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Efficiency of siphon

  1. Feb 18, 2008 #1
    Although I generally understand how a siphon works, I am wondering if a siphon would work in a vacuum or in a pressurized enviroment?
     
  2. jcsd
  3. Feb 18, 2008 #2

    mgb_phys

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    Yes in theory. In practice most liquids will boil in vacuum.
     
  4. Feb 19, 2008 #3
    No, a siphon can't work in vakuum, not even in theory. Liquid in a siphon travels against the force of gravity (upwards) because it is pushed by the pressure: pressure must decrease with hight, so that the upward force on the lower end of a small segment of liquid is greater than downward force on the upper end.
    If the pressure is zero on the start of the pipe, it can't decrease with hight since liquid can't stand negative pressures: so liquid can't flow against gravity.
     
  5. Feb 19, 2008 #4

    mgb_phys

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    A siphon works because of surface tension and intermolecular forces in the fluid.
    It's a bit like a chain draggin itself over a pulley.
     
  6. Feb 20, 2008 #5
    Can you explain this? Could you not make a siphon out of a glass tube to drain mercury (which does not wet glass) out of a container?

    My guess would be there must be at least enough pressure to support the column of liquid in the short arm of the siphon. Surface tension can be neglected for all sizes of tubes used in practice. In other words, you couldn't drain the water out of a bathtub on the second floor of the empire state building by running a hose up through a window in the 20th floor and
    then down to the ground again on the outside. (I've never been to the empire state building; I hope it has 20 floors!) Even if the hose were full to begin with, it would develop a vapour-lock when the vapour pressure of the water plus the pressure exerted by the
    water in the short end of the siphon equaled the external pressure of the atmosphere.
    You said it yourself: when the liquid "boils", the siphon does not work.
     
  7. Feb 20, 2008 #6

    mgb_phys

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    There is a limit on how high you can make the hump in a siphon without the drop in pressure causing the liquid in the top of the siphon to boil.

    Other than that it does depend totally on viscosity of the liquid, there are some long chain molecules that will siphon without a tube! If you hang a bit of it over the edge of a beaker the whole lot will flow out onto the floor. Basically the stuff falling down the long arm is heavy enough to pull the stuff in the short arm up.

    Interestingly superfluids (liquid Helium) will also siphon themselves out of a beaker without having to start it - but this is due to them having no viscosity so that any adhesion to the surface of the beaker, however weak, is stronger than the bonds to the rest of the liquid - so this isn't really a siphon but looks like one.
     
  8. Nov 18, 2008 #7
    Have been thinking about a perpetual siphon. Maybe I'm crazy.
    What if the volume in a long downward tube exceeded the container from which the liquid is being siphoned and the end tube was raised above the original container's surface level? Once the syphoning has begun, at some point would not the pressure created from the vacuum of the increased volume in the exit tube be of sufficient length and capacity to compensate for atmospheric pressure and the resistance of the volume of the container being siphoned as well as all other resistances. Unlike the chain on the pulley example, we have introduced vacuum as a force for the unimpeded continuance of flow.
    I am a complete ameteur at this, but I can't get this model out of my head and I think it will work. I should add that the flow from the end of the tube reenters the source tank. A spiral tube should accomplish the length aspect of this task, while limiting the volume of the source tank.
    In other words, can the vacuum force in the tube overcome all other forces if the volume be of sufficient girth? The key is the volume of liquid inside the depository tube.
     
  9. Nov 18, 2008 #8

    russ_watters

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    No, the only thing that adding these complexities does is distract you from the key issue: flow only happens when there is a pressure difference and the only way to have a pressure difference is if the source is higher than the sink.
     
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