gravity. You don't seem to grasp how energy and forces are related. Gravity is a force, but the static object in gravity has potential energy. A small volume of water has pressure energy stored in it. The static object in a gravitational field does not exert a force separate from the gravitation for. A static volume in a liquid does not exert a force outside the constraints of the liquid. In fact, since both energies are related to height, the concept of "head" is used to describe the stored energy. Pressure, gravity and velocity are described as "head" and it's always an energy measurement. Read Hydraulic head. The ball rolling down the incline can be solved from the forces exerted by the ground at various angles but in the end they are meaningless obfuscations and mgh is all that is required and gravity is the only force necessary to describe it. --DHeyward (talk) 19:11, 9 November 2014 (UTC)
Here:
Raise the horizontal tub that is just above the outlet (or better yet, connect the outlet to the bottom of lower container in the same fashion as the upper container). Now raise the upper horizontal bar. Pressure energy will drop as gravitational energy is gained. Flow is the same. All other forces are the same. The fluid connection between two liquid reservoirs separated by height drives the flow from higher energy to lower energy. If you didn't need the atmosphere to explain the initial case, it's extremely dubious to need it for raising the bar. -DHeyward (talk) 20:13, 9 November 2014 (UTC)
You keep saying I'm wrong about various things, but I'm not sure exactly which specific things you think I'm wrong about. In order to build a base of understanding what we agree and disagree on, let me know by number which if any of the following statements are not correct. This may look like a lot of questions, but hopefully this will make our discussion go a lot faster, and this covers a lot of territory we have discussed but that I didn't get clear answers from you on, and hopefully most of them you will agree with so you don't have to even mention them.
1. You believe that pressure energy can cause a liquid to go up a siphon at a steady speed (i.e. zero acceleration) against the force of gravity without exerting a force on the liquid.
It's not what I believe it's physics. A bucket of water has a constant sum of gravitational potential energy and pressure potential energy per unit volume (arbitraraily small). A liquid exchanges it much the same way kinetic energy can be exchanged with gravity energy.
2. Energy is basically defined as the capacity to do work.
3. Work is basically defined as the the application of a force over a distance.
4. Therefore Energy is basically defined as the capacity to apply a force over a distance.
5. Pressure is defined as force per unit area.
6. Therefore "Pressure energy" can basically be defined as the capacity to apply a pressure force to an area over a distance.
7. After a siphon of constant cross section has been allowed to run for a little while, the speed of liquid flow will settle to a constant speed, as long as the difference in height between the surface of the source reservoir and the exit stays the same.
No, it also depends on pipe diameter. Speed can be varied. Volume transfer is constant. Liquid speed determines velocity head which sums with gravity head and pressure head to equal the head in the source. Faster liquids have lower pressure sinve the gravitational head is constant.
8. The sum of the forces on a small volume of liquid going up a siphon equals the acceleration of that bit of liquid. i.e. F=ma
9. If the small volume of liquid is going up a siphon at a steady speed, then it has zero acceleration.
10. If it has zero acceleration then the sum of the forces on it equals its mass times zero, i.e. F = m * 0
11. The small volume of liquid going up the siphon has the force of gravity on it therefore (ForceOfGravity) + (OtherForces) = m * 0 = 0 therefore (OtherForces) = -(ForceOfGravity) therefore OtherForces cannot equal zero since ForceOfGravity does not equal zero.
So your tub never fills up?
12. If the difference in height between the entrance and exit of a siphon is only one millimeter, and the siphon is tall, say 5meters, and the siphon has been allowed to run until it reaches a steady speed, then the velocity of the liquid shortly after it has entered the siphon is no where close to enough to carry that liquid up to the top by its momentum alone.
It's a liquid and obeys the physics of a liquid which says that it has 1mm of head to work with. It's not compressible. Pressure energy from source is converted to gravitational potential energy.
13. The atmosphere exerts a downward force of about 100000 Newtons per square meter of liqud surface area at sea level.
Slightly less than the pressure on the drain side so flow is actually from high to low pressure. In fact, the surface of the outlet can be below the ocean surface while the input is a 1 meter sq. well. There's a lot more pressure across the ocean surface than the well. But because the only thing that matters is height difference, it's irrelevant. The output pipe diameter, likewise can be larger or smaller than the input. The amount of water on the input and output can be greater than the other as long as the outuput is lower.
14. If the entrance of a siphon is very shallow, say one millimeter below the surface of the upper reservoir, then the pressure at the entrance of the siphon due to hydrostaic pressure will be practially zero for our purposes, and the pressure at the entrance of the siphon will be almost entirely nothing but atmospheric pressure.
The depth and pressure on the inlet tub is irrelevant. Only the surface matters. That's the datum reference. This is a fundamental property of liquids. The sum of gravitational and pressure energy is constant. That's why the depth of the inlet tube does not matter because it's the same at the surface and depth.
15. The atmospheric force per unit area on the surface of the reservoir will transfer to this one millimeter deep siphon entrance through the liquid and be seen as a force per unit area at the entrance that can exert an upward force on the liquid in the entrance to the siphon.
Net force is zero until the siphon starts and doesn't matter what the depth of the inlet side of the tube is. If it wasn't, they would start without making it a fluid system. It's also all gravity as a syphon can drain an entire lake without external energy or forces. It uses it's own potential energy.
16. Atmospheric pressure pushes the liquid up a barometer.
Not a siphon and static head is maintained. The height difference and gravitational potential energy matches the pressure energy throughout.
17. Atmospheric pressure pushes the liquid up a drinking straw.
18. It is safe for me to assume that if you didn't call out any of the above numbers as incorrect, then you agree they are reasonably close to correct for our purposes. Mindbuilder (talk) 20:53, 9 November 2014 (UTC)
I called them all out as you lack the fundamental understanding to discuss the topic. Here's a wuestion: The maximum lift height of a siphon is less than a manometer. Can you answer why this is the case and any remedies? Assume ideal conditions. It's a fundamental property of fluids. --DHeyward (talk) 22:39, 9 November 2014 (UTC)
Are you saying you're calling out every one of the numbers as incorrect, even for example number 5? Just list the numbers you think are wrong, or if nearly all are wrong, the say that and list the numbers that are right. What I meant on number seven is does the speed settle to a constant speed for a given siphon, i.e. one particular instance of a siphon.
Actually a siphon can go much higher than a manometer. See Andrew K Fletcher's video of a water siphon going up 24 meters in the article references. But I'm interested to hear why you thought it couldn't.
I'm pretty sure I have the fundamental understanding to discuss the topic (but of course I could be wrong). You seem to have some deep seated misconceptions about physics that are preventing us from coming to an agreement. Remember, I've already dispeled several misconceptions you had. For example you thought gravity didn't do work on an object in freefall. I gave a cite to MIT and others to establish that. You thought the atmosphere couldn't do work, I showed the vacuum cylinder and piston with a car(or brakes) on top to prove that it is possible. You thought gravity couldn't do work in a closed losssless system, but I gave the example where a pendulum swings down and sets a flywheel spinning, to prove that it could. You would do well to consider carefully what I say and seriously consider that I might be right and you might have mistaken notions at the base of your physics understanding. You've been mistaken before in this discussion. Are you the kind of person who latches onto and convinces yourself of any odd excuse you can to prevent admitting you're mistaken. Or are you the type of person that takes a hard look at your own beliefs from time to time and are careful not to allow yourself to accept lousy excuses to continue believing your mistakes? The Dunning-Kruger effect predicts that when someone like you is confused about a subject, that you will look for reasons to convince yourself that you are not confused or suffering from the Dunning-Kruger effect. But you're not supposed to look for evidence that you're not confused and suffering from it, you are (and we all are) supposed to look hard for evidence that you ARE suffering from it. Thats how you help a little to avoid it. I have to admit that I could be the one confused here. It seems clear to me that I'm not, but I have to admit that there is a non-negligible chance that what seems clear to me is mistaken. Can you admit the same? If you can't admit a non-negligible chance that you're mistaken, then you have a serious problem with overconfidence. 00:40, 10 November 2014 (UTC) Mindbuilder (talk) 00:41, 10 November 2014 (UTC) Mindbuilder (talk) 00:45, 10 November 2014 (UTC)
No, your other examples were wrong too they just weren't applicable to siphons so I moved on. Your swinging pendulum doesn't do work and then do "negative work." You would fail that question as well as the others. I've asked you simple explanations that Bernoulli's equation explains quite well and you are lost. Namely draining without a siphon and then raising the middle of the tube tube. Use the picture above. The next question as why the lift limit of a siphon is less than a barometer. A siphon does not require the atmosphere to describe it's operation. Its lift limit does have an atmospheric component but so far it's eluded you as you don't understand the process. --DHeyward (talk) 00:53, 10 November 2014 (UTC)
And did you forget that you initially stated that gravity didn't do work on a free falling object, then you admitted that it did? Mindbuilder (talk) 01:07, 10 November 2014 (UTC)
Oh, maybe I'm mistaken. When I go back I see that you didn't actually state that gravity did work on a free falling object, you said "Object in freefall does work and it's from gravity." I just assumed that was a garbled way of admitting gravity was doing work. But maybe I misunderstood what you meant. So do you think that MIT professor was wrong in his question or answer or what? Because I agree with him. Are we both confused about that but you're not.
And That vacuum cylinder with piston pushing car up is very relevant to the siphon. Do you think the atmospheric pressure exerts an upward force on the piston face to push it up? I'm not asking now if the atmospheric pressure pushing the piston up is work, I'm just asking if the atmosphere is exerting a force on the piston, or if maybe you think the pressure energy of the atmosphere is causing the piston to go up without exerting a force on it. Mindbuilder (talk) 01:49, 10 November 2014 (UTC)
Energy can be stored as potential energy. For siphons, it gravitational potential energy. Your tub has an "up leg" and it has water all the way to the tap. Turn the tap on an it flows immediately. It has the same potential energy used in a siphon. It's not confusing to flow up. The water at the valve is at the same potential energy as the water in the city reservoir surface. The height difference between your tub and the reservoir surface is the only factor. A siphon isn't any different. I have no idea what Fletcher's "24m siphon" is. If it strays from the non-viscous ideal fluid or relies on capillary effects or ionic effects or other minutae it has factors that are beyond the normal siphon which is gravity, liquid pressure and velocity. Any ideal analysis that deviates from Bernoulli's equation is fringe and/or quackery. You still haven't answered the basic limiting height question in a siphon. I can write it mathematically but I'm ffraid you cannot as it is only your non-rigorous imagination rather than physics that drives your belief. --DHeyward (talk) 03:06, 10 November 2014 (UTC)
Sorry, I thought the name Andrew K Fletcher and the 24 meter number would make it easy to find the reference on the Wikipeida siphon page but I didn't realize it wasn't there. Now I'm not sure if it was removed or was never there. Any way, here is the link http://www.youtube.com/watch?v=sz9eddGw8vg I was not convinced he really did it at first, but then I learned more about liquid tensile strength and realized it was not inconsistent with other experiments like vacuum siphons, 100m trees, and the Z-tube, so now I don't have much doubt he did it.
You want me to supply the explanation for why a siphon can't go as high as a manometer, but inevitably I will fail at that since siphons have been demonstrated to go higher than manometers. Andrew K Fletcher's is one, several siphons have also been demonstrated in vacuum, where the manometer height is zero, a fairly easy height to beat for any liquid that doesn't have much vapor pressure. You needn't give a long mathematical answer, a one sentence description of your idea will likely suffice, since I very likely have already heard of your theory of why the siphon can't go as high as a manometer, because I have read a whole lot of false theories about stuff like that.
Now I see part of the disconnect between us about how filling the tub has relevance. I was thinking about a pumped pressure water supply and you were thinking about a gravity feed supply. So basically you were thinking of an inverted siphon, from say, a water tower, down low then back up to my tub. There is of course no mystery to anyone about how a pumped water supply can fill my tub. As for an inverted siphon, I can't imagine why you think that is the same as a regular siphon just because the flow rate is the same for equal altitude drop. In a regular siphon the pressure goes down as the liquid goes up the first half. In an inverted siphon the pressure goes up as the water goes down the first half. the pressure trends are completely OPPOSITE, not the same. To say the same thing is happening when the opposite is happening is twisted. It's like you're saying that since 5 + (-4) = 1 and (-4) + 5 = 1 then therefore 5 = -4. Just because the final sum comes out the same doesn't mean the way you got to the sum is identical or even effectively identical. The forces and pressures at many points in the siphon and inverted siphon are different in magnitude AND direction, NOT the same. Nothing you said above about how inverted siphons behave compared to regular siphons surprised me. I already knew it. I already understood it. I understand why it produces the same output flow. It does nothing to prove that atmospheric pressure doesn't push the liquid up a regular siphon. It does nothing to prove that pressure energy makes the liquid go up without exerting a force. You have a misunderstanding of the very basic physics of the situation. You think pressure energy can make the liquid go up without exerting a force. Discussing something as complicated and different as the inverted siphon won't help to clarify that much more basic issue for either of us. That's why I asked you about whether atmospheric pressure exerted a force (whether it ends up doing work or not) on the piston of my vacuum cylinder with a car on top diagram. That's a critical question that required only a quick yes or no answer, but you dodged it, or maybe just forgot. Mindbuilder (talk) 05:37, 10 November 2014 (UTC)
Air is not fluid. Anyway, go back to the "inverted siphon: and think about the up leg. Why does water go up? Does it have anything to do with air pressure? If the outlet is below the inlet and is 100 square meters and the inlet is above the outlet and is 1 square meter does it matter? Now flip it over for a siphon: does it matter for the siphon? (the answer is No for both questions because physically they are the same. The energy (including gravity, pressure and velocity head) is pretty straight forward to calculate. Bernoulli did it. --DHeyward (talk) 14:22, 10 November 2014 (UTC)
From Merriam Webster's - Fluid: "a substance (as a liquid or gas) tending to flow or conform to the outline of its container" And from Wikipedia Fluid: "Fluids are a subset of the phase of matter and include liquids, gases, plasmas and, to some extent, plastic solids."
I wouldn't bother about this, but I'm not sure I convinced you: did you see AKF's 24m siphon, and do you still think siphons can't exceed manometers?
The water goes up an inverted siphon because the pressure force beneath each molecule or small volume of water is sufficient to overcome the downward pressure force above it in addition to the downward gravity force on it. Air pressure is often a large component of the pressure force above and below the water molecule. In an inverted siphon, the atmospheric pressure component isn't needed because the pressure component from the liquid head of the down part is sufficient to push the water up, even if there is no atmospheric pressure. In a regular siphon, If there is no atmospheric pressure to supply an additional upward component of pressure force, then there won't be any upward force to push the molecule up above the reservoir against gravity, because the liquid head at the surface of the source reservoir can't supply enough pressure force to push the water up above the surface of the reservoir without the help of atmospheric pressure. That is why the two situations are different, even though they end up with the same output flow. Now of course my explanation makes no sense if you think there is no upward pressure force pushing the water up, because the pressure energy is making the liquid go up. But the pressure energy can only make the water go up by exerting a pressure force. That's the question we have to settle, and the inverted siphon doesn't help settle that question.
I think I came up with a much more direct way to get to the understanding we need. Lets consider a siphon that has been running for a while so that it has settled to a steady speed. Lets assume a constant tube diameter so the speed is about the same and constant throughout the siphon. And lets assume a slow speed, so for example the difference in height between the surface of the source and destination reservoirs is one millimeter. For that difference h, the formulas say the speed would settle to about sqrt(2gh) or sqrt(2*(10m/sec^2)*.001m) = .14m/s. Now do you think you could use Bernoulli's equation to calculate the pressure at various heights in the up tube? Not the pressure energy, but the pressure? Of course you could. Now imagine we let a small plastic cube flow into the siphon entrance, and the cube has a density equal to the liquid, so it would flow along with it. Now lets assume the cube luckily just happens to flow up the siphon with the sides straight vertical. The pressure of the liquid will exert forces on the sides of the cube that are equal and opposite, and so will cancel for our purposes. And since all the books say that an object floating in a liquid will experience a pressure force on all the sides, top, and bottom, From the Wikipedia article on Buoyancy "The upward force on the cube is the pressure on the bottom surface integrated over its area..." and since you can calculate the pressure using Bernoulli's equation, and your calculation will have a non-zero pressure, the liquid will exert a downward pressure force on the top of the cube and an upward pressure force on the bottom, right? They won't be equal, because the bottom of the cube will be at a greater depth, but there will be a force of pressure on the top and bottom of the cube, because that's what all the books say, right? Please actually answer yes or no if you believe there will be a pressure force on the bottom and top of the cube, don't just make a comment without confirming if there will be or not. Mindbuilder (talk) 00:14, 11 November 2014 (UTC) Mindbuilder (talk) 00:42, 11 November 2014 (UTC) Mindbuilder (talk) 00:48, 11 November 2014 (UTC)
Here's a relevant quote from the Wikipedia article on Force: "Moreover, any object traveling at a constant velocity must be subject to zero net force (resultant force)." No exception is mentioned for "pressure energy" or anything else. So if a bit of liquid is moving up the siphon at constant velocity, it must be subject to zero net force, i.e. there has to be a force other than gravity to cancel the downward force of gravity to zero or else it would be changing velocity. I have never heard of any exception to this in physics. A projectile launced upward can move against the force of gravity but it won't have constant velocity. Mindbuilder (talk) 03:18, 11 November 2014 (UTC)
Here's a quote from the textbook "University Physics": "The essential physical principle is Newton's first law: When a particle is at rest or is moving with constant velocity in an inertial frame of reference, the vector sum of all the forces acting on it must be zero." Again no exceptions are given for anything. Mindbuilder (talk) 03:52, 11 November 2014 (UTC)
Can you cite any credible source discussing any exception to Newton's first law for anything like "pressure energy"? (in classical mechanics like we're discussing here, not quantum mechanics or something like that) Note that a projectile launched upward into freefall isn't an exception to Newton's first law because it doesn't have constant velocity. Mindbuilder (talk) 05:33, 11 November 2014 (UTC)
I was looking around at various fluid physics pages and came on this one https://www.youtube.com/watch?v=uqyLOuAzbvo by Kahn. He applies Bernoulli's equation to flow through a tube consistent with a siphon. It's not a siphon, but he leaves the entrance and exit heights and the tube bends as abritary variables so it is quite analogous to the siphon. In it he discusses the work going into the tube and the work going out, and how they are equal, but he doesn't say there is no work in just because the work in plus the work out equals zero and the system is lossless. Indeed he states flatly the value of the work in, and fits it into Bernoulli's equation. He also talks about the force that is exerted into the entrance by the pressure, and doesn't claim there is no force because the pressure energy makes stuff happen. It's less than 10 minutes, and even shorter if you play it at increased speed. Maybe you should straighten him out since he seems to be confused in just the same way I am :) And remember, if you say there is work in but no force pushing the liquid up, you can just do the analysis starting at a plane that passes through his tube at the midpoint and analyze it half at a time, and see that what is the new entrance at what used to be the middle, will be having a force and work going into it. Indeed he makes no implication that the entrance and exits to his tube are open to ambient conditions. His analysis is just as valid if there is tubing leading to his entrance and tubing leading away from his exit. His analysis could just be thought of as an analysis of the up section of a siphon, or a small part midway in the up section of a siphon. Mindbuilder (talk) 23:18, 12 November 2014 (UTC)
Your confusion is that it's the atmosphere. It's not. The pressure difference is from flow (velocity) in the pipe and the static source reservoir. This is where pressure, gravity and velocity matter. It's not atmosphere. Atmosphere cancels because it is the same at both the inlet and outlet. Gravity would cancel if the output and inlet surface were the same. Once they are different, flow starts and pressure drops and Bernoulli's equation can be solved throughout. Read starting at page 22. --DHeyward (talk) 23:32, 12 November 2014 (UTC)
That udel pipeflow example is using gauge pressures. Guage pressures aren't real, they're just caclulated values to simplify some calculations. There are no actual negative pressures (i.e. tension) in that example. If there were actual negative pressures, and even a tiny bubble got in, then the siphon would collapse and it would be obvious that atmospheric pressure was providing some significant forces, not cancelling.
But setting aside the atmosphere for a minute, there is nothing in Kahn's analysis that assumes anything about an atmosphere. He could have started his video by saying that he was going to analyze a small section midway up the up side of an inverted siphon of vacuum oil on the moon, and the rest of his video could have been identical, including the pressure force doing work into that section of tubing from the section just below and the force pushing out to the section above. On an inverted siphon or a regular siphon in the atmosphere, the fluid, in a small segment of the tube half way up the up side of the siphon, doesn't know what caused the pressure below. It doesn't know if it was from velocity or a giant reservoir above or partly from the atmosphere. It only knows that it is experiencing a pressure force from below pushing into its segment. Also, the velocity of the analyis can be brought extremely low so that the pressure addition or subtraction caused by the velocity term becomes practially zero, and the pressure force pushing in will still be there doing work. But because the velocity is so low, the pressure force will be practically the same as in a siphon that is static because the valve on the exit is closed, and that pressure force will be far from zero. And of course if we barely crack the valve on the exit of a siphon and start letting a few drops out, We wouldn't suddenly expect all the hydrostatic pressure forces to instantly and completely dissappear. The effects that caused hydrostatic pressure forces would change with velocity but they'd still be there causing pressure in addition to the pressure caused by velocity, right? So doesn't that show that at least sometimes when we analyize a short segment of a siphon, that we consider there to be a pressure force pushing up into that segment (possibly with an opposite force pushing down mostly canceleing), regardless of whether that pressure came from velocity effects or hydrostatic effects or the atmosphere? Have we come to agreement on that point yet at least? Mindbuilder (talk) 01:07, 13 November 2014 (UTC)
You're making stuff up again. They just define the atmosphere as zero pressure because it cancels. Don't confuse how a siphon can start with how a siphon runs. You 1 mm below surface, low flow siphon creates just enough pressure drop from velocity in the "up" tube to create the slow flow (mind you the pressure drop is small so the flow is small). That's all there is to it. You cannot create the flow without trading pressure head for velocity head and ultimately gravity head. The pressure drop is between the static liquid in the source and the moving liquid in the tube. it's not a pressure difference between the atmosphere. Read the equations. They make sense. It's like your trying to argue a ball rolling down an incline has a different force acting on it if the incline is shallow. It's nonsense. Go back to the mercury in water siphon. Mercury rises from both reservoirs. When they meet and form a fluid, the direction of one tube completely changes. That's gravity expressed as the height difference. Atmosphere and vacuum are irrelevant except where the property of the liquid is changed (to either solid or gas). --DHeyward (talk) 01:51, 13 November 2014 (UTC)
I hope you see the light eventually, but in the mean time I've got to get back to business, so I'll not be back for more than a month, probably a lot more. Bye. Mindbuilder (talk) 06:21, 13 November 2014 (UTC)
"Neglecting air resistance, ... there are only two forces acting upon the pendulum bob. One force is gravity. The force of gravity acts in a downward direction and does work upon the pendulum bob. However, gravity is an internal force (or conservative force) and thus does not serve to change the total amount of mechanical energy of the bob."
"When the only type of force doing net work upon an object is an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. In such cases, the object's energy changes form."
It gives examples like we were discussing where "...the only forces doing work upon the objects are internal forces - gravitational and spring forces." The examples given include a ball dropped in free fall absent air resistance and mass hanging from a spring(a man on a bungee cord in their example).
Here is another good reference from Princeton to the forces in the fluid according to a derivation of Bernoulli's principle. http://www.princeton.edu/~asmits/Smits_text_part1.pdf Consider especially section 4.2.1 on page 78(pdf page 90). Some choice quotes: "If we neglect friction, the only forces acting on the particle are those due to its weight, and those due to pressure differences." "The force due to pressure differences acting in the s-direction[streamline direction] is ..." "Bernoulli’s equation indicates that the sum of pressure work, kinetic energy, and potential energy remains constant along a streamline." Notice that if a particle is moving straight up a siphon at constant speed, its kinetic energy isn't changing and its potential energy IS changing with its height. In order for the sum of pressure work, kinetic energy, and potential energy to remain constant while potential energy is changing and kinetic energy is constant, pressure work must be non-constant.
For an element of fluid going up a typical siphon, the element experiences a pressure force from the fluid below which does work on the element as it is pushed up, increasing the element's gravitational potential energy, though its pressure energy goes down an equal amount as it rises.
"Newton's laws of motion state that a volume of a fluid that is not in motion or that is in a state of constant velocity must have zero net force on it. This means the sum of the forces in a given direction must be opposed by an equal sum of forces in the opposite direction."
So in a siphon that has reached a steady flow rate and therefore in which the fluid is flowing up at a steady speed, Newton's laws imply that there must be another force besides gravity acting on the fluid. Mindbuilder (talk) 22:50, 30 May 2015 (UTC)
Liquid tensile strength preventing air leakage
The caption to figure 4 is incorrect. If the fluid has no tensile strength, then the molecules will fall individually, allowing air from the bottom to flow up, past the separate molecules and fill the space above the fluid. The air pressure above the fluid will remain at atmospheric pressure, and the siphon will not "start."
If the fluid that is shown in the lower leg of the siphon in the figure were to be replaced by dry sand, or ground pepper, the siphon, similarly, would not "start."
72.95.61.224 (talk) 15:58, 28 February 2015 (UTC) john dooley - jwdooley@aol.com
You can put a piece of pipe in a tensile strength tester, and see what force is required to pull the pipe apart. You can't do that with the vast majority of liquids, water for example. However water still has a surface tension, that makes all the water droplets stick together. Water can support only about 3 to 4mm of it's own weight and it does this with surface tension. — Preceding unsigned comment added by 124.185.24.72 (talk) 08:08, 25 March 2015 (UTC)
Surprisingly, the tensile strength of water has been demonstrated to be sufficient to pull itself up more than 14 meters above the barometric height in a siphon, and 100m in trees. See the reference in the Maximum Height section in the siphon article. Water has been demonstrated to have a tensile strength comparable to rubber when it adheres well to the tube walls. See the Z-tube. But in typical siphons operated at normal atmospheric pressure, the liquid is at positive absolute pressure at all points in the siphon, and therefore there is no pulling going on.
The article of the old consensus of Jan 2014 I think was much better than the current version. I expect DHeyward to come back here soon and undo my recent reversions back toward the old consensus. Would you support my restoration toward the Jan 2014 version against DHeyward's changes? Mindbuilder (talk) 18:04, 25 March 2015 (UTC)
I question the tensile strength theory.
We can't pull water out of a bucket like we can do with a chain. We can't put water in a tensile strength tester. We can see from a slowly dripping tap that water will support only 3 to 4 mm of it's own weight. We can slowly overfill a glass of water and see that it can rise above the top of the glass by 3 to 4 mm. They are all real tests that we can see with our own very eyes. Yet people will throw that stuff out and talk about trees lifting water 100 metres. Do we really know how trees get water to 100 metres in height? Z Tubes don't demonstate tensile strength, someone has simply made up a formula to make it look that way. Water will move depending on the pressures that it is being subjected to, and if there are pressure differences created within a pipeline, liquid will move accordingly to balance out those pressures. Those pressure differences can be created by a pump, by increasing/decreasing air pressure at one end of a pipeline, by elevation, by density. The last of these, density, is one of the key reasons why a siphon works, air and water have different densities. If they were the same, a siphon wouldn't work, even with gravity. — Preceding unsigned comment added by 124.177.145.35 (talk) 06:02, 26 March 2015 (UTC)
Re section Modern Research
Why is the following comment in there when it is just an opinion, no research has been undertaken.
Writing in Physics Today in 2011, J. Dooley from Millersville University stated that both a pressure differential within the siphon tube and the tensile strength of the liquid are required for a siphon to operate.[29]
The Pascal Siphon section still remains incorrect as per earlier advice.
I didn't believe the liquid tensile strength theory for quite a while either. It is still amazing to me. Everyone can see that water has a little stickiness when in contact with hydrophilic materials like glass, although it is not obvious that it is more than slight surface tension level stickiness. It is also clear that water pulls itself together while resisting boiling, especially when superheated above normal boiling temperature in a microwave, though again, it is not obvious that that is more than surface tension level tensile strength. It is clear that liquid tensile strength relies on adhesion to tube walls. Z-tube and tall siphon experiments confirm that greased or unclean tube walls prevent any tensile strength from being exhibited.
I can't blame you for being skeptical, as I was for quite a while.
But how else can it be explained when a siphon operating in vacuum doesn't collapse, as demonstrated in the Youtube video from the University of Nottingham? Or the 24 meter siphon of Andrew K Fletcher? And how does one explain that the water doesn't get flung away from the center of the Z-tube? The Z-tube isn't the end of liquid tensile strength research. There were also theoretical calculations of what tensile strength water could be expected to have, and experiments with tiny amounts of water trapped in small voids in crystals, getting up towards the theoretical numbers. One of the things that makes me believe that tall mercury and water siphons and Z-tubes really are operating in tension is when they collapse to their barometric height due to tapping or dirty tube walls. That shows me that they weren't just staying up because of an imperfect vacuum or unexpected pressure or something, there really was tension on the walls and in the liquid.
While I believe that trees pull water up, I still don't know how they generate the pull up at the leaves.
Dooley's opinion probably doesn't strictly qualify as a Wikipedia cite. Though it is a published opinion of a university professor, it was only published as an opinion, not peer reviewed as a verifiable fact. Still I don't plan to nit pick that cite. In fact I came to the conclusion a few days ago that the article needs to fully articulate the liquid tensile strength theory, even though it is clearly wrong for typical siphons. We should present the debate since it is a significant viewpoint in the peer reviewed literature.
The Pascal's siphon section is indeed still incorrect. I'll probably work on that eventually, but only after I've fixed up a bunch of other things. Mindbuilder (talk) 09:01, 26 March 2015 (UTC)
Re Dooley's Opinion. You missed my point. It is listed in Wikipedia under the section Modern Research
when no research has been undertaken. It is not research, just opinion so maybe Wikipedia should have an Opinion section. The point a raise is not the format that it was published in, but that Wikipedia is claiming it as modern research.
If you put a piece of pipe in a tensile strength tester, there will be a certain effort required to pull that pipe apart. If you fill the pipe with water, seal the ends, it will take no more effort to pull it apart. Put a chain inside that pipe, crimp the ends and the effort required to pull it apart will increase.
In tall barometers, above the atmospheric pressure height for the liquid, there can be a number of scenarios that will occur. If it is water, then above 10.2m height, the top part of the water will turn to vapor. Treat that water first by boiling it to remove dissolved air, and the water doesn't turn to vapor quite as easily. Above 10.2m, you then might expect the water to slide down leaving a negative pressure vacuum above the water level. But it doesn't, rather it enters an unwhat stable condition, filled to the top of the tube, but a simple rap on the tube will have it collapse back down. What is holding it up? Is the unstable condition because it is on the edge of it's tensile strength. No, it's because there is pressure differences occurring, as much as density and gravity want to make it head downwards, as it does that it is creating pressure differences within the liquid that also have the water want to even out those pressures, and go back the other direction. Rapping on the tube gets it to switch to vapor, and thus vapor then fills the upper tube.
If the siphon in a vacuum was operated by tensile strength, then drilling a hole in the top of the tube should make no difference to the siphon, instead it would stop operating.
Strange that those that advocate that it is gravity and tensile strength, and not atmospheric pressure, change there opinion when the bubble in the siphon is presented and then claim that is a special application where atmospheric pressure does play a part.
And those that advocate it is gravity and atmospheric pressure and not tensile strength, change there opinion when the vacuum siphon is presented and then claim that is a special application where tensile strength does play a part.
Rather than gravity, it should be referred to as gravitational potential head, combined with density creates pressure differences. Liquid moves because of pressure differences, and the liquid trys to balance those pressures out. Tensile strength is not needed. Atmospheric pressure is not need as shown in vacuum siphons, and in a practical siphon, plays the part of ensuring the liquid does not turn to vapor.
Z tubes have centrifugal force sending the water in one direction, and in doing so, creating pressure differences that want the water to move back in the opposing direction. No tensile strength is needed. — Preceding unsigned comment added by 58.166.173.42 (talk) 07:53, 27 March 2015 (UTC)
I'm not at all clear about how your theory uses pressure to explain why siphons in vacuum don't collapse.
In finite element analysis, a siphon is divided up into thousands of tiny elements, and the forces on the faces of those elements are considered. Lets consider a 20m tall siphon like Andrew K Fletcher's, filled with water, but with the surfaces of the reservoirs at equal heights so the siphon is not flowing. Do you believe such a siphon would collapse immediately, or could it stay filled for a considerable time as AKF claims his did? If we look at a tiny cube of water at the height in the siphon equal to the reservoir surfaces, the cube will have the force of pressure squeezing all its faces with a magnitude about equal to atmospheric pressure, is that right? If we look at cubes farther up the siphon, they will have less pressure on their faces, until we reach about 10m, where the pressure will have dropped to about zero, right? Looking at cubes still higher, the pressure will continue to decrease, right? But pressure less than zero is negative pressure, where negative pressure is basically just another way of describing tension, or attractive forces between the molecules rather than repulsion, right? Mindbuilder (talk) 19:06, 27 March 2015 (UTC)
Re pressure differences in pipelines.
If you have a horizontal pipeline filled with water that has zero pressure in it, and you increase the pressure at one end to 10 metres, then the water will start to move in that pipeline to even out the pressure difference. If you have a siphon that is operating, and you seal the outlet container and increase the air pressure in that container, the siphon will still operate, however the water will flow in the opposite direction if that air pressure is increased enough. The direction of flow is based on differences in water pressure. If a pump is capable of pumping 1 litre per second against a maximum head of 120 metres, and it is attached to a pipeline that has 150 metres pressure head inside it, it will pump no water because it can't create a pressure difference. That is not claiming pressure difference is a energy force, rather simply that water moves to even out pressure differences however they are caused, be it by a pump, raising the level of one reservoir or increasing/decreasing air pressure within a sealed reservoir at one end of a siphon.
Re 20 metre tube height.
For simplicity, if you have a simple tube, sealed at one end, 20 metres in height, filled with water and put it into a reservoir, then yes, in theory, the pressure at the base will be 10 metres absolute, and 10 metres height it will be 0 metres absolute, and in theory, negative 10 metres absolute at the top of the tube.
However, that raises a question, is it possible to have negative 10 metres absolute pressure. If you have a vacuum chamber, fill it with water then try and suck all the water out, the best you can do is get it to 0 absolute. At some point, the water will boil, turn to vapour and you would be extracting vapor, until all the vapor, and any air that was in the water, is all extracted, and then what....at best you get to zero absolute. It appears we can't create negative 10 absolute in that vacuum chamber.
Back to our tube, can we also have negative 10 absolute?. Normally what happens with tap water, is that it boils and turns to vapor, and then vapor fills the top 10 metres. With vapor, it does not have the density of water, and thus there is no change in the pressure as you travel up vertically from the 10 metre point to the top. Well, okay, it might have some minor density, about 1/1000 of water. So air or vapor changes the pressure situation in pipes because of density.
What if we prevent water turning to vapor by treating it before hand. We can then get the water to go greater height than 10 metres, exactly how much is open to debate, earlier experiments by Nokes didn't go much above the atmospheric pressure limit and there is some uncertainty about Fletcher's experiment. It would seem that yes we can go above the 10 metre atmospheric limit however the water becomes very unstable.
In a 20 metre height tube, the discussion then becomes about what are those options for what can happen. The water can turn to vapor and vapor fills the 10 metre void at the top is one. The water doesn't turn to vapor and simply slides down the tube, creating a 10 metre void of nothing. This doesn't happen, and would appear it can't happen. Water stays at the top in a very unstable condition, ready to fall back down with the slightest knock of the tube. After it does fall down, if the top 10 metres then was isolated and reviewed, what would it contain? Nothing? Vapor?. I'd say vapor due to the switch of water to vapor when the tube is knocked. What if an ionic liquid was used or something that didn't so easily turn into vapor? That is probably where experiments should be taking place. And raising the level of the siphon in the vacuum siphon is another experiment that should be done along with drilling the hole in the top of the siphon tube. Also, show what happens in a vacuum siphon if a small bubble was allowed to enter.
The unstable state is where water is wanting to move down because of gravity/density as well as move upwards because of pressure difference. If I have a horizontal pipe that has 2 pressure points of 20 and 10 metres, the water will move towards the 10 metre point. If 10 and 0, same again, it will move towards the 0. If somehow 0 and negative 10 absolute, then the water will move towards the negative 10. In a vertical tube, we then have to factor in pressure due to height, however the principal is still the same.
It is not possible to measure negative 10 metres absolute with a pressure guage. If you could somehow measure the pressure at the top of that 20 metre tube, you will find that it is not negative 10 metres absolute, rather it is still back at zero. It would be worth doing an experiment with a 10 metre tube and trying to determine pressures all the way up the tube from the bottom. — Preceding unsigned comment added by 121.222.66.159 (talk) 00:44, 28 March 2015 (UTC)
I'm pretty sure that when the water detaches from the top of a 20m water barometer, that there is a void left filled with water vapor.
Instead of imagining a small cube element of liquid water with an abstract boundary, lets consider something more solid. Consider a small glass cube with thin walls and nothing but perfect vacuum inside, except some lead weight to make it neutrally buoyant in the water. At the bottom of the 20m static siphon or barometer, the walls of the cube will tend to flex inward from the one atmosphere external pressure pushing in. At about 10m the external pressure will be zero and the walls will be neither pushed in to the vacuum inside the cube nor pulled out. But I believe that higher in the siphon, the walls of the glass cube will tend to flex outward as they are pulled outward by the negative pressure in the water. This is what I mean by negative pressure. We know that some substances can exert an attractive pulling force on the walls of a glass cube, glue being an example. Even uncured, thick sticky non-rigid glue has some obvious pulling capability despite being liquid rather than solid. I think water is a bit like glue in this respect, in that it adheres to glass somewhat. But unlike thick glue, thin water very easily narrows as you pull till it snaps with little resistance. This is why you need the water adhered fully to the inner walls of the tube all around, so that under negative pressure the water can't pull away from the walls and narrow and break with little resistance. That's why if the inner surface of the tube has oil on it then no negative pressure can be achieved in the tube, because the water can't stick to the walls of the tube.
One way to realize that this negative pressure or pulling on the cube surfaces must be happening, is to consider the forces on the cube. Near the bottom of the siphon, the pressure times the area of the bottom of the cube gives an upward resultant force on the cube. The pressure times the area of the top of the cube gives a resultant downward force on the cube, but a little less because it is slightly less deep in the water. The pressures on the sides cancel. There is also the force of gravity on the cube. If the cube is not moving then the sum of all the forces must be zero. Therefore the pressure force on the bottom minus the pressure force on the top must be equal to the weight of the cube. This analysis holds true up to the zero pressure line.
But consider if the bottom surface of the non-moving cube is exactly at the zero pressure line around 10m. There will be no pressure force on the bottom of the cube to hold it up against gravity. If the cube is not falling then there must be a pull on the top of the cube, equal to the weight, by the water above it. The same is true if we're not talking about a solid glass cube but rather an element of liquid water. Gravity is pulling down on all the water in the siphon, so if it is not falling, then there must be some upward force on it. A small liquid element has no way of knowing what the pressure is in other parts of the tube, it only reacts to the forces applied to it by other molecules touching it (in addition to the force of gravity).
The most direct demonstration of liquid tensile strength is probably the piston test. A capped cylinder mounted vertically has a piston inserted in the bottom and is filled with liquid, being careful not to let any bubbles in. The piston is pulled down and the negative pressure is measured at the top until the tensile strength of the liquid gives way and a cavity forms in the liquid. Search Google for "experimental apparatus for static tension test of oil acrylic" (without the quotes) to see a report of such an experiment on page 11 of the book "Recent Developments in Cavitation Mechanisms". Mindbuilder (talk) 05:56, 28 March 2015 (UTC)
I hope you see the light eventually, but in the mean time I've got to get back to business, so I'll not be back for more than a month, probably a lot more. Bye — Preceding unsigned comment added by 124.185.84.64 (talk) 09:14, 1 April 2015 (UTC)
I would have elaborated a little more about the piston test but I thought that was enough to settle it. But one more quick comment for when/if you come back, or in case anyone else reads this. Note that in the piston test, the pressure gauge at the top was a diaphragm at the top of the cylinder, and that the pressure measured by the downward pull on the diaphragm was more than negative two atmospheres absolute pressure. That means the diaphragm was not just experiencing zero pressure within the cylinder and one atmosphere pushing down from above, but rather at least one atmosphere of downward pull by adhesion (i.e. negative pressure) from the oil in the cylinder in addition to the atmosphere above. Mindbuilder (talk) 20:35, 1 April 2015 (UTC)
The added sentence "The reduced pressure is caused by liquid falling on the exit side" is an excellent step forward. I think that this entire discussion is an excellent example of how Wikipedia works. If I could add some thoughts without overstaying my welcome,
1)The falling water needs the tube (waterfalls don't siphon) so that the falling water can act as a piston to lower the pressure at the top of the siphon.
2)The siphon can start from rest, before the water starts to fall.
3)If atmospheric pressure is the driver, would it be fair to expect toilets to work better in Philadelphia than in Denver (where atmospheric pressure is about 15% less than at sea level)?
160.3.4.181 (talk) 19:20, 26 April 2015 (UTC)john dooley - jwdooley@aol.com
Although there is more atmospheric pressure to push the liquid up a siphon at the lower elevation of Philadelphia as compared to Denver, there is also just as much more pressure resisting the exit of the liquid. The pressure still has its effect pushing the liquid up, but it's almost balanced at the other end. You might think that if the atmospheric pressure is equal and opposite at each end, then it cancels. But it is critical to note that it doesn't completely cancel, because the intervening effects of the taller column of liquid cause a difference that is equal to the difference in the height between the upper and lower reservoir. If a siphon in Denver and Philadelphia have the same difference in entrance-exit height, then they'll operate about the same regardless of the difference in atmospheric pressure(unless the siphon is nearly as tall as the barometric height).
If atmospheric pressure isn't pushing the liquid up, then what force makes the liquid molecules go up against gravity in the flying droplet siphon? There is the downward force of gravity on the liquid molecules, but they still go up, so there must be an upward force on them. Where does that force come from? Is the air in the chamber pulling the liquid up? Is it possible for a gas to have significant pull? If you're thinking the liquid going up has something to do with the speed and momentum of the liquid going up, realize that the up tube of the flying droplet siphon can be made a very large diameter, and be very tall, and therefore the speed can be almost arbitrarily slow, and not nearly enough to carry the liquid up by momentum. Then where does the force to make the liquid go up come from? Mindbuilder (talk) 20:52, 13 May 2015 (UTC)
If you make the up tube large so that the speed is arbitrarily slow, then you won't have flying droplets,
rather the water would just dribble out of the end of the pipe. With a garden sprinkler and jet, as water exits the sprinkler jet, pressure energy is convert to velocity head. In the siphon, if your pipe is large and there is no jet, then there will be no flying droplets. You need either the water to be travelling at a reasonably speed if there is no jet, or you can have a large tube and slow speed if you fit a jet, however, as noted above, pressure energy is converted to velocity head energy as it travels through the jet, with the smaller jet size giving it the speed necessary.
I mean what makes the water go slowly up the fat up tube before it dribbles out (or squirts out, depending on the nozzle) into the air chamber? It's just the higher pressure at the entrance than in the air chamber that makes the water go up, right? Liquid flowing from higher pressure up to lower pressure. No mystery there. And if the entrance is shallow, say ten centimeters, then the pressure at the entrance is 99% from atmospheric pressure, and maybe one percent hydrostatic pressure(1/10th meter depth is 1% of 10m barometric height), right? Mindbuilder (talk) 04:42, 1 June 2015 (UTC) But whether it dribbles out or squirts out into the air chamber, it's certainly not being pulled up by the liquid on the down side, right? Mindbuilder (talk) 23:47, 1 June 2015 (UTC)
Right? No.
Pressure difference and atmospheric pressure are two separate issues. You can increase or decrease the atmospheric pressure, yet the pressure difference will not change. If you increase atmospheric pressure, the pressure of all the water increases, not just at the inlet or outlet, yet the pressure differences within the tube do not change. Same applies to decreasing atmospheric pressure. In fact, you can remove atmospheric pressure altogether and the siphon still operates.
Re the comment "......it's certainly not being pulled up by the liquid on the down side, right?"
Isn't that how you suggest a siphon in a vacuum works? — Preceding unsigned comment added by 60.231.96.58 (talk) 21:40, 2 June 2015 (UTC)
Yes, I think a siphon in vacuum uses cohesion tension to pull the liquid up. I should have specified that I was talking about the flying droplet siphon in my last comment, although some of it could also apply to a normal siphon. Would you agree that the flying droplet siphon would not work in a vacuum? Do you think the liquid is pulled up in the flying droplet siphon? Would you agree that it is atmospheric pressure that pushes the liquid up a barometer? If not primarily atmospheric pressure, what is it that you think makes the liquid go up in a flying droplet siphon? Mindbuilder (talk) 05:19, 3 June 2015 (UTC)
In a vacuum siphon, cohesion tension (or what some might call tensile strength) is not an energy source, it is just a property of the material, similar to the way a rope or chain has a certain tensile strength. With a rope or chain, if we put it over a pulley so it was shaped like a siphon, we would need an energy source to move the rope or chain on what we could call the upside. So what provides the energy in a vacuum siphon to move the water upwards on the upleg? The water in the upleg has a total weight, where is the energy coming from to move all of that upwards? — Preceding unsigned comment added by 124.185.55.205 (talk) 06:23, 3 June 2015 (UTC)
The energy comes from the fall of the liquid on the taller down side. This is more obvious in the chain model of the siphon. Mindbuilder (talk) 06:51, 3 June 2015 (UTC)
Are you saying the weight of the liquid on the taller down side, is pulling the shorter upside over the top of the siphon, the same way a chain would use its weight to pull the shorter upside over a pulley? — Preceding unsigned comment added by 124.185.81.110 (talk) 09:07, 3 June 2015 (UTC)
Last Talk Section Continued
I'm starting a new section continuing the previous section just for less scrolling.
Yes, that's basically what I'm saying, the heavier side pulls up the lighter side. Of course the chain model is not quite perfect even for a vacuum siphon. For example a vacuum siphon can have a fat up tube with more weight than the down tube, and it will still work if the down tube is taller. To fully understand that, you have to take into account the adhesion forces to the tube walls, particularly from the non-vertical surfaces of the tube. Though of course the adhesion/pressure forces exerted by the walls of the tube on the liquid don't do any work if we assume the walls don't move. Mindbuilder (talk) 13:52, 3 June 2015 (UTC)
You need to make up your mind. You say: "Yes, that's basically what I'm saying, the heavier side pulls up the lighter side. Then you acknowledge that what you have just said is wrong. Absolutely wrong. So how does a fat up tube siphon in a vacuum work? Clearly atmospheric pressure is playing no role. Clearly the heavier side will not pull up the lighter side. So why would you even say that the heavier side pulls up the lighter side when you know it is not correct for a siphon? Adhesive forces to the tube walls? If something is sticking to the tube walls, then it's not moving. Non-vertical surfaces? There is no non vertical surfaces, apart from the brief section at the top. And it's the same for both sides if the fat up leg reduces back down before it goes back over the top. And if adhesive forces are a good thing, then there is a lot more of them on the upside, given the greater tube surface area, than on the downside, yet the down leg still wins out. If it's cohesive forces, same again, more on the fat upside given there is more volume, more water. What is the only thing left to explain how a fat up tube siphon in a vacuum works? What is the universal truth about how liquids perform?
Oops, my previous post was stated poorly. What I was thinking is that in vacuum siphons, the taller side, which is usually the heavier side, pulls up the shorter side, which is usually the lighter side. But of course with a fat up tube siphon, the taller side might not be the heavier side, yet still the taller side does pull up the shorter side. As for the adhesive forces, I am talking about the brief sections of non-vertical walls above the fat section. The adhesive forces with the walls don't pull the liquid over or make the siphon go, but they support most of the weight of the fat side so that the taller lighter side can still pull the liquid up the shorter heavier side. For any vacuum siphon, adhesion to the walls is essential. Many experiments with siphons and liquids in tubes at negative pressures have shown that if adhesion to the tube walls is compromised by grease, or other things, negative pressure cannot be maintained. I don't know what you are referring to by the universal truth about how liquids perform. I'll post this now and post a better explanation of the fat up tube siphon in vacuum in a few hours.Mindbuilder (talk) 00:26, 4 June 2015 (UTC)
What I was thinking.....?
I think it showed you were only kicking ideas around in your head, thinking you had figured it out, that the heavier side was pulling the lighter side, only to finally realize that your theory was fatally flawed. And yet you still persist. You still think the taller side is pulling the shorter side. Yet the shorter side can weigh twice as much, 10 times as much, so if it was anything to do with being pulled over the top, the heavier side would win out. That clearly is not the case. The universal truth?
Well there is two truths. Firstly, if you really want to understand siphons, you have to be open to alternative ideas, even the ones you disagree with. You have to kick all those ideas around, trying to find proof that they are true, or proof that they are not. You need to look at all the different scenarios, which can rule out certain claims. The air bubble in a practical siphon rules out cohesive force. That water has a vapor pressure shows why a practical siphon stops operating at 10 metres vertical height. The fat up leg siphon shows it is not about weight, and the heavier side pulling the lighter side over. The fat up leg siphon in a vacuum is a good one to review, because, as noted, it takes out atmospheric pressure, weight and the chain model, and yet still it works.
The other truth, well, seriously, I can't help you. You have to see the light for yourself. You have to kick all the evidence around and reach the right conclusion.
When I first got into understanding the siphon, I found the cohesion tension theory hard to believe, but I eventually accepted it after learning of the various kinds of evidence for it. I do change belief when shown good reason or evidence, and I'll change again if I see a better explanation. And no I wasn't kicking ideas around about how they work. You may not have noticed that I was the person who created the fat up tube siphon diagram and added it to the siphon article more than five years ago. And I did figure out then how it would work for vacuum siphons as well. My post was unclear only because I had started out discussing the lighter and heavier sides of a siphon with constant cross section without specifying that cross section. And I still think it is basically true that generally the heavier side pulls up the lighter side, particularly in constant cross section, near vertical siphons. I don't agree with your characterization of that as "absolutely wrong" just because the fat up tube siphon requires some subtle refinements of that theory. It's like how I don't consider Newtons F=ma to be "absolutely wrong" even though it's not precisely right, and can be way off at relativistic velocities.
I wanted to get more into the fat up tube vacuum siphon this evening, but got caught up in other things. But I'd like to leave something to think about till I can respond more fully tomorrow. Consider a fat up tube siphon in vacuum with the two reservoir surfaces at equal height. Of course the siphon won't flow, even though one side is heavier than the other. The Youtube video of the vacuum siphon demonstrated this static condition, though with equal size tubes on each side. All the liquid in the siphon has the force of gravity pulling down, yet that can't be the only force on the liquid molecules, or by Newton's law, they would be accelerating downward. There is no atmospheric pressure to push the liquid up in vacuum. What do you think is the force that holds the liquid up? Is it a pulling force or a pushing force? To simplify even further, consider a cylindrical drinking glass, filled with liquid while submerged, inverted, and raised half way above the surface of a liquid under vacuum. The only force to keep the liquid from falling out of the glass would be adhesion to the ceiling (what was the bottom inside of the glass before being inverted) and cohesion of the liquid under tension, right? Mindbuilder (talk) 06:51, 4 June 2015 (UTC)
Re your comment
And I still think it is basically true that generally the heavier side pulls up the lighter side, particularly in constant cross section, near vertical siphons.
"Basically"?? "Generally"??
Either it is true or it is not.
"Constant cross section"??
We already know that the cross section doesn't matter in siphons, both practical and vacuum siphons.
"Heavier side pulls the lighter side"??
The fat up tube siphon, both practical and vacuum, shows this not to be the case, that heavier or lighter does not matter, that weight of the liquid when comparing the up leg and down legs doesn't matter.
"Near Vertical Siphons"?? Vertical, horizontal, undulating: they all make no difference.
"Requires some subtle refinements of that theory" ?? Subtle?? Like acknowledging it is seriously flawed.
A siphon works, even with changing the cross section of the up tube, changing which is the heavier side, changing the direction of the tube as it travels from inlet to outlet, it can be vertical, horizontal, undulating. — Preceding unsigned comment added by 124.185.11.73 (talk) 00:34, 5 June 2015 (UTC)
In a constant cross section siphon in vacuum with vertical tubes except for a small turn at the top, clearly it is flatly true that the heavier side pulls up the lighter side. Do you think it is unreasonable to say that Newtons F=ma is true even though it is not exactly true, and way off at relativistic velocities? It's generally basically true, though not exactly true, right?
The cross section matters as far as determining if it is the lighter or heavier side pulling up the other. But of course it is always the taller side pulling up the shorter in a vacuum siphon.
Please don't skip my question above about what holds up the liquid in the inverted glass under vacuum. Do you think there is a force holding the liquid molecules up? Do you think it is a pulling force or pushing force between the molecules like a magnetic or electric attraction or repulsion? You seem to be suggesting in higher up posts, if that was you, that it is the pressure differences that make the liquid go up a siphon. I'm not sure what you're thinking, but it looks like you think the pressure differences are something different than the atmospheric pressure or negative pressure(cohesion and adhesion tension forces). Is that what you think? I think its important to realize that pressure differences make things move or support things because of the pressures that make up the difference. It's not the differences that makes things happen, its the differences that allow the pressures to make things happen.
Do you think that siphons in vacuum have negative pressures at heights above the surface of the upper reservoir? Or do you think the pressures there are positive, zero, or what? Mindbuilder (talk) 04:18, 5 June 2015 (UTC)
Your opening sentence is wrong, better sort that out before you go further. And adding the words "clearly", "flatly" and "true" doesn't make your statement more true. Why not simply say:
"In a constant cross section siphon in a vacuum with vertical tubes except for a small turn at the top, the heavier side pulls up the lighter side."
As noted earlier:
"Heavier side pulls the lighter side"??
The fat up tube siphon, both practical and vacuum, shows this not to be the case, that heavier or lighter does not matter, that weight of the liquid when comparing the up leg and down legs doesn't matter. The vertical height of each does, but not the diameter or cross section.
When I have difficulty persuading people here on Wikipedia, it seems they often refuse to answer simple questions. I wonder why that is. I can't help but suspect that it is because they're concerned that if they answer my questions, that they might end up losing the argument, and they would rather not lose the argument than learn the truth. But maybe there's some other reason. Do I ask the questions in a rude way that triggers a stubborn resistance to answer or something like that? Is it that people just don't know the answer, and just quietly ignore them rather than reveal that they don't know? I try to keep the questions simple because I know that many questions have answers far to complicated to explain to me in a Wikipedia talk page. I try to ask questions with yes or no answers. I try to ask things like: do you agree with a particular point? That then gives me some idea what direction to go. The misconceptions people can have about physics are virtually infinite. I can't address or imagine every possible misconception. If people don't answer questions about what they're thinking, it takes far too long to figure out what to say so that we can come to an understanding. Mindbuilder (talk) 15:48, 5 June 2015 (UTC)
If you are wondering why you are having difficulty communicating with the world, well for a start,look at your opening line: "When I have difficulty persuading people......." There is a whole lot of arrogance right there. Has it ever occurred to you that you might not be right. Maybe you need to get out more. — Preceding unsigned comment added by 144.131.195.100 (talk) 00:42, 6 June 2015 (UTC)
The article says that a tensile-strength siphon has to be perfectly clean and bubble-free.
But I read somewhere that siphon sections of the California Aqueduct exceed the 10m barometric height
limit by 20m or so. The water is not perfectly bubble-free. Rather, the bubbles are periodically flushed
out by intermittently operating pumps. The bubbles normally in the system are too small to completely
break the water's tensile strength. Because the water is moving so fast, the bubbles do not have
time to grow. Well, sometimes things can go wrong, and the pumps are there for standby. But
the tensile-strength operation is crucial to overall energy-efficiency. — Preceding unsigned comment added by 108.185.178.7 (talk) 18:12, 18 July 2015 (UTC)
And, in fact, the momentum of the fast-moving water normally moves the bubbles out.
The biggest challenge in normal operation is keeping bubbles and debris out of the intakes. — Preceding unsigned comment added by 108.185.178.7 (talk) 18:15, 18 July 2015 (UTC)
And the pipe has to be very smooth so that bubbles can't get trapped on the seams. But hey, this
is working on a large scale. If only I could remember where I read all this... — Preceding unsigned comment added by 108.185.178.7 (talk) 18:19, 18 July 2015 (UTC)
Whoever claimed the aqueduct siphons water higher than 10m was very likely mistaken. The aqueduct does take the water over 10m, but with full time pumps, combined with possibly up to another 10m of help from the siphoning effect. Of course there are tall sections of "inverted siphons" as well, that bring the water up out of deep depressions, but that requires no negative pressure. The Edmonston Pumping Plant pumps water up almost 2000ft over the Tehachapi Mountains. Such an amazing fact as exceeding the barometric height in a large aqueduct would very likely be discoverable with Google. Mindbuilder (talk) 22:47, 19 July 2015 (UTC)
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