Australian physicist spots dictionary error
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From: Peter Webb on 13 May 2010 04:22 "Greg Neill" <gneillRE(a)MOVEsympatico.ca> wrote in message news:YhGGn.210918$kj4.147410(a)unlimited.newshosting.com... > Peter Webb wrote: >> "Uncle Al" <UncleAl0(a)hate.spam.net> wrote in message >> news:4BEAB508.B3DFD96E(a)hate.spam.net... >>> Peter Webb wrote: >>>> >>>> "Simple Simon" <pi.r.cubed-nospam(a)gmail.com> wrote in message >>>> news:obpGn.7674$Gx2.532(a)newsfe20.iad... >>>>> AFP - >>>>> "An Australian physicist has uncovered an error in dictionary >>>>> definitions >>>>> that has likely stood uncorrected for a century." >>>>> > http://www.france24.com/en/20100511-australian-physicist-spots-dictionary-error >>>>> >>>>> >>>> >>>> Picky, picky, picky. >>>> >>>> Siphons need gravity *and* air pressure. They don't work in vacuums or > in >>>> zero-g. >>> [snip] >>> >>> A siphon requires gravitation, a closed conduit, and a fluid with >>> tensile strength when inside the conduit. >> >> >> Wrong. "Tensile strength" has nothing to do with it. >> >> >>> A vegetable oil siphon in >>> hard vacuum is entirely reasonable if the oil is degassed beforehand. >> >> 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. > > Nope. A fluid with a tensile strength inside a conduit will > act like an Atwood Machine with distributed mass. This will > function just fine, even in vacuum. > Fluids don't have "tensile strength", unless you are referring to the almost microscopic contribution from surface tension. You can't siphon in a vacuum. Liquids are not "pulled through" the siphon; they are "pushed through" by air pressure. The only liquids which could siphoned in the absence of air pressure (or something else to provide the push) are superfluids such as Helium near 0 degrees K, and this is hardly what people are talking about when they refer to a siphon. >> >>> OTOH, try siphoning seltzer. >>> >> >> If by seltzer you mean carbonated beverage (eg Coca Cola), these siphon > just >> fine. > > Evolved bubbles of gas will collect at the high point > of the closed tube. Unless the flow is fast enough to > scour them away, eventually the siphon will be 'broken' > by the gas gap. > > Yes, but that isn't what we are talking about - that's a different time dependent effect, and its not because the liquid is carbonated per se, its because air (or rather CO2) gets into the siphon. What pushes liquids through a siphon is basically the same as pushes liquids through straws, and you can definitely drink Coca Cola through a straw. You could easily siphon out the contents of a bottle of coke.
From: Greg Neill on 13 May 2010 08:46 Peter Webb wrote: > > Fluids don't have "tensile strength", unless you are referring to the almost > microscopic contribution from surface tension. From "Advances in Food Research" by George Franklin Stewart, section VIII, "Tesnile Strength of Fluids": "... Wc = 2T Thus, if the surface tension of water at 20C is 72.8 dynes/cm, its Wc is 145.6 dyne-cm/cm^3. However, the distance at which molecular attraction remains significant is very small, on the order of molecular dimensions of 10^-8 cm. Then the force of cohesion or the tensile strength would be 145.6/10^-8 = 1.456 x 10^10 dynes/cm^2 or about 14,000 atmospheres. The theory of van der Waals indicates a value of about 11,000 atmospheres. The difficulty presented in determining this experimentally lies in grasping a bar of water at its ends." > > You can't siphon in a vacuum. Liquids are not "pulled through" the siphon; > they are "pushed through" by air pressure. You've missed the boat on this one. Fluids do indeed exhibit tensile strength. > > The only liquids which could siphoned in the absence of air pressure (or > something else to provide the push) are superfluids such as Helium near 0 > degrees K, and this is hardly what people are talking about when they refer > to a siphon. Nope. Do some research. Do a google search on "fluid tensile strength" for starters. > > >>> >>>> OTOH, try siphoning seltzer. >>>> >>> >>> If by seltzer you mean carbonated beverage (eg Coca Cola), these siphon just >>> fine. >> >> Evolved bubbles of gas will collect at the high point >> of the closed tube. Unless the flow is fast enough to >> scour them away, eventually the siphon will be 'broken' >> by the gas gap. >> >> > > Yes, but that isn't what we are talking about - that's a different time > dependent effect, and its not because the liquid is carbonated per se, its > because air (or rather CO2) gets into the siphon. Any attempt to decrease the ambient pressure of the fluid, such as by exerting tensile stretching, will cause the gas to come out of solution more quickly, rupturing the column of fluid all along its length. > > What pushes liquids through a siphon is basically the same as pushes liquids > through straws, and you can definitely drink Coca Cola through a straw. You > could easily siphon out the contents of a bottle of coke. Here you're assuming that there's ambient air pressure. No one is arguing that it requires a pressure differential to get an empty siphon started. But once started (the tube filled) the pressure differential is no longer required -- the siphon setup works like an Atwood machine.
From: Peter Webb on 15 May 2010 03:12 "Greg Neill" <gneillRE(a)MOVEsympatico.ca> wrote in message news:RwSGn.18924$Vl1.1333(a)unlimited.newshosting.com... > Peter Webb wrote: >> >> Fluids don't have "tensile strength", unless you are referring to the > almost >> microscopic contribution from surface tension. > > From "Advances in Food Research" by George Franklin Stewart, > section VIII, "Tesnile Strength of Fluids": > > "... > > Wc = 2T > > Thus, if the surface tension of water at 20C is 72.8 dynes/cm, its > Wc is 145.6 dyne-cm/cm^3. However, the distance at which molecular > attraction remains significant is very small, on the order of > molecular dimensions of 10^-8 cm. Then the force of cohesion or > the tensile strength would be > > 145.6/10^-8 = 1.456 x 10^10 dynes/cm^2 > or about 14,000 atmospheres. The theory of van der Waals indicates > a value of about 11,000 atmospheres. The difficulty presented in > determining this experimentally lies in grasping a bar of water at > its ends." > >> >> You can't siphon in a vacuum. Liquids are not "pulled through" the >> siphon; >> they are "pushed through" by air pressure. > > You've missed the boat on this one. Fluids do indeed > exhibit tensile strength. > >> >> The only liquids which could siphoned in the absence of air pressure (or >> something else to provide the push) are superfluids such as Helium near 0 >> degrees K, and this is hardly what people are talking about when they > refer >> to a siphon. > > Nope. Do some research. Do a google search on "fluid tensile > strength" for starters. > >> >> >>>> >>>>> OTOH, try siphoning seltzer. >>>>> >>>> >>>> If by seltzer you mean carbonated beverage (eg Coca Cola), these siphon > just >>>> fine. >>> >>> Evolved bubbles of gas will collect at the high point >>> of the closed tube. Unless the flow is fast enough to >>> scour them away, eventually the siphon will be 'broken' >>> by the gas gap. >>> >>> >> >> Yes, but that isn't what we are talking about - that's a different time >> dependent effect, and its not because the liquid is carbonated per se, >> its >> because air (or rather CO2) gets into the siphon. > > Any attempt to decrease the ambient pressure of the fluid, > such as by exerting tensile stretching, will cause the gas > to come out of solution more quickly, rupturing the column > of fluid all along its length. > >> >> What pushes liquids through a siphon is basically the same as pushes > liquids >> through straws, and you can definitely drink Coca Cola through a straw. > You >> could easily siphon out the contents of a bottle of coke. > > Here you're assuming that there's ambient air pressure. > No one is arguing that it requires a pressure differential > to get an empty siphon started. But once started (the > tube filled) the pressure differential is no longer > required -- the siphon setup works like an Atwood machine. > > I read it, but I still don't believe it. If you have a piston half full of water, and the space above it as a vacuum (or more accurately water vapour at whatever partial pressure water has at that temp), and try to expand the volume in the the cylinder, then the force that is required is a function of the outside air pressure, the same as if the cylinder had no water in it. This is one atmosphere (multiplied by the cross sectional area of the piston). If the cylinder is completely full of water, then the force is 11,000 times stronger. Yet at a microscopic level, the only difference to the forces inside the cylinder is the surface tension between the top of the water and the bottom of the piston. Surface tension is not that strong, and in any event depends upon the composition of the materials - if the piston was made out of Teflon, it would be microscopic. I cannot find a good description on the web as to what is really going on. Unless somebody can provide a compelling theoretical argument, the only way I would be convinced is through seeing this in action. Thanks for the posting, though; you have made me think about this, and quite possibly I have been wrong about this. Depressing to think that I cannot be sure how something even as simple as a siphon works.
From: Benj on 15 May 2010 08:38 On May 12, 3:54 am, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > Siphons need gravity *and* air pressure. They don't work in vacuums or in > zero-g. If I was to pick one of these as they best description of the > mechanism (and its a dictionary definition, not a physics lecture) I would > probably go with air pressure as dictionaries do now; what pushes the fluid > through the siphon is air pressure, it is the proximate mechanism. Note that > you cannot use a siphon to go over an obstacle greater than 30 feet in > height - this is neatly explained by air pressure, and inexplicable if you > only consider gravity. I find it very interesting that physicists can't even explain something as simple as a siphon without a bunch of meaningless jargon and hand-waving! Just show you how stooopid science is while pretending to have "all the answers". Can a siphon work over 30 feet? Yes it can. Consider a rope inside a teflon pipe. Does a siphon usually need air pressure to work? Yes it does! It is air pressure that CREATES the "tensile strength of water". In a vacuum the water will simply boil. So what IS the true "tensile strength" of a liquid? Well, first of all, you have to measure it in a vacuum! If it's measured under pressure then the pressure that is pressing the liquid molecules together and acts like a tension. Thus to split such a liquid apart you must overcome BOTH the "tensile strength" of the liquid and the compression. Liquids that boil in vacuum have NO "tensile strength"! ( or perhaps we should say NEGATIVE tensile strength) Liquids that don't boil do have a tensile strength. The easiest way to measure this is the very clever "cavitation" experiments. In other words what force does it take to rip a liquid apart (make bubbles). For a true load of ignorant rubbish check out the following site from Portland State University. They are clearly trying to take the crown from MIT for the "I have no idea what I'm doing" award. I'll bet they all believe in AGW too! http://web.pdx.edu/~d4eb/tensile/index.htm And for a lesser load of rubbish with much more jargon and fewer mistakes check out: http://www.aip.org/pt/feb00/maris.htm Note that BOTH these sources haven't a clue about how xylems work in trees. In fact, science in general has no clue about this ancient question. The latter, however, is far more correct than the former in that it notes the valves that allow trees to grow tall. If you take time to actually THINK (Yeah I know it's unAmerican!) about what I said above, the function of atmospheric pressure in tree sap will become apparent. Not apparent will be the ELECTRICAL functions of xylem pumping. Effects studied many years ago by J.C. Bose. You remember him? The TRUE inventor of radio instead of Marconi! But what do I know? You guys are all genius experts and I'm just a crackpot!
From: Peter Webb on 15 May 2010 11:06
"Peter Webb" <webbfamily(a)DIESPAMDIEoptusnet.com.au> wrote in message news:4bee497a$0$12241$afc38c87(a)news.optusnet.com.au... > > "Greg Neill" <gneillRE(a)MOVEsympatico.ca> wrote in message > news:RwSGn.18924$Vl1.1333(a)unlimited.newshosting.com... >> Peter Webb wrote: >>> >>> Fluids don't have "tensile strength", unless you are referring to the >> almost >>> microscopic contribution from surface tension. >> >> From "Advances in Food Research" by George Franklin Stewart, >> section VIII, "Tesnile Strength of Fluids": >> >> "... >> >> Wc = 2T >> >> Thus, if the surface tension of water at 20C is 72.8 dynes/cm, its >> Wc is 145.6 dyne-cm/cm^3. However, the distance at which molecular >> attraction remains significant is very small, on the order of >> molecular dimensions of 10^-8 cm. Then the force of cohesion or >> the tensile strength would be >> >> 145.6/10^-8 = 1.456 x 10^10 dynes/cm^2 >> or about 14,000 atmospheres. The theory of van der Waals indicates >> a value of about 11,000 atmospheres. The difficulty presented in >> determining this experimentally lies in grasping a bar of water at >> its ends." >> >>> >>> You can't siphon in a vacuum. Liquids are not "pulled through" the >>> siphon; >>> they are "pushed through" by air pressure. >> >> You've missed the boat on this one. Fluids do indeed >> exhibit tensile strength. >> >>> >>> The only liquids which could siphoned in the absence of air pressure (or >>> something else to provide the push) are superfluids such as Helium near >>> 0 >>> degrees K, and this is hardly what people are talking about when they >> refer >>> to a siphon. >> >> Nope. Do some research. Do a google search on "fluid tensile >> strength" for starters. >> >>> >>> >>>>> >>>>>> OTOH, try siphoning seltzer. >>>>>> >>>>> >>>>> If by seltzer you mean carbonated beverage (eg Coca Cola), these >>>>> siphon >> just >>>>> fine. >>>> >>>> Evolved bubbles of gas will collect at the high point >>>> of the closed tube. Unless the flow is fast enough to >>>> scour them away, eventually the siphon will be 'broken' >>>> by the gas gap. >>>> >>>> >>> >>> Yes, but that isn't what we are talking about - that's a different time >>> dependent effect, and its not because the liquid is carbonated per se, >>> its >>> because air (or rather CO2) gets into the siphon. >> >> Any attempt to decrease the ambient pressure of the fluid, >> such as by exerting tensile stretching, will cause the gas >> to come out of solution more quickly, rupturing the column >> of fluid all along its length. >> >>> >>> What pushes liquids through a siphon is basically the same as pushes >> liquids >>> through straws, and you can definitely drink Coca Cola through a straw. >> You >>> could easily siphon out the contents of a bottle of coke. >> >> Here you're assuming that there's ambient air pressure. >> No one is arguing that it requires a pressure differential >> to get an empty siphon started. But once started (the >> tube filled) the pressure differential is no longer >> required -- the siphon setup works like an Atwood machine. >> >> > > I read it, but I still don't believe it. > > If you have a piston half full of water, and the space above it as a > vacuum (or more accurately water vapour at whatever partial pressure water > has at that temp), and try to expand the volume in the the cylinder, then > the force that is required is a function of the outside air pressure, the > same as if the cylinder had no water in it. This is one atmosphere > (multiplied by the cross sectional area of the piston). > > If the cylinder is completely full of water, then the force is 11,000 > times stronger. Yet at a microscopic level, the only difference to the > forces inside the cylinder is the surface tension between the top of the > water and the bottom of the piston. Surface tension is not that strong, > and in any event depends upon the composition of the materials - if the > piston was made out of Teflon, it would be microscopic. > > I cannot find a good description on the web as to what is really going on. > > Unless somebody can provide a compelling theoretical argument, the only > way I would be convinced is through seeing this in action. > > Thanks for the posting, though; you have made me think about this, and > quite possibly I have been wrong about this. Depressing to think that I > cannot be sure how something even as simple as a siphon works. > > And Benj indirectly does make a good point. In a vacuum, the liquid would explosively boil away, making the question of whether a siphon would work in a vacuum even more moot. Indeed, the reason it boils away immediately and fully into gas molecules is because unlike in a solid there are very few forces holding it together. I still don't buy this tensile strength pulling it through the siphon; as far as I can see, its a push from air pressure. |