Science Buddies: "Ask an Expert"

Is it possible to create positive siphon flow between two containers of fluid where the output container has a higher fluid SURFACE level than the input container, provided the flow pipe inlet is higher than the flow pipe outlet? In other words: Is it the relative level of input versus output or relative surface level of the fluids which permit siphon flow?

Hi,

Great question! Here's a video that answers it better than I can: https://www.youtube.com/watch?v=CZmP0vsRBZ8

The force that pulls the liquid from one container to the other depends primarily on the weight of liquid in the delivery part of the tube relative to the liquid in the other part. The amount of liquid is measured from the bend of the tube to the surface, not to the end of the tube. Once you put the tube below the surface, the liquid in the receiver is pushing back on it and prevents the flow. When you operate a syphon in the normal way, with the liquid in the receiver below that in the other container, you see that the flow stops when the levels are exactly the same.

Hope this answers your question.

Sybee

Nice video, thanks.

Now, to complicate the question

If the outlet of the tube were exiting into a chamber of air trapped below the surface level in the receiving container, would the liquid then continue to flow or would the air pressure increase due to the addition of fluid around the trapped air stop the flow anyway from air pressure rather than liquid equilibrium? I am guessing it would stop anyway.

Yes, I'm pretty sure it would stop. The pressure from the liquid is transferred to the air which would prevent liquid from flowing out of the tube.

It is good to question the laws of physics. Einstein did some really great science by questioning Newton's laws of gravitation.

Sybee