Australian physicist spots dictionary error
in [Physics]
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From: jmorriss on 15 May 2010 23:04 On May 12, 6:10 pm, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > Wrong. You cannot siphon anything in a vacuum. This is because (as the > dictionary definition correctly states) it is air pressure which pushes the > liquid through the siphon. > So the air pressure is greater at the top opening of the sipon than it is at the bottom opening? Don't think so... The driving force of a siphon is the difference in fluid pressure given by the formula: Pressure = density x g x depth as calculated from both ends of the siphon for any particular point in the siphon. And the only component that <drives> the siphon is the part with "g" in it... Atmospheric pressure helps keep the calculated pressure from going negative at the top of the siphon.
From: Jerry on 16 May 2010 06:03 On May 12, 9:02 am, Uncle Al <Uncle...(a)hate.spam.net> wrote: > OTOH, try siphoning seltzer. I presume you mean that it is impossible to suck soda up a soda straw? I just finished using a trimmed flex-straw to equalize the fill levels of two glasses of cola. Experiment trumps theory. Jerry
From: Peter Webb on 16 May 2010 08:18 <jmorriss(a)idirect.com> wrote in message news:8758573c-c219-4999-8cc2-6a826cd9a314(a)s41g2000vba.googlegroups.com... On May 12, 6:10 pm, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > Wrong. You cannot siphon anything in a vacuum. This is because (as the > dictionary definition correctly states) it is air pressure which pushes > the > liquid through the siphon. > So the air pressure is greater at the top opening of the sipon than it is at the bottom opening? Don't think so... The driving force of a siphon is the difference in fluid pressure given by the formula: Pressure = density x g x depth as calculated from both ends of the siphon for any particular point in the siphon. And the only component that <drives> the siphon is the part with "g" in it... Atmospheric pressure helps keep the calculated pressure from going negative at the top of the siphon. _____________________________________________ You are simplifying and misrepresenting my argument. The water flows out the "down" half of the siphon through gravity. However, what pushes the water up the top half is air is pressure. In the absence of air pressure, the water would flow downwards from the top in both directions. You can see this if you lift the higher end of the siphon out of the water, the water in the siphon drains back into the top reservoir. It is not directly "pulled though" by the water falling into the lower reservoir; when the water starts draining out into the lower reservoir it creates a pressure differential between the pressure at the top of the siphon and air pressure, and the air pressure pushes the water up to the top of the siphon to replace that draining into the lower reservoir. If water had tensile strength, and this caused the operation of the siphon (as others had claimed), then you would be able to start the siphon with it going over an obstacle of greater than 10 metres high (= 1 atmosphere of pressure) and then lift the high point of the siphon until it is more than 10 metres high, and the tensile strength of the water would keep it operating. This does not work.
From: Greg Neill on 16 May 2010 09:52 Peter Webb wrote: > You are simplifying and misrepresenting my argument. The water flows out the > "down" half of the siphon through gravity. However, what pushes the water up > the top half is air is pressure. In the absence of air pressure, the water > would flow downwards from the top in both directions. You can see this if > you lift the higher end of the siphon out of the water, the water in the > siphon drains back into the top reservoir. It is not directly "pulled > though" by the water falling into the lower reservoir; You're 'cheating' in using that argument, since it is accepted that tension cannot be maintained without containment and maintaining a continuous column of fluid. Once you open the end of the tube you no longer have a siphon. Note also that for narrow tubed siphons working with a significant head, the suction caused by the water moving on the downward leg can be great enough such that very little or no water will return to the upper reservoir when the tube end is lifted above its surface. It behaves sort of like a 'wet-vac'. > when the water starts > draining out into the lower reservoir it creates a pressure differential > between the pressure at the top of the siphon and air pressure, and the air > pressure pushes the water up to the top of the siphon to replace that > draining into the lower reservoir. How could it create that pressure differential without tension in the column? You're defeating your own argument. > > If water had tensile strength, and this caused the operation of the siphon > (as others had claimed), then you would be able to start the siphon with it > going over an obstacle of greater than 10 metres high (= 1 atmosphere of > pressure) and then lift the high point of the siphon until it is more than > 10 metres high, and the tensile strength of the water would keep it > operating. This does not work. You make this claim, but can you back it up with empirical evidence? One can point to many examples of water tension operating over more than 10m height differential -- just look at a forest.
From: Peter Webb on 16 May 2010 23:44
"Greg Neill" <gneillRE(a)MOVEsympatico.ca> wrote in message news:NMSHn.279494$kj4.243910(a)unlimited.newshosting.com... > Peter Webb wrote: >> You are simplifying and misrepresenting my argument. The water flows out > the >> "down" half of the siphon through gravity. However, what pushes the water > up >> the top half is air is pressure. In the absence of air pressure, the >> water >> would flow downwards from the top in both directions. You can see this if >> you lift the higher end of the siphon out of the water, the water in the >> siphon drains back into the top reservoir. It is not directly "pulled >> though" by the water falling into the lower reservoir; > > You're 'cheating' in using that argument, since it is > accepted that tension cannot be maintained without > containment and maintaining a continuous column of > fluid. Once you open the end of the tube you no longer > have a siphon. > > Note also that for narrow tubed siphons working with a > significant head, the suction caused by the water moving > on the downward leg can be great enough such that very > little or no water will return to the upper reservoir > when the tube end is lifted above its surface. It > behaves sort of like a 'wet-vac'. > >> when the water starts >> draining out into the lower reservoir it creates a pressure differential >> between the pressure at the top of the siphon and air pressure, and the > air >> pressure pushes the water up to the top of the siphon to replace that >> draining into the lower reservoir. > > How could it create that pressure differential without tension > in the column? You're defeating your own argument. > I am treating tension as meaning the ability of the material to resist materials being pulled apart. Liquids are easily "pulled apart". The characteristic of water that I am using is its incompressibility, which may be considered a form of "negative tension". Most liquids have very strong forces preventing their compression, but none at all preventing them being expanded into a vapour in the absence of some othe forces (eg air or water vapur partial pressure). >> >> If water had tensile strength, and this caused the operation of the >> siphon >> (as others had claimed), then you would be able to start the siphon with > it >> going over an obstacle of greater than 10 metres high (= 1 atmosphere of >> pressure) and then lift the high point of the siphon until it is more >> than >> 10 metres high, and the tensile strength of the water would keep it >> operating. This does not work. > > You make this claim, but can you back it up with empirical > evidence? > Only the fact that pumps cannot pull water up more than 10 metres, or over obstacles higher than 10 metres, and this has been known since the early days of the steam engine when one of its first uses was draining water out of mines. > One can point to many examples of water tension operating > over more than 10m height differential -- just look at a > forest. > > Well, no. You can point to many examples of water being pumped over heights greater than 10 metres. You have not proved that for the respiration of trees the mechanism is based on water having some form of structural strength (resisting tension) or even what that mechanism really is or means. In the world of engineering, where we know exactly how thing work, we have lots of examples of water being pumped up greater than 10 metres - in dams used for storage of hydroelectricity, mining, and elsewhere. These always have the pumps at the bottom (if the height to be pumped is greater than 10 metres), and none of them as far as I know are based upon water having any tensile strength. They "push" liquid through the pipe, either from air pressure or a pump increasing the pressure at the bottom, not pull it using water's intrinsic "tensile strength". How trees suck up water is not known exactly. However, if you believe that water can be pulled through tubes using its tensile strength, then this would have a bazillion practical uses, the most immediate and obvious being that you could pump water up heights excluding 10 metres by putting the pumps at the top, filling the pipes with water, and then sucking the water up more than 10 metres using its tensile strength. I have never seen this at a dam or heard about it in a mine. Have you a single human engineering example where the supposed tensile strength of water is used to make siphons or pumps which will lift water more than 10 metres by "pulling" at one end (using tension) rather than pushing at the other end (using incompressibility) ? |