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Water Preasure Please answer

  1. May 6, 2009 #1
    Water Preasure Please answer....


    I'm from the uk, I am a complete novice (no physics PHDs) Please dont Flame me..., I just registered on the site to get an answer to a question that my mates are arguing about. I'm sorry if its trivial or posted in the wrong forum.

    My question is,

    If I filled a glass of water and inverted it and held it above the surface of the tank of water, the water stays in the glass. Now, if you were to imagine a miniature submarine that started at the top of the glass and descended down what would the pressure gauge read as it traveled down the glass?

    And then the pressure at the bottom of the glass level with the surface water of the tank?

    I would then expect the reading to increase in pressure as it descends into the tank as normal.

    If anyone can settle this argument much appreciated....

    adam a physics newbie.
  2. jcsd
  3. May 6, 2009 #2
    Re: Water Preasure Please answer....

    At the surface the pressure is equal to atmospheric pressure(H).As you descend into the water the pressure increase is given by....hdg(h=depth d=water density g=acceleration due to gravity).This gives a total pressure of H+hdg.If you ascend into the glass the pressure decreases by hdg(here h stands for the height above the surface)and the total pressure is H-hdg.
    Last edited: May 6, 2009
  4. May 22, 2009 #3
    Re: Water Preasure Please answer....

    Adam - You've made a barometer. When you invert the glass of water and expose it to the tank, some of the water will escape into the tank - enough to allow a partial vacuum to form at the 'bottom' of the inverted glass. The air pressure between the glass and water will be lower than the ambient air pressure (recall how a mercury or alcohol barometer works) - just enough to 'hold' the remaining water in the glass above the tank level. The pressure gradient from the surface of the glass water to the bottom of the tank is continuous as indicated in Dadface's post.

    The submarine will see a pressure lower than ambient air at the surface in the glass and then as it decends see the pressure increase at ~0.5 psi/ft (it will see ambient pressure at the tank water level as Dadface pointed out).
  5. May 28, 2009 #4
    Re: Water Preasure Please answer....

    I too am a novice on this site and am trying to find an answer to a similar question which is: I filled a tube 48" x 1.5" with water, sealing one end and then inverted it - I expected the water to remain due to the vacuum but it did not. Is there a certain ratio of ht. to dia. required to keep the water in the tube? I would appreciate any imput, guidance or direction on this....Thank you!
  6. May 28, 2009 #5
    Re: Water Preasure Please answer....

    Try again but make sure the tube is completely filled with water.Hold your hand or something suitable over the open end and don't remove it until it is completely immersed under water.Atmospheric pressure is capable of supporting a column of water about 10 meters high so it should support a column of the length you have.The height to diameter ratio does not matter unless the tube has a tiny diameter in which case surface tension effects become more appreciable.
    Last edited: May 28, 2009
  7. May 28, 2009 #6
    Re: Water Preasure Please answer....

    ralph - if you are simply "turning your pipe upside down" chances are the water will just pour out (especially with a pipe as big as 1.5 inch diameter). The key is to keep the open end of the pipe underwater. If you could hold the filled pipe underwater (say while standing in a swimming pool), and then raise it up to a vertical position from below the water, you'd probably have better luck. Also, you need to be sure the closed end is well closed - if air can leak in there it will ruin the effect by allowing the pipe to drain.
  8. Jun 13, 2009 #7
    Re: Water Preasure Please answer....

    Well this seems to be a good site for me as I too am a novice from Canada with a problem. I need a "simple" formula to determine water pressure.

    I have installed a 2 inch high density polyetholine pipe underground for a distance of 2km. The grade on the pipe is between 0.7% and 0.9%. I have estimated I will require about 6.6 ton of water to fill the pipe. How do I determine how much water pressure will be at the lower end of the pipe? The OD of the pipe is 2 inches while the ID is 1.75 inches.

    I have installed valves at both ends of the pipe so I feel confident that I can control the flow of water, but I am worried that when I open the upper valve the pressure will exceed the pressure rating for the fittings at the lower end.
    Last edited: Jun 13, 2009
  9. Jun 13, 2009 #8


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    Staff: Mentor

    Re: Water Preasure Please answer....

    It's just p=mgh. Do you know what the actual height difference is between the two ends of the pipe?
  10. Jun 13, 2009 #9
    Re: Water Preasure Please answer....

    Yes. The top end is 37 ft asl and the bottom end is 2 ft asl. The exact distance between the two points is 5,284 ft.
    Last edited: Jun 13, 2009
  11. Jun 14, 2009 #10


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    Staff: Mentor

    Re: Water Preasure Please answer....

    Well, p=mgh. Since the weight density of water is 62.4 lb/cu ft, p=62.4*35= 2184 lb/sq ft or 15.2psi.

    Often, though, in English units we just use the height as a unit of pressure: that's 35 ft of static head pressure.
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