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lyner

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How does a siphon work?
« Reply #25 on: 17/10/2008 23:17:53 »
Quote
Add a pinch of salt and this diagram comes to life without the need of a pump!
But that 'pinch' of salt has to be lifted up (requiring gravitational potential energy - the same amount that your pump would supply) and it would also need significant energy to get it to the site in the first place. It isn't free energy, is it? Of course a column of dense liquid will fall when there is a less dense liquid on the other side of the tube. It would be really surprising if it didn't - and for all the well known reasons.

The varicose vein behaves very much like any soft walled tube partially filled. Hold  up one end and the air pressure will press it inwards and the hydrostatic pressure will make the water fall to the bottom; it will go flat. Isn't that just elementary stuff? We all know that the heart doesn't provide all the pumping action for the circulatory system - Leg muscles plus valves in the veins and also the arteries help in the process. What are you proposing which is significantly different? (Actual figures would help.)

The only aspect of your work which is potentially revolutionary is your apparent experience with very high siphon tubes. You would need to repeat it with scrupulous reliability and verification to get people to listen to you.  Credit to you for producing your movie but it will need more than that to convince the world.

Your 'small scale' version is an entirely different matter; it just verifies what we would all expect. You can't scale an experiment validly if you don't also scale the ambient atmospheric pressure.
 

Offline Bored chemist

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« Reply #26 on: 18/10/2008 01:22:36 »
Andrew, you clearly don't understand proper experiments.

I don't need to pick an argument with "name ignored by most people", I just need to point out that they are hardly independent.
 

Offline Andrew K Fletcher

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« Reply #27 on: 18/10/2008 09:39:56 »
That pinch of salt does not need to be lifted. It is a result of evaporation altering the density at an elevated point. This in turn generates the force required to drive fluids to greater heights as shown in the U tube experiment that illustrates different levels and indeed on the video on youtube that shows the actual experiment. The 24 metre experiment and indeed the other experiments were designed to show that circulation takes place

1. Our varicose veins are not partially filled.
2.  we lift the end up as you suggest and the blood does not pool at the bottom as you suggest but either increases or decreases circulation
3. While asleep we are not using the leg muscles so we can rule that out.
4. People who have had veins stripped surgically still observe veins going flat while on IBT so we can rule that out too.
5. The experiment is not a siphon. A siphon does not work at this height. Lowering one vessel will just apply sufficient force to break the bead of water and will not generate a siphon effect. Not surprising really considering the amount of times people have tried to do it for all sorts of reasons. Without the salt in one side you are relying on pressure to move the water and not the flow. Difficult to explain without showing it, so guess we need to do more experiments. I do have more video footage from my experiments on video tape. This will require buying some computer component to transfer the footage to digital.
6. The experiment relies on causing each water molecule to follow the next in a chain reaction generated by the falling salts. Again shown in the simple experiment on Youtube with liquids, salts and sugar dropped in a vase of water. Picture the bead of water as a length of string instead of water for a minute. Clip on a weight at the top of one side and the whole length of string rotates, not because of pressure but because it is string with a weight on one end. The resulting pressures are from the flow, not the flow caused by the pressures.
7. The only aspect of my work is? Again may I refer you to the photographic evidence in the varicose vein study. This study was designed to show the effects of IBT and indeed the pressure changes taking place inside the veins, in a way that ignorance is not an option. IBT has provided some incredible results for people with neurological conditions, yet no matter how incredible, people will find all sorts of excuses for not taking on board the evidence that is put before them. Varicose veins and oedema are simple enough to clearly show the effects of IBT.

You also state that this is common knowledge.  But then have the courtesy to ask what is different, supported by actual figures.

Firstly this is not common knowledge, if it were then every single person with these conditions would be sleeping on an inclined bed.  In fact the common knowledge relates to tilting a bed in the opposite direction so that the legs are higher than the heart. Ask any doctor, surgeon or scientist that has been taught about posture and varicosity, they will tell you the same.

There is a move towards showing how solutes alter the pressures inside vessels at long last. The late Professor Harold T Hammel (Ted) to his friends wrote a paper and indeed forwarded it to me when he learned about my experiments in Brixham.

Am J Physiol Heart Circ Physiol 268: H2133-H2144, 1995;
0363-6135/95 $5.00
AJP - Heart and Circulatory Physiology, Vol 268, Issue 5 2133-H2144, Copyright © 1995 by American Physiological Society
Roles of colloidal molecules in Starling's hypothesis and in returning interstitial fluid to the vasa recta
H. T. Hammel
Department of Physiology and Biophysics, Indiana University, Bloomington 47405, USA.
To begin to understand the role of colloidal molecules, a simple question requires an answer: How do the solutes alter water in an aqueous solution? Hulett's answer deserves attention, namely, the water in the solution at temperature and external pressure applied to solution (T,pe1) is altered in the same way that pure water is altered by reducing the pressure applied to it by the osmotic pressure of the water at a free surface of the solution. It is nonsense to relate the lower chemical potential of water in a solution to a lower fugacity or to a lower activity of the water in the solution, since these terms have no physical meaning. It is also incorrect to attribute the lower chemical potential of the water to a lower concentration of water in the solution. Both claims are derived from the teachings of G. N. Lewis and are erroneous. Textbook accounts of the flux of fluid to and from capillaries in the kidney and other tissues are inadequate, if not in error, as they are based on these bogus claims. An understanding of the process by which colloidal proteins in plasma affect the flux of nearly protein-free fluid across the capillary endothelium must start with insights derived from the teachings of G. Hulett and H. Dixon. The main points are 1) colloidal molecules can exert a pressure against a membrane that reflects them and, thereby, displace a distensible membrane; 2) they can alter the internal tension of the fluid through which they diffuse when there is a concentration gradient of the molecules; and 3) only by these means can they influence the flux of plasma fluid across the capillary endothelium. However, the process is complex, since both the hydrostatic pressure and protein concentrations of fluids inside and outside the capillary vary with both position and time as plasma flows through the capillary.
http://www.fasebj.org/cgi/content/full/13/2/213
Evolving ideas about osmosis and capillary fluid exchange 1
H. T. HAMMEL 2
Department of Physiology and Biophysics, Medical Sciences Program, Indiana University School of Medicine, Bloomington, Indiana 47405-4401, USA
ABSTRACT
When a solute is dissolved in water at (T, pel), the temperature and external pressure applied to the solution, the water in the solution is altered as is pure liquid water at (T, pel -  H2Ol). The liquid water and the water in the solution are in equilibrium when  H2Ol is the osmotic pressure of the water in the solution. Every partial molar property of the water in the solution at (T, pel), including its vapor pressure, chemical potential, volume, internal energy, enthalpy and entropy, is identical with the same molar property of pure liquid water at (T, pel -  H2Ol). This elementary fact was deduced by Hulett in 1903 from a thought experiment; he concluded that the internal tension in the force bonding the water is the same in both solution and pure liquid water, in equilibrium, at these differing applied pressures. Hulett's understanding of osmosis and the means by which the water was altered by the solute were neglected and abandoned. Competing ideas included the notions that the solute attracts the water into the solution and that the solute lowers the activity (or concentration) of the water in the solution. These ideas imply that the solute acts on the solvent at the semipermeable membrane separating the solution and water. Hulett's theory of osmosis requires that the solute alter the water at the free surface of the solution where the solute exerts an internal pressure on the boundary of the solution retaining the solute. Fluid exchange across the capillary endothelium is influenced, in part, by colloidal proteins in the plasma. The role of the proteins in capillary fluid exchange must be reinterpreted based on Hulett's view, the only valid view of osmosis.—Hammel, H. T. Evolving ideas about osmosis and capillary fluid exchange.
FROM 1960 UNTIL his death in 1980, Professor Scholander and I began preparation for this lecture honoring August Krogh. Of course, we did not know then the circumstances that would become available. Few persons admired Krogh more than Scholander did. And no one, I believe, admires Pete Scholander more than I do.
During this lecture, I shall attempt to be a teacher and a provocateur. I hope to increase your understanding of osmosis with some old and some new ideas about the osmotic process. I will reexamine Starling's experiment (1) and suggest new mechanisms to account for fluid exchange across the capillary endothelium as blood flows from one end to the other. My suggestions will be incomplete; so I challenge the reader to search for additional mechanisms whereby to account for fluid exchange between a capillary and its surrounding interstitial fluid.  http://www.fasebj.org/cgi/content/full/13/2/213
 

Offline Andrew K Fletcher

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How does a siphon work?
« Reply #28 on: 18/10/2008 09:46:28 »
Bored Chemist:  A name ignored by most people?
Hardly independent? What does this mean exactly? What are you saying about this person?
 

Offline RD

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« Reply #29 on: 18/10/2008 12:47:57 »
Re: "How does a siphon work ?"

Imagine a bucket on a table containing a long heavy chain, the chain is much longer than the height of the table.
Pulling one end of the chain out of the bucket and allowing it to fall to the floor could set in motion a process which could pull all the chain out of the bucket.

I believe siphoning is like this chain-in-a-bucket analogy.
Tension is possible in liquid, i.e. it can be pulled like a chain (or string), e.g. xylem tubes in trees...

Quote
Imagine a narrow tube filled with water and running to the ground from a treetop 360 feet in the air. Water is free to move in the xylem, and the walls of the xylem tube provide no direct support to the water inside. The support comes instead from the water itself. Its internal cohesiveness makes the column of water act like a long suspended string, and the tension on the molecules at any point in the column must support the weight of all the water below them. Expressed as a pressure, or force per unit area, the tension on the water in the xylem is surprisingly high: for every thirty feet of tree height, the tension increases by roughly fifteen pounds per square inch. For a xylem tube 360 feet high, the tension at the top is 180 pounds per square inch.
http://findarticles.com/p/articles/mi_m1134/is_/ai_n13606614


I believe it could be possible siphon in a vacuum. It would be necessary to pump some of the liquid initially to start the process,
 (analogous to pulling an end of the chain out of the bucket and allowing it to fall to the floor), but once started the flow would be maintained by gravity, i.e. air is not necessary for a siphon to work.
« Last Edit: 18/10/2008 12:53:50 by RD »
 

lyner

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How does a siphon work?
« Reply #30 on: 18/10/2008 14:39:08 »
RD

So are you saying that a liquid is the same as a solid? You imply that water is just like a chain. Inter molecular forces in liquids are  small -  enough to produce surface tension, which is a very small effect. You tow your car with a chain and not a column of water.
Do you have evidence to the contrary?
« Last Edit: 20/10/2008 10:45:55 by sophiecentaur »
 

lyner

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« Reply #31 on: 18/10/2008 15:10:04 »
AKF, your post goes on too long for me to answer all the points but there are some things I need to clear up.
The salt gets up there because energy has been transferred one way or another. It's not magic just heat.

Without muscular activity in the legs, guardsmen faint and airline passengers develop dvt. The veins are distended and go flatter when the legs are elevated. Scout first aid knowledge. The veins are partially full, as I said.
If this tension exists then how can it not show itself everwhere where water is involved?
If you believe anything about the nature of molecules then you have to accept that a molecule can only interact with its close neighbours. You continue to ignore my challenge to tell me how molecules in your model 'know' that they are in your u tube and not in a simple syphon.
« Last Edit: 18/10/2008 18:31:13 by sophiecentaur »
 

Offline Andrew K Fletcher

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« Reply #32 on: 18/10/2008 18:11:00 »
Without muscular activity? A guardsman’s muscles are not relaxed. The pressure on the base of the foot is compromising the circulation. This is why they shift their weight from one foot to another to relieve the pressure and assist the circulation to get past the obstructed vessels.

Airline passengers compromise their circulation by sitting in one position for too long, often falling asleep for hours on end, The dry environment further accelerates evaporation adding to the back pressure as dissolved salts flow down to the restricted vessels. Again pressure on the buttocks compromises circulation, this same pressure is responsible for haemorrhoids developing while sitting on the loo seat for  too long, or as granny used to say “don’t sit on a cold hard surface for too long as it will give you piles.

It does show itself everywhere water is involved. Here is the ultimate demonstration for you:
feature=related
 

Offline RD

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« Reply #33 on: 18/10/2008 18:41:24 »
RD
So are you saying that a liquid is the same as a solid? You imply that water is just like a chain. Inter molecular forces in liquids are her small -  enough to produce surface tension, which is a very small effect. You tow your car with a chain and not a column of water.
Do you have evidence to the contrary?

The xylem example above is evidence of the tension water can withstand...
 
Quote
tension at the top [of 360 foot tree] is 180 pounds per square inch.

180 psi is quite substantial, not a "very small effect". If you had a tube of water with two freely moving,
 (and snugly fitting) pistons, you could tow your car with it if it had a diameter of about three inches.




(the I shapes are the freely moving pistons).
« Last Edit: 18/10/2008 19:39:27 by RD »
 

lyner

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« Reply #34 on: 18/10/2008 18:48:36 »
Quote
Without muscular activity? A guardsman’s muscles are not relaxed. The pressure on the base of the foot is compromising the circulation. This is why they shift their weight from one foot to another to relieve the pressure and assist the circulation to get past the obstructed vessels.

Airline passengers compromise their circulation by sitting in one position for too long, often falling asleep for hours on end,
So are you agreeing or disagreeing that muscular activity (that means movement) aids circulation?
I'm afraid that you've been wasting my time. You give me a link which is supposed to be demonstrating the existence of tension in water. I patiently looked at it all (btw, it was a Kids programme and not a Scientific Presentation - which is about all you can expect on U tube). Nowhere in the link does 'Dr Bob' mention tension. He merely talks in terms of density and classical hydrostatic pressure effects. You have chosen to quote a phenomenon as proof of a fanciful idea you have. That is no evidence, in any way, that you are correct. It can be explained perfectly on the basis of density changes and a resulting pressure difference.

As for the phisiological bits, if you want me to I could calculate the power available from the pressure difference on different sides of your circulation system. The estimated output power of the heart is a couple of Watts (look it up - there's a lot of stuff about it). I should estimate your 'osmotic' power to be in the region of milliWatts. So the heart dominates - other effects are there and probably measurable but . . . so what (WATT)?
Can you accept arguments based on things like power and energy with actual quantities quoted? Or does it smack too much of 'conventional, chauvinistic' Science?
Again, I ask you to give me some evidence of a properly documented phenomenon in which 'tension' is the only explanation?
 

lyner

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How does a siphon work?
« Reply #35 on: 18/10/2008 18:54:09 »
Quote
tension at the top [of 360 foot tree] is 180 pounds per square inch.
That is not evidence - it is a statement.

Quote
If you had a tube of water with two freely moving, (and snugly fitting) pistons,
 you could tow your car with it if it had a diameter of about three inches.
That has been explained in terms of atmospheric pressure by Otto von Guericke years ago. See this link among many others.
http://en.wikipedia.org/wiki/Magdeburg_hemispheres
 

Offline Andrew K Fletcher

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« Reply #36 on: 18/10/2008 18:58:28 »
A californian redwood?

The cohesion tension theory?

My experiment at 24 metres?

No matter how much evidence is before you you will never accept it!


The following letter came from Professor Hammel:

INDIANA UNIVERSITY

SCHOOL OF MEDIICINE date September 6/ 1995

Dear Mr Fletcher:

I received the information you sent me regarding your ideas about fluid
transport in trees, in tubing and in the vascular system in humans.

I will study your ideas and comment upon them as soon as possible. A Quick
scan of your Brixham experiment prompts me to ask if you conducted this
experiment with boiled water without any solute added to the tubing on
either side of the central point which you raise 24 meters? I expect that
you could raise the tubing to the same height with or without solute in the
water. In any case , your experiment confirms that clean water (water that
is unbroken water, water that is without a single minute bubble of vapour)
can support tension of several hundreds of atmospheres. The record tension
obtained experimentally is 270 atmospheres. At 10 degrees C. (c.f. Briggs,
L. Limiting negative pressure of water. Journal of Applied Physics 21:
721-722 1950).

I expect even this tension at brake point can be exceeded by careful
cleansing of the water, to remove even the most minute region of gas phase.
When the water is already broken, as occurs when gas is entrapped on
particulate matter in ordinary water, the water will expand around even a
single break when tension (negative Pressure) is applied to the water. When
you boil the water, prior to applying (2.4-1) ATM negative pressure to the
water in the highest point of the tubing, you eliminate some of these breaks
in ordinary water. I expect that dissolving NaCl or other solutes in the
water will have little or no effect on the way you measure the tensile
strength of water.

I am enclosing some reprints that may interest you. Some of these deal with
negative pressures we have measured in tall trees, mangroves and desert
shrubs. Other reprints deal with how solutes alter water in aqueous
solutions and how colloidal solutes (proteins) affect the flux of protein
free fluid between plasma in capillaries and interstitial fluid.

Sincerely H.T. Hammel Ph.D.

 

Offline RD

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« Reply #37 on: 18/10/2008 19:15:34 »
Quote
If you had a tube of water with two freely moving, (and snugly fitting) pistons,
 you could tow your car with it if it had a diameter of about three inches.
That has been explained in terms of atmospheric pressure by Otto von Guericke years ago. See this link among many others.
http://en.wikipedia.org/wiki/Magdeburg_hemispheres

It is intermolecular forces, not atmospheric pressure, which permits water to withstand tension.

If you want an experiment try separating two sheets of wet glass each about a foot square.
The intermolecular forces between the water molecules act like glue holding the two sheets of glass together,
atmospheric pressure is not required: this glass-sheet demonstration would work in a vacuum.

(The Magdeburg hemispheres demonstration would not work in a vacuum).
« Last Edit: 18/10/2008 19:17:48 by RD »
 

lyner

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« Reply #38 on: 18/10/2008 20:10:41 »
Quote
this glass-sheet demonstration would work in a vacuum.
That, again, is merely a statement. Have you any evidence of this.
You might bear in mind that the  force which has been measured can be explained by atmospheric pressure.
What value do these inter molecular forces have, btw? Can you quote a value from somewhere. You imply that it has been measured. It would have huge implications on things like the boiling point of water, for instance.
 

Offline RD

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« Reply #39 on: 18/10/2008 21:04:15 »
Quote
this glass-sheet demonstration would work in a vacuum.
That, again, is merely a statement. Have you any evidence of this.

The wet glass sheets stick together because of the cohesive (and adhesive) properties of water..

Quote
Cohesion (n. lat. cohaerere "stick or stay together") or cohesive attraction or cohesive force is a physical property of a substance, caused by the intermolecular attraction between like-molecules within a body or substance that acts to unite them. Water, for example, is strongly cohesive as each molecule may make four hydrogen bonds to other water molecules in a tetrahedral configuration. This results in a relatively strong Coulomb force between molecules. 
http://en.wikipedia.org/wiki/Cohesion_(chemistry)

The water really does act like strong glue. Glue does not require atmospheric pressure to function: glue works in a vacuum.

What value do these inter molecular forces have, btw? Can you quote a value from somewhere. You imply that it has been measured.
 It would have huge implications on things like the boiling point of water, for instance.


The strong intermolecular forces between (polar) water molecules are why water has higher boiling point than similar sized molecules, e.g. water (H20) is liquid at room temperature, whereas ammonia (NH3) and methane (CH4) are gases at room temperature.
Hydrogen cyanide (HCN) boils at 26oC.
« Last Edit: 18/10/2008 21:20:16 by RD »
 

Offline Bored chemist

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« Reply #40 on: 18/10/2008 22:20:34 »
Quote
this glass-sheet demonstration would work in a vacuum.
That, again, is merely a statement. Have you any evidence of this.
You might bear in mind that the  force which has been measured can be explained by atmospheric pressure.
What value do these inter molecular forces have, btw? Can you quote a value from somewhere. You imply that it has been measured. It would have huge implications on things like the boiling point of water, for instance.
The value has been measured- it's about 2.5KJ/g
 

lyner

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« Reply #41 on: 18/10/2008 23:28:49 »
Quote
The value has been measured- it's about 2.5KJ/g
KJ/g is not a unit of force.
Do you have a reference so that I could look at your source? It may make more sense than the bald statement.
 

lyner

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« Reply #42 on: 18/10/2008 23:55:58 »
Quote
The strong intermolecular forces between (polar) water molecules are why water has higher boiling point than similar sized molecules, e.g. water (H20) is liquid at room temperature, whereas ammonia (NH3) and methane (CH4) are gases at room temperature.
Hydrogen cyanide (HCN) boils at 26oC.
But water boils at room temperature at an ambient pressure at about 0.1 Atmospheres.  A volume of water inside the cylinder would boil once the pressure difference was reduced to that amount - with about 90% of the force which would be needed to separate the Magdeburg Hemispheres under the same conditions.

It is very easy to show the effect of depressing the boiling point of water under reduced pressure - I have done it in a bell jar with lukewarm water. It just boils when you reduce the pressure with a vacuum pump. If you conduct your experiment at room temperature, why would the same thing not happen at an appropriate pressure?

 

Offline RD

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« Reply #43 on: 19/10/2008 01:27:45 »
« Last Edit: 19/10/2008 01:34:30 by RD »
 

Offline Bored chemist

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« Reply #44 on: 19/10/2008 10:25:11 »
The figure I gave is the latent heat of evaporation. It's a measure of the energy required to separate the molecules against the force that holds them together. As such it is an indirect measure of that force.
To convert it to a force is tricky because you need to know the form of the force/ distance curve so you can differentiate it.
However you can convert the figure from KJ/g to KJ/mol then to J/ molecule and, knowing how many molecules there are in a given area (from the density etc) you can work out the energy required to separate the molecules in a given area. If you assume that the potential is roughly linear over some small distance you can get an estimate of the force required.

I gave that figure just to show that the force and be calculated (it will be very large).
RD has cited a direct experimental value.
The point is that the force can be (and has been ) measured and calculated.

The other point- that the boiling point of water is dependent on atmospheric pressure is a red herring. The other liquids' boiling points also depend on pressure. However water will always have a much higer boiling point because it has strong bonds holding the molecules together whereas things like methane don't.
« Last Edit: 19/10/2008 10:31:20 by Bored chemist »
 

lyner

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« Reply #45 on: 19/10/2008 18:16:47 »
That's fair enough.  I was trying to relate it to a possible 'tensile strength' so the number would be handy.
I don't think that pressure vs boiling point is irrelevant. Doesn't it relate to the energy needed to make a surface molecule break free? If there is even a hint of a surface anywhere within the liquid bulk then it can form a bubble at a low enough pressure and any tension you might have had will not count. This argument would not prohibit dynamic tension, as long as the load is applied briefly enough.

But, if a normal lift pump will not operate  over a greater height than that which corresponds to atmospheric pressure for the liquid density, then how can an inverted u tube support a greater head?
I appreciate that, in a small bore tube, the effects of the tube surface could make a difference but, in a bulk liquid, what can keep a column above that which is supported by the AP difference?
The paper quoted above agrees with my point - it doesn't work for static pressures below the saturated vapour pressure.
 

Offline Andrew K Fletcher

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« Reply #46 on: 19/10/2008 22:07:44 »
6 mil bore tubing was used. The reason the inverted tube works is the bead of water remains unbroken s0 at no point is water relying on adhesion to a surface other than the walls of the tube which does not affect the tension applied to the water inside it.
The water has no surface to be pulled from. Having a pump at the top would require the adhesive properties of water to stick to the diaphragm or piston, which in effect is the same as capping one end of a water filled tube and raising it vertically. Adhesion in water while very strong does not come close to the cohesive properties of water molecules.

The inverted tube worked as it was envisaged to do and supports the weight of two columns of water suspended vertically in the smooth bore tough nylon tubing.

The bead of water remains intact for a long time until water vapour forms into bubbles, these join together and cause the beads of water to separate. Now both levels fall back to the 10 meter limit at sea level and the space above is vacuum. As was the case in the original water pump problem that Galileo and Toricelli were faced with at the castle of the Grand Duke of Tuscany. The barometer was born later from trying to resolve this problem, but the problem to this date stands unless we change the parameters a little by using an inverted single open ended tube.

Andrew


That's fair enough.  I was trying to relate it to a possible 'tensile strength' so the number would be handy.
I don't think that pressure vs boiling point is irrelevant. Doesn't it relate to the energy needed to make a surface molecule break free? If there is even a hint of a surface anywhere within the liquid bulk then it can form a bubble at a low enough pressure and any tension you might have had will not count. This argument would not prohibit dynamic tension, as long as the load is applied briefly enough.

But, if a normal lift pump will not operate  over a greater height than that which corresponds to atmospheric pressure for the liquid density, then how can an inverted u tube support a greater head?
I appreciate that, in a small bore tube, the effects of the tube surface could make a difference but, in a bulk liquid, what can keep a column above that which is supported by the AP difference?
The paper quoted above agrees with my point - it doesn't work for static pressures below the saturated vapour pressure.

« Last Edit: 19/10/2008 22:36:51 by Andrew K Fletcher »
 

lyner

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« Reply #47 on: 19/10/2008 23:35:55 »
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: 19/10/2008 23:38:58 by sophiecentaur »
 

Offline RD

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How does a siphon work?
« Reply #48 on: 20/10/2008 09:12:23 »
I wish you could tell me the difference between the top of a U tube and the top of a single tube...


With reference to my chain-in-a-bucket-on-a-table analogy...

If an end of the chain is lifted to the brim of the bucket and released it will fall back into the bucket.

If an end of the chain is pulled over the brim and out of the bucket and below the level of the table then released, it could set off a process which pulls the entire chain out of the bucket.

The top of a siphon's U tube is analogous the brim of the bucket, and the water, (which can have tension), analogous to the chain.
« Last Edit: 20/10/2008 09:27:57 by RD »
 

lyner

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How does a siphon work?
« Reply #49 on: 20/10/2008 10:40:55 »
RD. All you have done is to repeat the 'analogy'. That is not an explanation in any sense. Of course what you say about a chain is correct. We can all measure the tensile strength of a chain; it's what we call a solid and the molecules are very well stuck together. If the tensile strength of water was as high then it would behave like a solid in many ways - it wouldn't flow, for a start. Or perhaps you would like to reclassify materials in some way.

Siphons don't work in a vacuum, in any case. You can't ignore the way a mercury barometer functions; it actually tells you about how the atmospheric pressure is acting. Without pressure pushing on the reservoir surface, the column collapses. Are you suggesting that a siphon would work at the same rate irrespective of the ambient atmospheric pressure? Is there something about the conditions in the tube leading up to a lift pump which is different from the conditions in the 'up' tube of a siphon? I ask again "How do the molecules 'know' what they are supposed to do in each case?" Do they know where they are? Can they ignore the atmospheric pressure in one case and yet totally rely on it in another case?
Dip your hand into a bowl of water. Can you draw up a thread of water by 'tension?

If you want to justify / explain how this phenomenon is going to work as you predict then you have to say, not only how these intermolecular tensions work to keep the column of water stuck together but what happens on the TOP surface / interface with the material of the tube?  Are the molecules stuck to that too? If not, why won't they drop off due to the low (negative?) hydrostatic pressure in that region and form a void? Forces act in three dimensions and all directions count.

You could, perhaps, also tell me what the limit to this effect could be. You could imagine making the top section tube wider and wider  until the width of the top channel was almost as great as the total height of the loop. Your 'tension' should then also be present. When would the effect stop? At the very last instant when the lower face of the horizontal section dips under the level of the water in the reservoir?

AKF, at least, has seen a phenomenon which needs some explanation and which doesn't confirm normal textbook models. All that you are doing, RD, is to assert a naive idea which is not in accord with the overwhelming opinion and a huge body of evidence.

BTW, there is a great little demonstration to show that air pressure is needed in order for water to be 'drawn' up a tube. If you try to suck up a drink through a straw when an airtight lid (sealed round the straw too, of course) is put on the beaker. If you didn't need atmospheric pressure, then you could suck all the water up with no problem. Try it. Try getting your outboard / lawn mower motor to operate without undoing the breather into the fuel tank. Air has to get in or the reduced pressure will soon stop liquid flowing.
 

The Naked Scientists Forum

How does a siphon work?
« Reply #49 on: 20/10/2008 10:40:55 »

 

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