Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: in on 08/11/2006 12:35:39
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Hi I was reading with interest a page on the idea of a vertical pipe stood in a pool of water, and a vacuum pump applied to the upper end. I am interested in this subject.
http://www.du.edu/~jcalvert/tech/fluids/hydstat.htm#Maxh
Does the column of water have any affect on the pressure at the bottom of the beaker it is standing in. For instance if a column of water was to be closed by a door from the liquid in the beaker, would the pressure of the water be changed. does the column of water have no affect on presure of water in the beaker it is standing in. I think this is an important scientific question and i can find no answer to it. It would be common sense that standing a column of water would mean a higher pressure excerted on the beaker, but is that not the reality. As the atmposheric pressure was already excerting pressure. Does anyone have the technological equipment to do this experiment. As i don't.
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You want to do an experiment ? All you need is a beaker, a test tube (or a tube with a stopper on one end), some water and some other liquids (with other densities mainly). To remain on the safe side, use saturated salt water as the other liquid (density is about 1.20) Your test tube (or stoppered tube)will replace the long tube with the door (lid ?) at the bottom.
The door or lid at the bottom of the tube prevents the pressure in the tube from being transmitted to the liquid in the pond, just as the bottom of the test tube prevents the pressure from being transmitted to the liquid in the beaker.
One question remains : how are you going to measure the pressure in the pond - or in the beaker in the experiment ?
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My gut feeling is that if the tube is supported in some external manner the weight of water is wholly supported by the beaker and the fact that some is sucked up the tube is irrelevant.
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I think what you are forgetting is the atmopheric pressure pusing down on the water in the beaker. This is holding up the water in the pipe - as many physics teachers have repeated -there is no such thing as suction.
So the weight of the water is exerting pressure on the beaker, but it must necessarily be the same as the pressure the air is applying to the beaker (otherwise the water would move to equalise the pressures).
Thus if you closed a door on the bottom, nothing would change.
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My gut idea is of course wrong, if the tube is supported externally the water "sucked" into it is in effect removed from the beaker hence the weight of beaker plus water is reduced.
If the tube is supported by the beaker (sits on the bottom) it is a different matter the water in the tube is then supported by the beaker and the weight of the water plus beaker is unchanged.
If we consider pressure, the water "sucked" up the tube will reduce the level of water in the beaker hence the pressure it exerts on the bottom of the beaker.
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Hi. I think this has to be proved in an experiment. Does anyone have a watch barometer. It would have to be the same amount of water in the beaker as was before the water pulled up the water. So extra water should be poured in to replace that in the column. My theory is that pressure will be different. After all if the atmposhere is pressing down, and keeping the water up. Yet it can only produce a column of water 34 feet high. Then when the column is not 34 ft high then the column may be putting lower pressure on the beaker than the atmosphere would.I used to htionk the pressuyre would be higher, but I worked out a few weeks ago it might bne the other way around. Does anyone have a barometer that could work underwater to proove this. I tried the straw tactic.
In response to the 1st response i tried an experiment but i have no expensive equipment. I tried the straw exerpiment where you breath into a straw at different depths, but it is not accurate enough.
Would anyone be prepared to do an experiment themself, and give me the answer.
I tried another experiment where i filled a bottle of water dilluted it with vimto, so that it would be heavier than the liquid in the sink then closed the bottle top, then opened the bottle when it was standing upside down in the sink. The vimto water was difusing with the water in the sink. Which makes me wonder if the water can diffuse then surely there must be a pressure affect too.
I thought there was such a thing as suction. Some say gravity is suction. The old joke gravity Sucks.
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The pressure of the water on the internal base of the beaker will be that which will be produced by the depth of water within the beaker plus the atmospheric pressure.
If the beaker is sitting on a spring balance variation in the atmospheric pressure will have no effect on the indicated weight as air pressure pushes not only down on the top but also up on the bottom.
If the depth of water is reduced by sucking up some of it into a tethered tube the weight will be reduced, the only effect air pressure will have is that it will control the possbile height of water within the tube (about 34 feet at ground level)
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Have you done an experiment to proove this. I would like someone to do an experiment? Anyway i don't think the water is sucked up. It is held up by the pressure.
I am asking if anyone had a watch barometer. Do you have one? And what would the pressure in the water in the beaker be just below the door, when the door was opened. I think there would a difference in this case. Has anyone tried to do an experiment? I already said the column would be 34 ft.
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It is irrelevant whether the water is sucked up or held up by pressure all that matters is that a quantity of water is removed from the beaker and is now supported in the tube, if you plugged the bottom of the tube and let the air in at the top of the tube the result would be just the same all that is relevent to the pressure at the bottom of the beaker is the level of water in the beaker plus of course the atmospheric pressure