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A doubt about pressure

  1. Jul 12, 2015 #1
    Hi, I wonder if what I think is in according to the theory in Why the liquid falls. Check out the image I've uploaded. (sorry my bad english)

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  2. jcsd
  3. Jul 12, 2015 #2
    No, this is nothing to do with atmospheric pressure (the same thing will happen in a vacuum) and the liquid will continue to flow until the surface in each container is at the same level (ie the depth in the lower container is greater).
  4. Jul 12, 2015 #3


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    The reason liquid flows in a siphon is because the weight of the water in the left side of the hose outweighs the water in the right side.

    Imagine, instead of water, you had a long piece of string in the hose. At the right end of the string there is one marble tied to the string, at the left end, there are TWO marbles tied, weighing twice as much. Discounting friction, the string and marbles will slide into the lower bucket.
  5. Jul 12, 2015 #4
    yes, I know that. But why it happens? Is it not because the difference between the pressure in each container?
  6. Jul 12, 2015 #5


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    No. Either side (or both) could be a 5 gal pail or an ocean. It would make zero difference.

    What would make a difference is the height. A siphon falling 50 feet will be very powerful. A siphon falling 6 inches will be very weak, because there's very little pressure differential. That's also why - when the water level on both sides is equal - the siphoning process stops completely.

    ?? It happens because of the weight of water in the pipe.
  7. Jul 12, 2015 #6
    good DaveC. I was typing my comment when you did your post. (sorry my bad english). Thank you.
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