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Gigantic Crazy Straw

  1. May 28, 2007 #1
    Gigantic Crazy Straw!!!

    I just have a quick theoretical question. If a gigantic tube was made out of some super-strong material, and all the air was sucked out, and then it was sealed on both ends, then one end was placed in the vacuum of outer-space, and the other end was placed in the ocean, would the water get blown into outer-space?

    Edit: I forgot to add that after the tube is in position with one end in space and the other in the ocean the ends would be unsealed.
     
    Last edited by a moderator: May 29, 2007
  2. jcsd
  3. May 28, 2007 #2

    Hurkyl

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    What would blow the water?
     
  4. May 29, 2007 #3

    chroot

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    It won't get blown into outer space, but it will rise about 33 feet up the tube. What you've invented is a gigantic barometer.

    - Warren
     
  5. May 29, 2007 #4
    However, you said that the ends were sealed? How would the water get up the tube if it's sealed?
     
  6. May 29, 2007 #5
    Why 33 feet? Is that assuming a certain tube diameter?

    According to Wikipedia (http://en.wikipedia.org/wiki/Barometer): "Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir."

    In the case of my question the "reservoir" would be the entire ocean, wouldn't it? So then the weight of the water in the tube would continue to increase until it equaled basically the weight of earths entire atmosphere, right?

    If not, would it work better if the end which was in the ocean was placed deeper, like at a depth of a few miles, so there is also the pressure of the ocean, and not just the air?
     
  7. May 29, 2007 #6

    chroot

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    Okay, so you've looked up how a barometer works. The pressure at the top of the tube is vacuum, and the pressure at the surface of the reservoir is 1 atmosphere.

    There's a relationship between the difference in pressures and the height of the column:

    [tex]P_2 - P_1 = \Delta P = \rho g h[/tex]

    Where [itex]\rho[/itex] is the density of the fluid (1 g/cm^3 for water), g is the acceleration due to gravity (9.81 m/s^2), and h is the height of the column. If we assume a barometer operating on earth, P2 is one atmosphere, and P1 is zero atmospheres (vacuum).

    If you solve this equation for h, you'll find that it's about 33 feet. In other words, a column of water 33 feet tall has the same weight as a same-sized column of air all the way to the top of the atmosphere.

    Notice that neither the circumference nor the shape of the tube matter. If you increase the diameter of the column of water by a factor of two, you increase its volume by a factor of four. If you increase the size of an imaginary column of air by the same factor, you increase its volume by the same factor. The ratio of the weights will remain the same. The result is that neither the volume of the columns, nor their size and shape, matter.

    Neither does the depth to which the tube is submerged. If you submerge the tube further in the fluid, the pressure at the end of the tube does go up linearly, but so does the length of the tube. The two effects cancel out, so it doesn't matter how deep you put your tube into the ocean.

    - Warren
     
    Last edited: May 29, 2007
  8. May 29, 2007 #7
    Okay, I guess that makes sense, now that I think about it, because if an 80 mile tube was filled with water and stood on end the water pressure at the bottom would be massive, so it would just flow back into the ocean, until the pressures were the same as everywhere else. Thanks for your explanation.
     
  9. May 31, 2007 #8
    Hi!

    I suggest that some other effects should be taken into considerations:
    eg. water has a saturation pressure, at a given temperature in a sealed tank with liquid water in it the pure gas of water reaches the saturation pressure. So, i believe that the water top of the tube would constantly phaze into gas, till the system reaches some equilibrium.
     
  10. May 31, 2007 #9

    chroot

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    This is a good point. A lot of water would certainly vaporize under these conditions, but more water would flow into the tube until the system reaches equilibrium. In this case, since the vacuum end of the tube is open, the equilibrium that will be reached will be hydrostatic equilibrium, with the greatest water vapor pressure at the bottom of the tube. The water molecule is too heavy to effectively escape the planet's gravity at normal temperatures.

    This is a somewhat small effect, though. The water won't climb a full 33 feet up the tube, but nearly so. The vapor pressure of water at 25C is about 24 mmHg, so we're talking about a change of perhaps 3% in the final height of the water column.

    - Warren
     
  11. May 1, 2008 #10
    Temperature

    Also, would the water not freeze when it got to a certain height in the atmosphere where the temperature would be 491.67 Rankine? Would any added pressure of being in the vacuum contribute to a change in the freezing point?
     
  12. May 2, 2008 #11
  13. May 5, 2008 #12
    would it matter that gravity is not constant throughout the length of the tube?
     
  14. May 6, 2008 #13
    ...and what if we used fresh water instead of salt water? Would we see the same results with distilled water?
     
  15. May 6, 2008 #14

    russ_watters

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    No. The water only rises about 33 feet and the change in gravity is negligible over that distance.
     
  16. May 6, 2008 #15

    russ_watters

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    Just about - except that distilled water is slightly lighter than salt water, so it would rise slightly higher.
     
  17. Jun 6, 2008 #16
    Wouldn't the point be though, that by unsealing the end in space, the air in the tube should vent into the extremely low pressure of space to equalise, thus leaving nothing to act against the pressure of the water at the bottom of the tube?

    X
     
  18. Jun 6, 2008 #17

    Doc Al

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    Nothing except the pressure of the atmosphere, that is. Which can support a column of water about 33 feet high.
     
  19. Jun 6, 2008 #18
    Oh I get it... :)
     
  20. Nov 23, 2008 #19
    Re: Gigantic Crazy Straw!!!

    vacuum is vacuum
     
  21. Nov 23, 2008 #20

    russ_watters

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    Re: Gigantic Crazy Straw!!!

    Welcome to PF - it's been 5 months since the last post in this thread...
     
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