I was going to make relate some boring-assed comments and questions about my girlfriend, a clawfoot tub, and a large bath towel.
I can see I’m outta my element here, though.
I had pictures, too. 
Oh, well.
Peace,
mangeorge
By what standard of measure?
- You are not Cecil.
- You have no one’s respect here.
- Don’t take number (2) personally, it is true. Respect is earned, not granted by divine decree.
- While you can affect a curmudgeonly and comically cranky attitude successfully, no one will have any idea that that is your “standard operating mode”, or that you mean well overall, in your first 5 posts on the board.
“Cecil” is human, and is human enough to learn from others. I know "Cecil does not know more about coal than I do - or maybe, in reality, he knows exactly as much as I do about coal, should someone ever ask him a question about it. Undersand what I am saying?
IMO, “Cecil” is a construct of the knowledge of experts here and elsewhere, combined with a need and a drive for eradicating ignorance, and topped off with a healthy serving of wry humor.
**Hussman, in your expandable bladder experiment, are you bladders at zero pressure? If so, will the water really remain liquid? If not, how can you say there’s no influence of the atmosphere? **
**
[/QUOTE]
Because there is no atmosphere inside the bladders, and the pressure outside the tanks is equal, hence they balance. You are correct, if the entire system were pulled at both ends, increasing the volume while the mass remains constant, and go to a steam plus water system.
Every thermodynamicist will tell you that the science is based solely on observation. Fortunately, as far as science has noted, each identical system under identical conditions has behaved identically, hence it’s a reasonable assumption the water will behave as described. If you should discover otherwise, please let the world know.
Be careful when you call people dumb, sometimes they may be talking about something you haven’t learned.
If you read the question, you’ll see the author was aware of Bernoulli’s equation.
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How does a siphon work? I understand that it allows liquid to be moved to a lower altitude, so there’s no problem in terms of potential energy.
And you’ll notice the hydrostatic pressure equation is simply the mechanical energy balance reduced to ignore velocity, and the density does not cancel out. (Check it yourself, I did).
He is asking how water (or whatever liquid) can go up then go down, is it pulled or pushed? Cecil argued pushed, Al (and I) argue pulled.
Your chain analogy is not correct, any mechanical engineer will tell you it is preferable to pump a liquid than to apply suction. Water is not a chain, it is a collection of H2O molecules held together (in liquid phase conditions) mostly by hydrogen bonding of one molecule to the oxygen of another molecule.
hussman, why do you respond to ZenBeam’s question about the bladder (01-05-2001 10:10 PM) yet ignore his very cogent arguments regarding tensile strength from the same message? I bring this up because you later continue to contend that you and “Al” argue that tensile forces are the dominating mechanism.
Perhaps a slightly different analogy will help put this “tensile strength” business into proportion.
There is a “juggling” prop known as the “cigar box”, which probably used to be made out of actual cigar boxes. The nature of the tricks performed with these props involves holding three or more of them in a stack, usually horizontally. See http://www.media-tainment.com/mcltjci.htm for an example.
The simple act of moving this stack to the juggler’s right (that is, not describing the loss-of-contact “trick” shown in the video) involves an imbalance of forces. The pressure between the blue and yellow boxes is greater than that between the yellow and red boxes. At neither interface is tension involved. It is simply an imbalance of compressive forces. And if no motion is involved, there is no need to involve “tensile strength” to explain why the three boxes stay together as your message of 01-05-2001 11:29 AM argues is why liquid water stays together.
Water will vaporize if the pressure drops below its vapor pressure at that temperature. Vaporization means the molecules stop remaining adjacent to each other and start moving freely. If they “stick” together below the vapor pressure, it is by definition (the definition of vapor pressure) not an equilibrium condition, and I doubt that
it would account for more than an inch or two of possible additional siphon height.
The logical conclusion of this last paragraph is that a siphon in a “vacuum” will only raise water over a very small height, and would be a non-equilibrium condition.
As for Cecil’s contention that air pressure is the operative principle… this is a very misleading statement to present as the answer to Bob Murphy’s question, “… how
do the individual molecules know that they are going to end up at a lower altitude?” The molecules, like the yellow box, each experience a differential between two compressive forces and move through the siphon as a result of the differential. The difference in air pressure between C and D is NOT a significant factor (the pressure at the inlet is determined by the height of water from A to D, while the pressure at C is atmospheric), and his explanation does not clearly debunk this hypothesis. The Encyclopedia
Brittanica argues against the air pressure differential, but then immediately falls into a variant of the “tensile strength” trap.
This discussion reminds me of the “centrifugal force” fallacy.
Spaghetti? Aaarggghhh!!
hussman, the relevance of the atmosphere isn’t whether the air comes into contact with the water, it’s that the water is at atmospheric pressure. In you bladder experiment, the water is always under positive pressure, so the tensile strength of water never comes into play.
Here’s a thought experiment you can apply to your bladder experiment. Suppose it’s filled with “special” water, which has zero tensile strength. Won’t the water still flow into the lowered bag? How will the experiment behave any differently?
The same holds for the siphons. If “special” water is used, the siphons will still work up to 34 feet. Where is the relevance of any tensile strength water may have?
Finally, here’s something I haven’t completely thought through, but maybe others can expand upon it. Suppose I pull the opening, B, above the water level, D. If water has a strong tensile strength, won’t the siphon keep working? And won’t the distance I’m able to raise the siphon before it breaks be directly related to how much of the maximum siphon height is due to water’s tensile strength? I haven’t performed this experiment, but I have a can’t imaginw raising the opening even an inch above the water surface before the siphon breaks.
Oops, one last thing: I’ve brewed beer in the past, and siphoning it from one container to the other is an important step. It’s not unusual to get bubbles in the siphon hose, and the siphon keeps on working. If tensile strength were what keeps a siphon going, why didn’t the siphon stop working? Perhaps an experiment to settle this question would be to measure how tall a siphon can be with no bubbles, and then measure how tall a siphon can be sustained when a bubble is inserted using a hypodermic needle at the siphon inlet.
I want to clarify in my last post, that I’m not talking about tiny little bubbles like you see in a glass of beer. The bubbles in the beer siphon hose take up all or most of the diameter of the tube, and could have a length several times the tube diameter. The tube inner diameter is about 1/4", I believe. When the flow of beer was slow, the bubbles could stay in place, with the beer flowing around the bubble on all sides.
Ooohh, good point ZenBeam. As a matter of fact, it led me to walk out to the lab and perform the following experiment:
I got about three feet (that’s one metre) of clear plastic tubing, and filled it nearly full of water. I put a thumb on each end and bent it into an inverted “U”, with one end higher than the other (like a siphon, natch). After inverting, an air bubble rose to the top of the curve. The bubble was big enough that it separated the water on the left and right. I let my thumb off the lower side, and only a trickle of water came out (most was held in the tube by air pressure). Then, I let my thumb off the higher side. Nearly all the water in the lower side rushed up, over the curve, and out the lower side. So, the siphon worked, despite there being an air bubble separating the water in half.
So here’s an example where “tensile strength” cannot be the root cause of the siphon. Personally, I like ZenBeam’s earlier explanation. What say you, hussman?
RM Mentock: Spaghetti! Spaghetti, spaghetti, spaghetti! Alfredo sauce! Boo!
In the article, Cecil writes
I was just tugging my fingers, making my knuckles crack. (I never do this, so I’m feeling kinda creeped out right now…) Anyway, as I understand it, the bubble remains behind for a while, preventing further cracking. I didn’t notice my knuckles being significantly looser after cracking than before, which would be evidence of the tensile strength of the fluid in the joint. I suggest the “considerable force” may just be evidence of ordinary air pressure and/or the strength of your tendons, neither of which would be affected by the small bubble.
(*) Where Cecil writes “the collapse of which”, shouldn’t that be “the formation of which”? Is this a typo, or am I missing something?
:thinking to self: Do not link the spaghetti threads. Do not link the spaghetti threads.
I, too, brew beer at home. There are two steps that require siphoning the beer into another container so as to leave the sediment (dead yeast, hops, protein from the malt) behind. The higher the input vessel is from the output, the faster the flow. Also, when I start the siphon by sucking on the output end, the beer leaves a large void in the tubing on the output side. Eventually, the beer catches up and closes the void. Once the tubing is full of beer, it goes faster. The quality of the initial sucking also determines how long it takes. Occasionally, we won’t be paying enough attention, and some of the sediment gets sucked up. Yet neither this nor quick introductions of air by removing the input end from the beer stops the siphoning. All this leads me to think that air pressure is involved in getting the siphon going, but after that gravity kicks in. Alas, I don’t remember covering siphons at all in my high school physics class. Pity, most everything else I do recall.
Also, point of clarification needed: Perhaps I misread, but did Cecil contradict himself by saying that the output end must be below the input end (good so far) and then later saying that one could siphon up to 34 feet? Or is that just how high the top of the siphon can be before the output has to turn back down?
Everyone STOP trying to incorporate all of these anecdotal evidence as to how a siphon works. You are all making this much more difficult than it already is. And it isn’t that difficult.
First how do you get things moving? Apply a force.
What possible forces could be acting on the water? Gravity and Air Pressure.
Which of these forces is responsible in making the siphon work? Both.
A siphon is a perpetual straw. (Given an infinite resevoir to work with.) If you don’t know how a straw works look it up. It is very simple. Your mouth or a pump creates a negative pressure (relative to the air pressure) and this difference of pressure creates a force that pushes the liquid up the straw. This is so well known and proven that to argue is futile. But, what causes the negative pressure in a siphon you ask?
The answer again is simple. Gravity. As the water falls down the tube it creates a pressure difference. The pressure difference needed to maker the straw work. The longer the tube or difference in height the faster the water falls (again simple physics) which causes a greater pressure difference and therefore a higher flow rate.
I will not go into the no atmosphere siphon again. It is too difficult for this thread. Remember the siphon this way. Pressure is the engine that drives it and gravity is the starter.
The question is not whether gravity plays a role, the question is what drives the upward leg of fluid up the tube while the downward leg of the tube is being pulled out by gravity. If it were just gravity, then the moment you released the opening, the fluid should part at the top of the U and fluid fall out both ends. But it continues to flow in one direction. Why? Either the fluid from the leading end pulls the rest of the fluid through by some sort of tension/suction effect, or the fluid behind pushes it’s way up.
As for the 34 ft comment, the latter interpretation is correct - the U can only reach up to 34 ft before it won’t siphon. The output end must be lower than the top of the input reservoir. Or rather, the top of the output reservoir, if you want to run from one tank into another with both ends submerged.
[hijack]
Dr. Dennis Hussey :
This will sound stupid, because it must be impossible, but can you explain to why this is impossible:
If you can raise water 30+ feet or 18 feet even, why can’t you use this to raise water up to fill an overhead container?
Say you hung a simple trough a few feet over a mountain stream.
Now throw a hose/siphon over the trough.
The water will flow up and over the trough, from the uphill side of the stream to the downhill side.
So far, so good. But now the hose springs a leak, not at the top, but midway on either side.
Couldn’t you collect the falling water in an overhead container?
[/hijack]
Hello,
There are a few ways to explain your question about what drives the siphon. This time I will try and explain it using fluid dynamics.
At the end of the tube sitting in the resevoir there is some sort of pressure if the siphon is in an atmosphere. We will assume that the water is not moving in the resevoir. The water can be moving but for that complex of a problem you need to take a fluid dynamics course to understand. So let us assume that there is no velocity. The siphon is working and there is flow in the tube.
P(air) = P(tube)Velocity(Water in Tube)
This pressure has a gradiant in the tube because of the acceleration due to gravity so it isn’t as simple as that but the mechanism is the same. To get around this all you have to do is take the velocity reading on the out take of the tube. This measurement incorporates the pressure gradiant. Pressure is the driving force that pushes the water up the tube and gravity pulls/push the water down the other side.
Again, the question comes up about pulling the water. Let us ponder pulling things. Internally the molecules are attracted to each other be it atomic force or magnetic force. Water is no different. Its molecules are attracted to each other. When pulled they resist the pulling. However, this resisting potential is extremely small. Exapmle: A water skipper (A small bug) can stand on the surface of water but a person can’t. You may think this is a compression problem but it isn’t. Now as the water is accelerated throught the tube the molecules resist the pulling apart. If the acceleration isn’t too great then the water does not seperate. If the water accelerates too fast then cavitation occurs, water seperation due to pressure/velocity/acceleration differences. If seperation occurs in the tube the siphon will keep on working. As the bubble example proves. It is the pressure difference NOT the molecular attraction that is the overwhelming force behind the siphon. Do NOT even try and understand hussman’s explanation about cavitation and vapor pressure because these are not the driving forces behind a siphon. These things can affect the pressure difference but they are not the drivers. Lastly the molecular attraction is what would make a zero pressure sihon work but, again, it seems to be too complex for this string. If you want to understand it then read my first post.
TheBrain, I don’t believe Irishman was asking what drives the siphon, I think he was trying to explain to you why the thread was continuing. Since he apparently failed, I’ll give it a shot.
You write
This ignores the possibility that the tensile strength of water is a factor which hussman, at least (if you had read his last post), believes is a factor:
I agree that it shouldn’t be that hard, but different people often have to look at things in different ways. Finding a way that “clicks” with a person can sometimes be the hardest part, hence the “anecdotal evidence”.
y’all are still argueing about this??
i thought i set everybody straight a few days ago.
-Luckie
On reflection, I think some of the comments on this topic (including Cecil’s) seem to have missed some basic physics concepts that might be worth reviewing. My last post didn’t discuss the model far enough to create a clear picture of the whole siphoning process. Hopefully the following discussion will do that, though I do not have experimental evidence to confirm the boundary cases of vaccuum or very tall siphons.
hussman gets the hydrostatic equation right, but doesn’t make clear that “dP” is the difference in pressure between some two points and “h” is the difference in height between those same two points. I think things are a little clearer if you reformulate the equation as P=rhog/gch+Po, where P is the absolute pressure, and Po is the pressure at some reference point from which h will be measured. If we measure h downward from the free surface of the water (D in Cecil’s diagram), then Po is “atmospheric pressure”, which is effectively constant over the 30-odd feet of height we are concerned with here. The formula checks out because h is zero at the free surface so P=Po at that point.
Let us assume for a moment that the siphon has filled a container at the output, so the free surfaces of the input and output containers are at the same level (D and C’), and interpret what this equation means at different points along a path through the water from the input free surface to the output free surface. At the input free surface D, the water molecules are being compressed together at atmospheric pressure. As we look at points along the way from D to A, the pressure increases above atmospheric pressure. Proceeding back up through the siphon pipe to a point level with D, the pressure drops back to atmospheric. Above that point, the pressure continues to drop. That is, the pressure in the siphon above D is below atmospheric pressure (though not negative). At some height the pressure will drop to the vapor pressure of water at that temperature, and the water will start to “boil”, but that is most likely above the height any of us have personal experience siphoning over. Since the vapor pressure is above absolute zero, we know that for normal siphons, there is no point in the system where the pressure is below absolute zero pressure, so there is no point at which molecules are being pulled away from each other, and therefore there is no point at which tensile forces are involved.
Since there is no tension involved, using a “lamp chain” as a model of the water in the siphon is misleading. A slightly better model would be filling the siphon with marbles, and holding them together at the ends. Pushing slightly more on one end of the line of marbles than the other will cause them to move through the tube.
However, the marbles model doesn’t really do justice to the problem until friction is introduced. In the absence of friction, the force imbalance from the inlet to the outlet would accelerate the marbles to infinite speeds eventually. The actual stream of water will drag on the walls according to hydrodynamic principles to balance out the pressure difference at a certain fluid velocity. To round out the marbles model, we need to regard each marble as having a force from the marble behind it, another from the marble in front of it, another from the friction with the tube, and gravity. All of these will balance out when the marble moves fast enough, because fluid friction increases with velocity. When the forces balance, the velocity won’t change any more, and the siphon will continue flowing at that rate.
Note that marbles lower in the tube must support the weight of marbles above them, so the marble-to-marble forces will be greater at lower elevations than at higher ones. This is the principle that underlies the hydrostatic equation shown above, but it must be modified when the water is moving. Unmodified, we could start at atmospheric pressure at D and figure the pressure at B, or start at C’ and go to B, and obtain different answers for the pressure at B if the height of C’ is different than D!
Friction accounts for the difference. The pressure at B is below the D-to-B hydrostatic case, and above the C’-to-B hydrostatic case, and everything works out. Before the marbles/water moleecules speed up, the pressure difference will be unbalanced, and they will accellerate until the friction does balance out the forces.
So while you cannot look only at the pressure variation through the siphon to explain the siphon effect, you need not call in “capillary action” or “cohesiveness/tensile strength” or air pressure differential or “impurities” to explain the siphon effect for water.
To address some effects previously mentioned:
In the case of liquid helium, which will “siphon” itself over the lip of a beaker, capillary action is the dominant force. In that case, the fluid weight density is so low compared to the surface tension that it draws itself along the surface against gravity. However, at no point does the pressure drop below ambient as it does in the water siphon, and this is an extreme combination of material properties unrelated to water’s normal properties.
Capillary action is only significant at the fluid boundary with the air, because only there is a net force parallel to the wall. Away from this boundary, the molecular forces between the fluid on one side of the molecule balance with the forces from the fluid on the other side, leaving only the net attraction toward the wall. Thus, once the siphon filled, capillary action could no longer drive the siphon anyway. Finally, we know from experience that you can siphon water with a garden hose, but sticking a piece of hose into a bucket will merely create a slight curl of meniscus… it won’t pull a column of water up noticeably anyway.
This theoretical discussion is in agreement with others’ observations that reasonably small air bubbles will not interfere with the siphon process. However, the lower density of larger bubbles can affect the overall balance of mass between B and C enough to counteract the pressure difference from D to C’.
TheBrain discusses gravity and air pressure as driving the siphon, but in using terms like “negative pressure” and the speed of “falling water” the explanation gets muddy. In any event, the he does not answer the posed question, which is why molecules near A move upward only to arrive much later at a lower altitude. The explanation that the molecules are “pulling” is deficient.
Also, I have to respectfully disagree with Jearl Walker’s assertion that intermolecular forces pull the water over the hump, since as I have shown no such assertion need be made to explain the phenomenon. I will grant that intermolecular forces may temporarily support a small negative pressure in the fluid, so that siphoning may work for a small height in a vaccum, but this effect would only come into play on Earth at the top of a siphon that was above 33 feet (or whatever height corresponds to the vapor pressure at that temperature) from the surface of the water, and it is a non-equilibrium condition anyway. (Jearl admits that air pressure helps in his solution, though with no apparent justification he regards “intermolecular forces” as dominant over the behaviour supported in an atmospheric environment anyway.)
I think I have beaten this discussion out of my system. 