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_________ __ / ___ \ | | | / \ | | | | | | | | | | | | | | | | | | | | | |__| |__| |__| | | | | | |
|__________| |__________| <- concentrated solution | | | | | | | | .......... | | | | |__| |__| | | | | <- water |________|
quote:Rosy, the concentrated sap in the phloem that is not actually taken up by the trees increasing growth cycle, is rediluted by incoming water from the soil, under a negative pressure generated by the falling sap.
quote:As the Brixham experiment demonstrates, it is possible to raise the water in excess of 14 metres, but eventually the water bead will break. When I say a siphon will not work, it is because I have actually tested it!
quote:When me and Rosy are talking about negative pressure, we mean negative absolute pressure - where a vacuum is zero pressure.
quote:The reason why GCSE textbooks tell you that a syphon will not work above 33 feet is that atmospheric pressure is enough to lift water 33feet, so up to this point the water is under compression by the atmospheric pressure. If you go above 33feet the water is in tension (a negative pressure) and should therefore boil or cavitate or something breaking the syphon.
quote:Now as you have found out real life is rarely as simple as GCSE textbooks, and water can actually survive a negative pressure if it is continuous, there are minimal dissolved gasses (which you removed by boiling) because of the cohesiveness of the water. It is not stable like this and a small bubble will cause it to cavitate. However if there are no gasses this is unlikely enough for you to do your experiment in Brixham.
quote:Now if you are using the same liquid in both tubes the pressure in the tube is only dependent on how much weight there is pulling on it, and the chance of cavitation is just dependent on the pressure. So the only difference between a normal syphon and your syphon is that the extra weight is provided by an extra length of water rather than salt.
quote:What would you expect to happen to the water levels in the tube, when you remove the both ends of the tube from the bottles, while it remains suspended above the 33 feet limit?
quote:Originally posted by l_kryptoniteOr you could get involved in the incredibly complex discussion being held in the general science section. I need to do about 3 years of study before I get back into that one though. Way out of my league.
quote:Your tube is pretty rigid, but if you squeeze it really hard I expect it will deform a little bit, you would only need a 5-10 percent deformation to cause a .5m movement in the water. Are you using the flexible clear PVC tube or the translucent white much more ridgid stuff?
quote:Hang on a minute - do I understand you correctly in that the bottom half metre of the tube empties and is full of air, and then stops?
quote:Have you done anything else other than removed the demijohn? because just removing the demijohn will not alter the pressures at all - so you haven't done anything to the water column apart from let water fall out of the bottom,
quote: eg you shouldn't have altered the tension in the elastic band (whether the elasticity is due to water stretching or the tube deforming) as you haven't changed the size of the weights on each end... did you pull the ends out of the demi-johns by lifting the whole apparatus or just the ends of the tubes?
quote:ps. by the way my calculation above was considering the force from the whole earth on the hydrogen atoms in a water molecule. Perhaps I should have said that on a molecular scale the earth's gravity is a very small force.
quote:Yes, itís the rigid translucent stuff! The softer walled tube will simply neck )( under the negative tension. This rigid stuff does not neck and therefore the diameter internally will not reduce as a result of the negative tension. If I were able to squeeze it and alter its shape, it still would not alter the volume, as in order to do this one would have to compress the tube equally from all directions and this would take a huge force.
quote:Just the ends of the tubes!Not quite correct Dave, there has been a reduction in the weights, because the water in the two bottles has been disconected, and thes do have considerable weight. Consider the water in the bottles as part of the mass of water inside the tubes and you begin to understand how trees draw water and mineral from the surounding soil into their roots, or into a cut stem or trunk, with no roots.
quote:I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.
quote:Did you lift the tubes up or down when you removed the tubes?
quote:So if you pull the tube out of the bottle, unless the level of the end of the tube is different to the level of the water in the bottle nothing has changed.Did you lift the tubes up or down when you removed the tubes?
quote:I am not sure what you mean by homeostasis, as it is not in the oed and the only definition I can find is that it is a biological system that is stable due to negative feedback. Some systems acting under gravity are stable due to negative feedback - eg water in a glass is stable, but to say everything acting under gravity is under negative feedback is ridiculous - there is no way that a cricket ball in the air is going to be held in position... I am confused by what you mean.[/quote][/quote]ho∑me∑o∑sta∑sis (hm--stss)n. The ability or tendency of an organism or a cell to maintain internal equilibrium by adjusting its physiological processes. The processes used to maintain such bodily equilibrium.fits ok with this paradigm and discussion on trees and plants?Your cricket ball is in the air, because gravity holds the atmosphere in place, and it will eventually come back to earth and its ultimate resting place, due to the inevitable effects of gravity, no matter how hard you throw it. Just like a nuclear explosion is brought back under control by gravity"The explanation requiring the fewest assumptions is most likely to be correct."K.I.S. "Keep it simple!"
quote:Sorry, I did not make myself clear about the requirement of compressing the whole tube equally. I was relating to the negative water causing the tube to collapse equally, as this would be the case with a liquid under tension. Not at all like a finger and thumb compressing it.
quote:ho∑me∑o∑sta∑sis (hm--stss)n. The ability or tendency of an organism or a cell to maintain internal equilibrium by adjusting its physiological processes. The processes used to maintain such bodily equilibrium.fits ok with this paradigm and discussion on trees and plants?