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Water held in straw by vacuum. Is this possible on grand scale?

  1. Apr 20, 2012 #1
    We all know the trick of holding water in a drinking straw by holding your thumb over the top creating a vacuum I know its also to do with air pressure below. I managed this on a bigger scale using a 2inch pipe, a water pump and a valve i could close at the top of the pipe to replicate the thumb. Now im wondering if this could be achieved on a grand scale ? I would love to create a water feature in my home a 6 or 10 foot wide pipe cut short in the ceiling filled with water that people could walk under, touch or even take a glass of water from! Is this possiblle before i turn my house into one giant puddle ?
     
  2. jcsd
  3. Apr 20, 2012 #2
    Take a glass of water put your hand on it ,turn it and release your hand. I think think this answers your ceiling pool question.
     
  4. Apr 20, 2012 #3

    rbj

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    and i seriously question if you had a vertical 5 cm pipe holding water with the no bottom on the pipe.

    it works for a straw that is thin enough because of the surface tension of the fluid. but a 5 cm pipe will have water trickling down one side with air replacing it on the other side. then that air will bubble up to the top and your vacuum is degraded.

    so frankly, i don't believe it.
     
  5. Apr 20, 2012 #4
    If inside the "straw" or whatever you had a vacuum above the water then I guess you could say the weight of the water will pull it down and the force from atmospheric pressure will try to keep it up.

    mass of the water would be volume*density: ∏r2*H*1000

    so the weight will be ∏r2*H*1000*9.8

    Pressure*area = force

    force= 101325*∏r2

    so we have 101325*∏r2 = ∏r2*H*1000*9.8

    101325 = 9800H

    H ≈10.3m (calculations were done using SI units)

    So I guess you can hold up to 10.3m of water before it falls, independent of how wide the pipe is.

    Who knows though, maybe I'm missing something important when I thought of this.
     
  6. Apr 20, 2012 #5
    A cup won't have both sides open so I'm not sure what you mean?
     
  7. Apr 20, 2012 #6

    HallsofIvy

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    Staff Emeritus
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    And I'm not sure what you mean! A straw doesn't have "both sides open" either- if you have the sides open, the water will not stay- and that's the whole point here.
     
  8. Apr 20, 2012 #7
    martiwood,

    That sounds like the perfect opportunity for a scientific experiment. A straw is a just a tube. You could try getting a series of tubes with different diameters, then seeing at what diameter you could no longer hold the water with air pressure.

    Of course you might want to think about what other variables might have an effect... maybe the tube length too? Or you could try the experiment with different fluids, maybe honey or vegetable oil something :D

    (I am not suggesting you make a mess of the house ;) )
     
  9. Apr 20, 2012 #8
    One might also ask "what would the surface tension of a fluid have to be to pull this trick off in a 10 foot diameter" and then see if any such fluids exist.

    This 10 foot water feature would work in zero to very low G.
     
  10. Apr 21, 2012 #9

    rbj

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    is the bottom of the pipe stuck into a pail of water, like a water barometer? that will work, independent of the diameter of the pipe.

    but i can poke a 1/4-inch diameter straw into water (or soda or juice), put my thumb over the top, pull the straw out (holding it vertically, the same as it was poked into the liquid) and the water does not fall to the table until i lift my thumb offa the top of the straw. (this is different from how a tube of fluid is made into a barometer because the bottom is still inside the pool of fluid.)

    and what i am saying is that you can't do that with a 2-inch diameter pipe unless the liquid is molasses or something like that.
     
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