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Unexpected siphon behavior

  1. Dec 3, 2015 #1
    Hello everyone I'm new here:D I'm doing a project on siphons and I'm trying to understand how a practical siphon works. I did a hell of a lot of reading and the explanations havd gotten jumbled up due to so many misconceptions. So anyways I conducted some experiments and a few strange things happened.

    One, I switched round the long and short sides of the tube and the siphon still worked(suspect that has to do with tensile strength or something, my tube was only as wide as a straw), then I was measuring how changing the height of liquid effects the rate of water flow and I got a curved graph any explanations?

    And as for the driving force, the one that's convinced me the most is atmospheric pressure, indirectly due to the formation of a partial vacuum in the bend of the tube.

    Lastly I tried blocking out the atmospheric pressure expecting the siphon not to work, it did (tell me thats not suposed to happen and that I must have done something wrong)
    -thanks in advance
  2. jcsd
  3. Dec 3, 2015 #2


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    Bernoulli equation has a gravitational potential term = 1/2 v^2 / 2 + g h + pressure / density = constant. Ignoring velocity and compressiblity, then

    g h = constant - pressure / density

    As height of a fluid decreases from the upper surface of a fluid, pressure increases linearly.

    As shown in this youtube video, atmospheric pressure isn't needed for ionic fluid.

    As long as the intake side of the siphon is submerged, there's some pressure related to the height of the fluid above the intake. For any fixed height above the ground, the pressure on the intake side is greater than the pressure on the outlet side because the upper surface of the fluid is higher on the intake side than it is on the outlet side. This pressure differential is what drives the fluid from the higher upper surface side to the lower upper surface side. It doesn't matter how far the siphon intakes or outlets are submerged on either side, as long as both remain submerged.

    Even with ionic fluid in a vacuum, there's still a limit to the height of the siphon above the intake side fluid level. The limit occurs when g h = constant - 0, since once the pressure drops to zero, a vacuum gap will form.
    Last edited: Dec 3, 2015
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