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Author Topic: How does a siphon work?  (Read 88255 times)

lyner

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How does a siphon work?
« Reply #50 on: 20/10/2008 10:44:27 »
AFK. Re your Brixham experiment. Did you, at the same time, do a control experiment in which you lifted a single sealed, water-filled tube up to the same height? It would be interesting to have compared the two situations.
 

Offline Andrew K Fletcher

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« Reply #51 on: 20/10/2008 11:40:36 »
Sophiecentaur

Your domed tube is still a capped end tube. This still relies on the ability of water molecules to adhere to the dome. The weight of the column of water will tear it away at the 10 meter limit. There is no point repeating what is known.

There is adhesion to the top of the U tube, but it equates to the same adhesion the water has on all of the internal wall of the tube. We know that if the top end is open the water flows freely out so adhesion to the walls of the tube is not required to hold the water up. However adhesion is important to prevent the water from necking as it is placed under tension. in that I mean water molecules being pulled away from the inside of the tube so that vacuum can replace it. Adhesion here is important. What we have with the inverted U tube is water molecules cohesively joined to water molecules in an unbroken column of water from one vessel to another.

If we raise our water filled tubes open ends from the two vessels while the tube is suspended vertically the water in both sides appears to be elasticised and rises up both sides equally.

Now the water could flow out one side or the other as one would expect based on current thinking about liquids. Yet it does not! It remains suspended because the water has been stretched down both sides held only by the link to other water molecules.

If we then blow up one side to increase the pressure you would expect the water to flow out the other side immediately. It does not! The water level on the side you increase the pressure rises but not enough to tip the balance and cause water to empty from the other side.

These experiments have been repeated many times, even at greater heights than the 24 metres at Brixham.

If you would like me to bring a demonstration to your school I would be delighted to do so. Or you could phone Mr Smith (head of science at Paignton Community College and ask him what he thought about water flowing up to the top floor of the college. Mr Smith added; ďI have no problem whatsoever with this experiment. This is exactly how trees lift water, but what can I do about it? I still have to teach the curriculum!Ē

To understand the experiment one needs to abandon pre conceptions about how water behaves under tension and think of it as stretching and contracting.

The video link about the ocean circulation was readily dismissed by yourself. That video shows clearly how a sinking denser ocean water can cause a dragging effect on water from the equator pulling up warm less dense water thousands of miles.

This is highly significant in understanding the U tube experiment. The same downward flowing salt in the one side causes the dilute fluid to flow up the other side. Nothing to do with pressure either. We can eliminate air / atmospheric pressure in the closed loop experiment.
 


I wish you could tell me the difference between the top of a U tube and the top of  a single tube. Particularly if the tube had a 'domed' top.  Assume that the vertical height is greater than the 10m , conventional, limit.
The molecules need to stick to the top surface whether it's a U or just the top of the tube.
On the attached diagram, the region in the upper section of both the single vertical and the U tube are under exactly the same conditions. How is the water supposed to stick to the top of one yet not to the other? How do the molecules 'know' that they are in different bits of apparatus so that they can behave differently? A loop of string stuck to the top of either curve would pull away from the upper surface just as easily.  Whatever the tension may have been, the liquid would not 'stick' to the top any easier for either case.
Can't you see my problem?
« Last Edit: 20/10/2008 12:05:05 by Andrew K Fletcher »
 

Offline BenV

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« Reply #52 on: 20/10/2008 13:07:19 »
I still don't see how your U tube experiment relates to how water is pulled up a tree - a tree is not a closed system - we know that water transpires from the leaves.  Fair enough, this would increase the solutes in the leaves, which would then flow back down, but as it's not closed, it can't be the mechanism you suggest.  Would your tube experiment still work if you put holes in the top of the tube, out of which some water could evaporate?
 

Offline Andrew K Fletcher

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« Reply #53 on: 20/10/2008 13:19:43 »
Ben
This flow and return system does not need any tubes to operate. The experiment was to show how strong the tension is and how efficient the density flow system is, not to represent a perfect mechanical tree.

If my experiment was inside a sleeve filled with water to represent an outer bark, then yes we could have pores in the top of the tube and yes it would cause water to exude at the top as the downard flow would drive the dilute fluid higher as shown in the video exp on youtube.

In fact as I said before Strasburger's experiment used an actual tree that had been killed by picric acid and showed circulation continuing for weeks after the tree had died, eliminating any living processes involved with transport. Killing the tree inevitably liberated solutes and this trickle down of salts from top to bottom maintained the circulation, generating the suction at the base of the trunk submerged in the tub of acid required to draw in the fluid. acid exuded from the dead branches for several weeks and this can also be addressed by using the U spirit level demonstration again on Youtube.
« Last Edit: 20/10/2008 13:33:57 by Andrew K Fletcher »
 

lyner

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How does a siphon work?
« Reply #54 on: 20/10/2008 13:48:55 »
The problem with this thread is that it involves three or more  topics which have been drawn together in order to 'prove' something. In reality, they should be discussed separately  as they do not necessarily support each other.
Much of the previous post (the reply to my post, that is) is fine BUT. You still do not say how the molecules (I assume you subscribe to molecular theory) behave.
Pressure in a fluid acts equally in all directions  (I think you will agree). There are forces between the water molecules and the inner surface of the tube. If there were not, then there would be nothing to keep them in contact when there was low / negative pressure in the tube.  Take a piece of dough, say, and stretch it. The sides move in as the length increases. That is the effect of tension. If you wanted to oppose these forces, then you would have to stick the sides to the inside of a tube - with some other forces. If your column of water is to stay at the same width (i.e. fitting the inside of the tube) there must be forces keeping the surface of the water in contact with the tube wall. These can either be due to pressure (pushing)  or adhesion (pulling) but they have to exist. You have already said that there must be negative pressure because it is at a height greater than 10m so the forces must be adhesive. These forces must exist whether the tube is single or a U.

The temporary tensile strength of an unbroken volume of water is one issue and it has been shown and measured. The conditions for it to work are very critical and it is not a common phenomenon to observe because, once the saturated vapor pressure is reached, evaporation can occur and cause bubbles. That's fine.
BUT you need to do much better than you have done if you want to explain the totality of your experiments in the terms you have done so far.
You surely agree that we need any explanations to be as consistent as possible over all situations and that the behaviour of a substance depends upon local molecular conditions.  Liquids can exist as liquids even when their temperature is above the normal boiling point - well known. That situation breaks down as soon as there is a nucleus around which a bubble can form. Well known.
The situation of a molecule next to the wall of a tube of any shape, under the same pressure conditions must be the same. How can you dispute that? If you cannot tell me how the situations are different then how can your explanation be satisfactory?  I take it that you did not try the experiment with a single tube? You 'assumed' the outcome of that experiment?

Quote
The video link about the ocean circulation was readily dismissed by yourself. That video shows clearly how a sinking denser ocean water can cause a dragging effect on water from the equator pulling up warm less dense water thousands of miles.
Your argument would also imply that it is the hot air 'pulling' cold air in underneath it that causes convection - i.e that even gases can exhibit tension. The more sustainable explanation is that  the more dense, cooler air displaces the warmer, less dense air due to pressure.
Why do you need tension to explain the ocean circulation when all you need is to realise that there is a bit of excess pressure (the more dense stuff sinks and PUSHES the less dense stuff upwards)? You seem to be confusing cause and effect, here.

Benv's comments make good sense. The structure of the equivalent 'tube' in a tree is very different. Not only is there some venting at the top but the Xylem  'tubes' are much thinner than your plastic tubing. Furthermore, they are very irregular and they have connections with the plant all the way up via  perforations.  It is surely the extremely small size of the tubes which gives a clue as to how they work. Adhesive forces between water and many substances are stronger than cohesive forces between water molecules. Narrow tubes (capillary) in the Xylem make use of this difference.
« Last Edit: 20/10/2008 19:30:04 by sophiecentaur »
 

lyner

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How does a siphon work?
« Reply #55 on: 20/10/2008 21:49:15 »
For anyone who doesn't know exactly 'how trees work' or has their own ideas about it, I suggest you take a look at this link. It is a Google Book review and does not show all the pages but there is plenty of evidence which you can read of a well thought out bit of Science which takes the magic out of the mechanisms used by plants to raise water.
The main point about it, as far as I am concerned, is that it depends upon the Xylem tubes being extremely thin. Cavitation is always a problem and can stop the process.

The mechanism does not rely on 'flow' or inverted U tubes. It is described and explained in terms which make sense and do not go against any established ideas.
 

Offline Andrew K Fletcher

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« Reply #56 on: 21/10/2008 08:54:38 »
A nice sweeping blanket statement as per usual. For a start, you do not know exactly how trees work!

Cavitation is not a problem for trees and does not stop the process, because every single xylem will cavitate and repair the cavitation!

It is not discribed in terms which make sense. Why do you think there is as much debate on the acent of sap in tall trees as there has ever been? Just because it is in the curriculum does not make it correct!

BTW: Didn't assume anything. Tried a T junction at the top of the U tube and found it failed at 10 metres as the junction simply caused the water to fall rapidly to the 10 metres and vacuum above it as expected. Thought I could have got away with it. However, even with the valve closed the join caused a seed point for the water to boil.

It is possible that there is a delay between a sink and a return flow that could be measured to prove whether it's tension or pressure changes that causes the Atlantic conveyor to function the way it does.

Gas could be tested in the same inverted U tube to determine if it also has a tension added to it. Something I have thought about doing with Co2 due to itís weight. If the gas flows out of the U tube then you are correct. What do you think should happen if we test Co2 in this way? BTW, I am not sure what will happen with gas either so would rather wait to see what happens. Though the atmospheric pressure will equalise at the same height as the water so we would need a much higher U tube to compensate for this. Interesting.

Andrew K Fletcher




For anyone who doesn't know exactly 'how trees work' or has their own ideas about it, I suggest you take a look at this link. It is a Google Book review and does not show all the pages but there is plenty of evidence which you can read of a well thought out bit of Science which takes the magic out of the mechanisms used by plants to raise water.
The main point about it, as far as I am concerned, is that it depends upon the Xylem tubes being extremely thin. Cavitation is always a problem and can stop the process.

The mechanism does not rely on 'flow' or inverted U tubes. It is described and explained in terms which make sense and do not go against any established ideas.

« Last Edit: 21/10/2008 08:57:18 by Andrew K Fletcher »
 

lyner

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How does a siphon work?
« Reply #57 on: 21/10/2008 10:20:53 »
I am, basically, a reductionist. If there is a theory which explains a phenomenon and it doesn't involve needlessly new complications then I tend to find it acceptable. Science, in general, looks to explain the World with a minimum of 'laws'.
The book in that link manages to give explanations for the phenomena involved with tree sap movement which don't need to introduce any new 'fanciful' ideas. Actual numerical values are quoted and that always reassures me that someone knows what they are talking about. The effect of adhesion and cohesion, taken together is considered and there is a very reasoned discussion of the actual forces involved and the requirement for tubes of the sort of size that Xylem uses. No magic and nothing actually new - just an intelligent approach which uses values drawn from elsewhere in Science.

I mention cavitation because that is something which couldn't be dealt with if the cavities were large.

I did not suggest a T piece should be added; it would obviously be a nucleus for local boiling. I suggested a smooth termination to the top of the tube. Your use of an unbroken length of u tube gave you the best chance of a smooth internal surface in the higher sections. Your explanation that you need tension all round the loop is not the only one which explains the phenomenon.
Yet again, I ask you to tell me the difference in the situation for my two molecules at the top of the two tubes. If you can't either assure me that you did the experiment I suggested or come up with some adequate theory about my two molecules then your explanation (being the 'new' one) is not proven.
As far as I am concerned, there is no need to use your 'cohesion - not- adhesion' idea until you prove it. Conventional arguments about tree sap  use adhesion and cohesion and make good sense.

Measuring the delay in the Atlantic Conveyor could be difficult; the time involved is over 1000 years. Why do you want to find tension everywhere? Pressure differences explain all these phenomena - even the Brixham experiments (the indoor and the outdoor ones) where the two ends are exposed to the atmosphere and one column just happens to be heavier than the other. If you adjusted the heights of the two reservoirs, you could stop the flow or even reverse it. The pressure at the bottom of a column of fluid is ρgh (h is height and ρ is density) If ρ is greater then h will be smaller for the same pressure.

It is not surprising that there is still some 'debate' about the tree sap thing; it is not straightforward, there is a sort of 'magic' about it because it is counter intuitive, in many ways and it is the sort of topic beloved of fringe Science.

There is also a lot of aimless debate about Income Tax, personal health and the Moon Landing 'conspiracy'. It doesn't meant that all views are equally valid.

Have you heard of the Van der Vaal forces? They apply in gases and in liquids. They modify the gas laws, particularly at high pressure and are caused by the asymmetry of charges around electrically neutral molecules. What sort of experiment did you have in mind which could reveal gas tension? Gases are usually only too happy to expand as much as you let them. They have already boiled.


 

Offline Andrew K Fletcher

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« Reply #58 on: 22/10/2008 11:03:23 »
Quote
I am, basically, a reductionist. If there is a theory which explains a phenomenon and it doesn't involve needlessly new complications then I tend to find it acceptable. Science, in general, looks to explain the World with a minimum of 'laws'.
The book in that link manages to give explanations for the phenomena involved with tree sap movement which don't need to introduce any new 'fanciful' ideas. Actual numerical values are quoted and that always reassures me that someone knows what they are talking about. The effect of adhesion and cohesion, taken together is considered and there is a very reasoned discussion of the actual forces involved and the requirement for tubes of the sort of size that Xylem uses. No magic and nothing actually new - just an intelligent approach which uses values drawn from elsewhere in Science.


1st Paragraph beginning with the word Basically adds nothing intelligible.

Quote
I mention cavitation because that is something which couldn't be dealt with if the cavities were large.

2nd paragraph on the other hand admits there is one big problem for the cohesion tension theory as it stands today. Cavitations. Yes, these tiny bubbles have a habit of forming bigger ones and when they form in the fine xylem tubes, which incidentally are much larger than capillary tubes used experimentally to show capillary action, fail to function because of the same 10 metre limit shown in the barometer and Water pump puzzle: Galileo, Toricelli.

This completely destroys the tension theory as it stands. Another problem is and I have said it time and time again, there are 40 metre trees here in Devon, Larch and some Ash-growing in close proximity to each other that have few branches on the top and very few leaves, yet obviously are capable of surviving for many years. The argument for the cohesion tension theory is that the collective loss of moisture from the leaves causes more water to be pulled up and out of the tree. How pathetic is that?

Capillary action. Well is the tree towering a hundred metres able to cause water to be soaked up through the massive trunk of a Californian Redwood at 5 thousand litres a day? I donít think so. If this were the case then rising damp in walls would do the same and it does not.

Root pressure: Donít even get me started on this one.

And then we have Strasburgerís experiments which you failed to address in my previous post. Just in case you missed it. Here it goes again. Take one large tree. Suspend it over a bath of picric acid after it has been recently removed from the soil. While submerged in the toxic soup, saw off the roots, or indeed leave them intact. The acid rises and kills all the living processes in the tree. Now for some 3 weeks or more the tree continues to draw acid up and transpiration continues, even though for all intensive purposes the tree is a skeleton.

Now somewhere I think you mentioned a straw analogy. Take one straw, say 10 metres or more tall. Suck as much as your lungs can bare to suck and se if water rises and flows out the end. But this is not quite like the leaves and branches on the tree is it? As you pointed out in other posts my experiments are too simple to represent a tree. (which incidentally was never my intention) But let us make our straw we are frantically sucking on more like a tree. The leaf for example has pores in it that allow gas to escape. So we stab a few holes in it. Some are closed and some are open, so like a flute we place a few fingers on some of the holes and suck. Do we suck air in or do we suck water up when we couldnít even suck water up without the holes in it?

Now let us look at the barometer. Here we have a glass tube as you suggested we should try. Mercury has been added instead of water so that the it can be scaled down, after all mercury is a liquid much denser than water and it was good enough for Toricelli and the barometer, which incidentally was a happy accident while trying to sort out the age old 10 metre limit problem, has indeed already got the rounded end you suggested should be tried with water. Let us not forget also that Galileo used a longer tube with water. You fail to accept that what this brilliant man concluded about his own experiments and your suggestion was indeed correct in a single suspended vertical capped tube. The barometer does not show mercury adhering to the top of the tube but shows vacuum above it. Toricelli was trying to recreate the pump problem using this set up and failed, but found the reason why it fails and there was born the barometer.

Now for your analogy of lowering one vessel to initiate a siphon.

Our suspended U tube filled with water, ignore the salt for a minute.

We lower one vessel. Remember the observed elastic properties of water I mentioned in a previous post, again ignored by yourself? Well all that is going to happen is that the water will stretch until it breaks. Note you will not pull the water up the other side. Yes I have tested it.

And no there is no ďsort of magicĒ about how trees lift water just a lot of stupid people that canít see the wood for the trees.

Andrew K Fletcher
« Last Edit: 22/10/2008 17:31:16 by Andrew K Fletcher »
 

Offline Andrew K Fletcher

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« Reply #59 on: 22/10/2008 11:41:09 »
----- Original Message -----
  From: Andrew K Fletcher
  Sent: Friday, June 02, 2006 2:15 PM
  Subject: [IAWA Forum] More questions on Circulation within a tree.



  A while ago, the question of density changes in residual leaf and
branch fluids as a direct result of the efficient transpiration from the
leaves of a tree was put to the group. Judging from the responses which
were also posted at the request of one of the groups members, it would
be fair to deduce that there is a general acceptance that density
changes would be an inevitable consequence of the evaporation of 98% of
the water from the leaves.

  several members also began to question what would happen to the sap
once the density had increased and indeed it was suggested that it would
be acted upon by gravity and that the sap would be moved as a result of
this interaction with gravity.

  This brings me to the next part of this important question for the
group.

  Explaining the results of Eduard Strasburger's experiment
  Andrew K Fletcher
  Evaporation from the leaves alters the density of the sap at the leaf,
and gravity pulls the denser sap down. This generates a positive
pressure in front of the falling sap, and a tension / negative pressure
behind the falling sap, which initiates a simple flow and return, much
the same as found in a simple flow and return domestic central heating
system, where the heat from the boiler alters the density of the water
causing the heated water to rise, where it is cooled inside the hot
water tank via a coiled copper tube, returning the cooled water back to
the boiler.
  The German botanist Eduard Strasburger's famous experiment - where he
killed all of the cells in a tree by cutting off the roots, while
submerged in a bath of picric acid - demonstrated that transpiration and
circulation was maintained for three weeks, after the death of the tree.

  I put it to the group that either the picric acid or the copper
sulphate solution used by Strasburger, caused the minerals and sugars
held within the dying leaves and branches to be released over the 2
weeks and that this was all that would be required for a simple flow and
return system to maintain the circulation and transpiration.
Furthermore, the experiment does suggest that no living process need be
involved in the bulk flow of a tree.

  This would result in a downward flow caused by the liberated solutes
and this would in turn generated suction at the base of the tree
sufficient to draw in more dilute solution from the bath, and that this
flow would continue until the liberated salts and sugars had either all
reached the picric acid / copper sulphate bath, or that the liberated
salts and sugars had changed the density of the fluid within the tub to
counterbalance any falling solutes.

  Andrew K Fletcher, UK

http://4e.plantphys.net/article.php?ch=4&id=98

Strasburger himself was an adherent of the school of physics and provided some strikingly efficient demonstrations of water being lifted to considerable heights without any involvement of living cells (Figure 1). He showed that woody stems with their lower end immersed in concentrated solutions of copper sulfate or picric acid and severed by a cut made below the surface of the liquid, will readily suck the solution up. Immediately upon contact, the poisonous fluid kills all living cells in its way, but the copper or the acid arrive in the transpiring leaves and kill them as well. The uptake of the solution and the loss of water from the dead leaves may continue for several weeks, and new solutions of a different color may be lifted in a dead stem.
« Last Edit: 22/10/2008 18:03:43 by Andrew K Fletcher »
 

lyner

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« Reply #60 on: 23/10/2008 17:08:09 »
It's a shame that you don't appreciate my point about Science being reductionist. But, having seen your approach, which tends to be the inverse of this, I should not be surprised. If one is going to have any chance of improving one's understanding of Science then you need as few factors as possible in the treatment of any situation.

I do not propose to have any strong opinions about the way trees work and I think that there are too many facets for the amateur to understand the process in detail.

What I do feel qualified to have an opinion on is the basic way in which your experiment with elevated tubes experiment may work.

Adhesion is not going to happen with a liquid metal because of the delocalised electrons. You won't get any bonding forces like the water molecules exhibit. That's why you get an inverted meniuscus with mercury. It wouldn't stick to the top a a tube any more than it sticks to the sides. It's not a candidate for a scale model. 
I notice that, yet again, you avoid the issue of what molecules actually 'do'. Toricelli, brilliant as he was, knew nothing of molecular bonds so he can't be expected to get it all right. In 2008, I should expect anyone to include the idea in explaining virtually anything.

Do you 'suck'in the air or is it pushed in by the external pressure? Interestingly, in my suggested experiment, there is no air pressure and no water goes up the tube and, in your suggested experiment, there is air pressure and the water moves.

Quoting endless instances of very high trees doesn't make your case better - just one tree would be enough and we all accept that the phenomenon does happen. What we are after, I thought, was some sort of explanation which is consistent with other explanations of other phenomena. That's what Science is all about isn't it?

There are so many differences between your experiments and what goes on in trees that, without some more detailed treatment (involving the molecular level) there is little chance of really improving our understanding of what goes on.

Trees may function after they are dead, for a while,  but the structures and materials were put there whilst it was alive - millions of years of development.
I'm afraid that I find it very hard to give credence to many of your personal Scientific models because you just don't apply the very basic concepts of cause and effect in your explanations. Dense fluids SINK through less dense fluids - explicable in historical and modern terms. Yet, on your video, you claim that they are pulled down. This is right, if you say that it is Gravity which is pulling - but Gravity is pulling everything down. It is the difference in density which makes it happen, the same as in the Oceans.

I leave you with my usual question. "How do the molecules know where they are?" I look forward to an answer. Don't bother with stuff about trees. Let's do one thing at a time.

"Wood for the trees" nice one (LOL), does your theory hold water tho?
« Last Edit: 23/10/2008 17:19:48 by sophiecentaur »
 

Offline Andrew K Fletcher

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« Reply #61 on: 23/10/2008 20:44:16 »
Your question about how molecules know where they are has been ignored because molecules cannot know where they are. If you could re-word your question so it makes more sense, maybe I can understand where you are coming from better.

I have tried to explain that the bead of water behaves how water and other liquids behave under tension. This is cohesion in water. The ability of water to stick to water is stronger than water sticking to the tube. This is why the loop works and the single upright tube close at the top either rounded off or flat, or twice as big at the top than the bottom is only as strong as 1 molecule sticking to the tube. As soon as one molecule pulls away more follow, just like blue tack pulling away from a wall, the water tears away from the top of the tube because of the weight of the water column below. The higher the tube goes the greater the weight of the water column. Obvious really.

The unbroken bead of water has a continued unbroken water cohesive bond, at no point is this bond relying solely on adhesion as is the case with your rounded closed end tube thought experiment.

Having a glass tube made to test your experiment is impractical, will inevitably fail,  And I see no point in considering testing it given the failings of Galileo on the same subject and many more that have followed.

Itís difficult to ask trees whether this flow and return system is holding water. On the other hand it is easy to ask people if it holds water in human physiology and circulation. More to the point it is even more fruitful to ask people with varicose veins to photograph them as they shrink using an inclined bed, when doctors and nurses predict the opposite will happen.

I would love a chance to talk to you and show these experiments so you can see them for yourself. They are quite humbling to physicists, one in particular a good friend and water engineer to boot sat on the floor with his hands on his face shaking his head saying this cannot be. Why have I not been shown this before. Why have I accepted without question what I have been taught? This same man also benefited tremendously from tilting his bed when he realised the significance of this discovery.

Andrew   tel: 01803524117
 

lyner

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« Reply #62 on: 24/10/2008 00:08:05 »
Quote
Your question about how molecules know where they are has been ignored because molecules cannot know where they are. If you could re-word your question so it makes more sense, maybe I can understand where you are coming from better.
Here lies the crux of our problem. The molecules in the water are what determine what it is going to do. A molecule at the top of your U tube will be in exactly the same situation as one at the top of a single tube; they will both have water molecules below them and plastic molecules above them. You have kept on insisting that they will behave differently in each case. I ask the question because there is no difference in the situations for each of the molecules so how can they behave differently? i.e How could they know where they are? You have acknowledged that that is a nonsense question.

Try to read the following carefully and, if you want to answer it, please keep strictly to the content. Neither trees nor human bodies  play any part of the argument or the context here.

Let us consider what is happening at the top of your u tube. First of all,  a liquid will flow and cannot 'keep its shape' without some sort of container - that's the  definition of any fluid. Furthermore there is a fundamental principle of pressure / stress  in fluids and that is it acts in all directions, equally. If you don't accept those two then we must part company - don't bother to read on.

At the top wall of the tube there can either be positive, zero or negative pressure.
If there is positive pressure then the force on the walls will be outwards and balanced by the force from the walls of the (rigid) tube.
If the pressure is negative then there will be an inwards force as well as one 'along' the bead. The liquid will flow 'inwards' or away from the walls unless it is acted on by some other, balancing, force . This force must be due to the attraction of the molecules of water to the molecules on the sides of the tube.
The third option is that there is no force at all - in which case you could take the wall of the tube away and the bead of water would stay intact and behave as it did before.

You must believe that the presence of the tube is necessary for the thing to work - that's the only totally "obvious, really"  thing about any of  this, actually. We can agree that option three does not apply, I'm sure.

The pressure can hardly be greater at the top than at the bottom. I think you would agree - so the first option cannot apply either.

So we are left with the fact that there must be  negative pressure at the top - whether it is a U or a single tube- once the 10m height has been reached. The molecules and the way they interact with their neighbours are what determines how the water will behave. Left to itself, with or without tension between the water molecules, the bead of water would become thinner and thinner (the 'tension' acting equally in all directions) unless the molecules at the interface stick to the molecules on the tube surface. Unless you acknowledge that you may as well say that water is like a solid and that you could replace it with a steel wire running through the tube and expect the same behaviour for steel as water when it emerged at the other end.

Your model implies that the tube is not interacting with the water - or you would have taken the tube into consideration - involving the forces on the molecules. That just cannot be correct or, by the same argument, you would be able to scoop up a handful of water and raise a column - like treacle (and even that will flow back down unless contained within a tube).
If water were like a' chain' then it would not need a tube to guide it - you could just pass it over a pulley. The tube must be doing something which you don't seem prepared to discuss in Scientific terms.
 

Offline Andrew K Fletcher

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« Reply #63 on: 24/10/2008 09:50:08 »
Didn't think anyone was interested in my experiments on the forum enough to go into them in more depth. I agree with the molecules of water interacting with the molecules of the inside of the tube. And indeed called this adhesion. Adhesion in a resin requires the same interaction between the adhesive and the tubular wall.

Trees must make use of this by producing resins in the sap. This must also contribute to the heights achieved by some tree species. One only has to have this resin between the finger and thumb and pull them apart to see how the resin forms fluid strands, much the same as a spiders silk.

Without any tube. The flow and return will take place in or out of a vessel as gravity causes a density flow, which in turn causes other molecules to be dragged behind it, completing a flow and return. Constant evaporation from the surface of a container filled with fluid containing denser solutes will alter the density at the surface.

There are a number of other anomalies going on in the tubular experiment too, for example, when the salt solution flows down and becomes diluted down the flow leg of the U tube, the flow appears to accelerate. I am not sure what is happening here, but having a stab at it, I think that gravity is able to align / polarise the salt molecules and the water to improve the flow. Conjecture I know, but have no idea what else could be introducing a boost because the upward flow of pre boiled / degassed water is maintained, and no further salt is added to the downward flow side. Another possibility is that gravity interacts better with the same amount of salt as it moves down towards the ground (sea level). Any ideas?

The simple boiled sugar solution video shows how the concentrated solution at the bottom of a pan of water on a gas ring operates a flow and return system that prevents water vapour from reaching the surface. Looking closely we can see active boiling below the surface yet the surface is free from bubbles breaking the surface.

Cavitations have been observed in a scaled down flow and return system consisting of a catheter bag, a bladder wash bag, a drip bag, and a network of vessels designed to show how circulation works in the body at the London International Inventions Fair 1997. This circuit worked very well with the drip bag introducing coloured salt solution at the top of the tubular circulation system. The downward flow caused the whole system to circulate and could be operated by, and indeed was, many thousands of visitors. The salts flowed down through a T junction at the bottom was a bladder wash bag to collect the salts, the coloured solution flowed through a catheter bag using a T junction again and the concentrated solution flowed to the bottom of the bag, as clean water flowed out to replace the salt solution lost. A convenient tap on the bladder bag represented the urine being released from the system. Many doctors, nurses, inventors, scientists and government officials for several countries were entertaining the circuit and discussing the implications with me. Yet none of those people were prepared to rock the boat and do something with it. Anyway, thatís history now. The point I was making is that this flow and return model produced itís own cavitations. Everyone could see that the flow continued around them with out problem. Bubbles forming, even 5 mil long were observed to flow down instead of up using small pulsate amounts of salt solution released from the bag to drive the circuit.

Once you have observed this and look at a diagram of either the human body, and animal, insect, plant, tree, or the ocean current, or indeed the experiment at Brixham, it all comes alive in front of you. You donít see inanimate drawings any longer, but a coherent flow and return. It is everywhere in nature, one just needs to understand it to see it.

 

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« Reply #64 on: 24/10/2008 18:47:05 »
There is no point in my replying to your comments because you have avoided addressing the one serious scientific point which would help to solve what is really going on. Another string of instances have not advanced the topic in any way. Clearly your Science is too fuzzy to discuss the issues I brought up. Did you not read what I wrote?
Pity.
 

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« Reply #65 on: 24/10/2008 18:58:34 »
Would anyone else like to contribute some sense?
 

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« Reply #66 on: 24/10/2008 19:06:18 »
You are stating the obvious. Of course the water interacts with the tube, why else would we have performed this experiment had it not?

The analogy of a rope or chain is merely to show the dragging effect of each molecule on it's neighbour. This reaction to movement will make the water flow up or down and even up and down in the same side of the tube if required. Yes a two way flow in the same tube. So how does this fit with your tubular interaction?
 

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« Reply #67 on: 24/10/2008 21:13:11 »
If, as you say, the cohesion is stronger than the adhesion, why does the water not stick to itself and pull away from the tube walls to form an ever-narrowing bead?
Why should the water interact differently with the walls of the U tube and the single tube? I don't think that you have actually considered this in detail.
Most of the links I can find imply that the adhesion between water and glass (as an example) is stronger than the cohesion between water molecules. That explains the positive meniscus with water in glassware. The negative meniscus for mercury is because the adhesion is much less.
But you clearly don't want to discuss what is happening between molecules. You'd rather use  generalised 'chat' ideas which don't help with the Science at all.
What is this 'reaction' you quote in your last post? Is it the Third Law 'reaction' or what?
The only reference I have read from elsewhere which actually discusses your 'tension' (several posts earlier) implies that it is 1. Dynamic (it won't last) and 2. Less than the adhesion. Neither of those facts predict your u tube behaving any differently to a smoothly terminated single tube.
BTW, i will repeat another question - how wide can your u tube before the effect will cease? From your arguments, I would infer that there is no limit - even the total height of the U.
And, please, no more trees until this is sorted out.
« Last Edit: 24/10/2008 22:35:44 by sophiecentaur »
 

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« Reply #68 on: 24/10/2008 23:18:09 »
As you said earlier, the tension must be acting on all the water, so why do you expect it to be acting closer to the wall? The fact that cavitations take place and can be observed taking place is good enough for me. Even with water vapour bubbles in the column of water the bead does not give up immediately. It is irrelevent whether you believe it or not unless you go ahead and repeat the experiments.
But it is worth considering that a molecule has an affect on it's neighbour. This must also apply when the bead of water fails.

How wide will it go? This is a new one for me. I have no idea having never considered it. We have gone higher than 24 metres though only by a few metres more, though with careful hoisting we could I am sure go higher still. The other video's I have show the experiment more clearly. I must find a way of changing them to digital format for the web.

We have not tested the experiment with a larger bore than 6 mil. But I suspect the diameter increasing more will reduce the stability of the experiment due to the inevitable increase in the weight of each column placing more tension on the water if this is what you mean. So diameter should be on a sliding scale to height achieved. logially that is.
 

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« Reply #69 on: 25/10/2008 00:20:41 »
Quote
As you said earlier, the tension must be acting on all the water, so why do you expect it to be acting closer to the wall?
It acts everywhere, as I said. The molecules which are in the bulk of the water will only act on the water molecules immediately around them. The tension will be acting in all directions there but not 'on all the water', just on adjacent molecules. I was interested in what happens at the interface - because that will be where there are molecules of water right next to molecules of plastic. If there are no attractive forces between these molecules then the water will part company with the plastic, pulled away by cohesion. You don't seem to think that this is relevant but, once the water and plastic are separate, the bead will collapse due to its cohesion.  If you try this with mercury it won't work at all because the cohesion in mercury is so much higher than the adhesion to the tube.
How can you not see this? If the tension 'within' the water is greater than the adhesive forces (stress) then the column will collapse and fall. Whatever the shape of the water container below the surface we are considering this will apply.  As long as there are no rough edges, the column will be supported in both the U and the single tube (I have said this so many times) because the molecules on the surface are under precisely the same conditions. What is your problem with understanding that - or in seeing the relevance of it?
If you believe that a liquid can behave in any other way then you must also believe that water ( and liquids in general) behave totally differently from the way they actually do. You seem to be so attached to the notion that the U tube is essential, rather than just happening to work better because of its inherently unbroken surface and because it is so much easier to fill with liquid without bubbles etc.

If you were to melt the end of some tube and blow a good, smooth, bulb on the end, it would allow just the same thing to happen as happens at the top of your U.
Just because you have produced an interesting and novel effect it doesn't mean that you have explained it correctly.
 

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« Reply #70 on: 25/10/2008 08:46:29 »
You are totally and utterly wrong about the shape of the tube not making any difference.

Let me put it in simple terms for you.

On either side of the tube we have suspended a column of water, which stretches like elastic under tension! The walls of the tube cannot collapse and account for the changes in water level when the open ends of the tube are pulled out of the container filled with water.

Now let us deal with the two plates of glass sticking together due to what you, et al say is showing the strength of adhesion in water molecules.

You are obviously adhering to what you teach in physics relating to adhesion being the stronger force so let me clear things up a little for you.

Question 1. Are you assuming that when two plates of glass are pressed together with water between them, the glass is close enough to have only the molecules that are stuck to each other and in contact with the glass. Or are we saying that there is water between the glass which has many water molecules that do not make contact with the glass too?
 

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« Reply #71 on: 25/10/2008 11:08:52 »
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The walls of the tube cannot collapse and account for the changes in water level when the open ends of the tube are pulled out of the container filled with water.
Can we not agree that a force is needed in order for the tube to be having an effect? If there's no force involved then you could take the tube away and there would be no difference - clearly nonsense, so this must be true.
The tube must be doing something. Just 'being there' does not describe what it does.

What does "like elastic" mean?
Elastic ( I think you mean a rubber band) is a solid which needs no container for it to stay in shape. As you stretch a piece of rubber, you distort the bonds throughout the bulk until the forces balance out. The length increases and the width reduces - keeping the volume constant. What is there in a piece of water which will achieve the same thing? All the water I have seen (0-100C) flows, except for small water drops - which we aren't discussing here.

How can your forces only apply 'along' the length of column yet not apply  across the width of it? ("like a piece of elastic")

Re your 'glass plate' question: An ''ideal' pair of glass sheets, which are totally flat (to within the thickness of one molecule) would probably stick together in any case, without water - welding, effectively. In practice, there will be many spaces between. The addition of water molecules would increase the adhesive force because they would provide thousands of times more molecules 'in contact' with each face. That will increasing the 'temporary' bonding between the faces - adhesion at work???

If  you look at any picture of menisci at a glass - water interface, you will notice that the water is pulled upwards at an angle of greater than 45 degrees. How can you explain this if you deny that the water molecules attract each other more than their attraction to the glass molecules? Also, how do you square your idea with the behaviour of mercury in glass? Do you subscribe to the normal ideas of vectorial addition of forces or do we part company here also?

Real Science does not use Metaphor and Simile to describe what is going on. Can you give a reply which doesn't rely on either of these?
 

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« Reply #72 on: 25/10/2008 12:13:26 »
I put it to you that the two not so ideal sheets of glass have many more molecules of water between them, many of which are not in contact with the glass on either side and only have contact with each other. So we are not only looking at adhesion but also at cohesion and each is as strong as each other until the water parts company with the glass. In fact there is little chance of the water being able to share 1 molecule with the other sheet of glass is there? So cohesion conveniently ignored here is very relevant. And when the water does separate from the glass sheets it is because water has been pulled away from the glass, not pulled away from other water molecules.
« Last Edit: 25/10/2008 12:21:30 by Andrew K Fletcher »
 

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« Reply #73 on: 25/10/2008 12:16:43 »
I do get a little tired of sneering remarks about the teaching of Science. Unlike the mass of fringe and fanciful approaches to Science which ignore selected bits of conventional Science yet use other bits, where they suit, good Science and Science teaching try to be consistent and rigorous. If a new idea comes to a 'true' scientist, it will be examined thoroughly to see where it agrees with and where it clashes with the rest of our experience. THAT is the sort of questioning that advances our understanding. Any serious Science teacher will use the conventions because they have stood against many tests, unlike 'alternative' and unproved ideas which come along and pass as frequently as No. 39 buses.
Alternative proponents tend to have an amazing arrogance about their abilities to understand things. We have to protect young minds from a lot of misinformation which presents itself as a crusade against the establishment.


 

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« Reply #74 on: 25/10/2008 13:44:07 »
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and each is as strong as each other until the water parts company with the glass
That is merely a statement. How do you 'know' that water will part company with the glass before some water molecules part? On what evidence is it based? A simple piece of evidence which refutes what  you say is the angle of a water meniscus. Does that not weigh in your assessment? Are you going to ignore the most basic ideas in mechanics, now?
 

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