Does a siphon create energy? Once flow is sustained, the water rises against gravity and piping friction unitl it passes a maxima point where it then freefalls out of the siphon. Does a siphon create energy? I know it can’t, but it sure appears to defy physics!
Can someone explain how a siphon can sustain a flow against gravity, etc?
You can’t siphon uphill. The reciever has to be below the sender so to speak.
If they level up the siphon will stop.
We clean our fish tank with aid of a siphon with NO moving parts. (Some siphons have a hand pump of some kind to prime the siphon.) A few thrusts of the wrist will prime the siphon and get water flowing uphill until it rises a few feet to the peak before draining up and out of the fish tank. Once flowing, it will continue. We’ve done it with our kiddie pool, too…
The apparent suction is provided by the displaced water, but it is flowing uphill and against piping losses! Sure beats me! Where’s Archimedes to explain this?
Siphoning works because of gravity, not in spite of it.
As mentioned, the net flow is downhill. Rather like skiing, you can go uphill some (once you have momentum), but you always end up downhill from where you started.
Unlike skiing, siphoning often (always?) starts out going uphill (for reasons of practicality). This is only accomplished by adding energy to the system, for example by sucking on the siphon hose to get the flow started.
Another way to add energy to the system to get the flow started is to use a few thrusts of the wrist.
Stranger
As others have said, it may go up a little bit, but the end of the siphon the water is coming out of HAS to be lower than the end the water is going into or it will not work. Period.
And to clarify, the energy you add to the system is in the form of a pressure differential between the air inside the tube and the air pressing on the liquid.
Is this similar to a box-full of chain? If you drag enough of an end of the chain out of the box, it’ll all spill out. The chain goes up over the side of the box to do it, just like the siphon.
Or a simplified explanation if you don’t want the equations: If water flows down from one end of the siphon without water flowing up the other end, you’d end up with a vacuum in the middle. The pressure differencial between the tank and the vacuum would then cause water to flow up from the tank.
That’s not quite an accurate picture, because really water is flowing up and down simultaneously such that no vacuum is formed. But it is accurate to say the flow of the water causes a pressure differencial between the two ends of the siphon.
Curse you and Stranger both. While I was drawing up my neat, simplified explanation of a siphon you posted the answer. :mad:
Yeah, but on the downside, I misspelled “differential” twice. I had changed it from “differential” (spelled correctly) to “difference”, then changed it back (mostly).
I think they disabled the EDIT feature just to frustrate perfectionists like me.
And I was working quite fast so I’m not sure that the height difference in my drawing is correct. It doesn’t matter though because there is certainly more water in the pipe on the right than on the left. If not, the thing stops. It also won’t work on the moon.
So your top arrow for “h” should extend up to the surface of the water in the tank on the left.
Seems everyone forgot one crucial part. The siphon does not “create” energy because you need to account for the energy it took to get the water there (wherever “there” is) in the first place. That is, it took energy to pump water into your fish tank or gas tank or whatever. The siphon is merely recouping some of that power as the liquid moves to a lower place but you will always need to put more energy in to get the water “up” then you will ever get back out (or at least get back in a fashion that you can use…technically all the energy is there but inefficiencies ensure you never get all back that you put in).
[nitpick]
The relative lengths of pipe inside and outside are immaterial. As long as the pipe outside the the tank is below the surface of the water in the tank, the siphon will operate until the tank level falls to the level of the effluent pipe opening.
[/nitpick]
I knew that.
Raise the effluent pipe end above the water level in the tank and the flow reverses.
Not so! A siphon over a barrier of approx. 32 ft. high will be vapor locked due to the temperature and pressure conditions of the water. Water CAN NOT be siphoned under such conditions. 
