Help understanding how a siphon works?

AdamX

Hello everybody. I am somewhat confused about how a siphon works. Attached is an image which will hopefully help me explain. In the top left corner of the image, a classic siphon is shown. My question is however directed at the second image. If i was attempting to siphon water over a fence from one swimming pool to another, and the pool reservoirs were equal in height. Would that hinder the process? Or would the siphon still work so long as the downward momentum of the water was greater than the uphill flow due to the downward section of the siphon tube being longer than the uphill section of the siphon tube?

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JaredJames

One reservoir must be lower than the other.

You can bluff this initially by having one end of the hose lower than the other (suck water from the top of the full pool and have the hose in the bottom of the other, but once the pipe's reach equal height it will stop (and reverse).

Your description regarding "uphill / downhill" is irrelevant, it is the vertical height difference that matters only. Horizontal distance only comes into play with resistive losses, in which case the longer, the worse it is.

AdamX

Thank you sir i now understand.

Fr34k

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

If you would like to understand the physics behind.

JaredJames

What? That has nothing to do with it.

Drakkith

Yeah, that link has nothing to do with how a siphon works. It's simple gravity is all. The weight of the water in the tube exerts a greater amount of force on the water coming up the tube than gravity does, which sucks it up. If the two resevoirs are equal in height, then the force equalizes and water will not flow either way.

rcgldr

As long as the water level in the source reservoir is higher than the destination reservoir, the water will flow through the siphon towards the lower reservoir. If the siphon rises above the water level in the source rreservoir, the flow will not occur if there's is an air gap at the top of the siphon. There's also a limit as to how high the siphon can rise above the water; if it's 33 feet or higher, the pressure drop versus altitude results in the water turning into vapor, and a near vacuum at the top.

The reason the water can flow upwards and then back down again is because the pressure at some height (altiitude above ground) on the "downflow" side of the siphon is lower than the pressure at the same height on the "upflow" side of the siphon, as long as the "downflow" side exits below the water level of the upper reservoir, or exits into a reservoir with a lower water level. This pressure difference results in a net flow towards the lower reservoir until the reservoirs level equalize, which will result in equal pressure versus height within the siphon on the "upflow" and "downflow" sides and the flow stops.

cragar

I just want to say that you cant siphon water in a vacuum.

JaredJames

I'm just going to put this in as an explanation to your above: http://www.physicsforums.com/showthread.php?t=402634

Fr34k

JaredJames and Drakkith: would you like to enlighten me why am i wrong?