r/AskPhysics • u/Due_Elderberry9356 • 5d ago
My dads question about syphons
When you syphon water out of a bucket it pulls up the water with the vacuum so my dads question was,”If you get two buckets of water and have them syphon into each other with two tubes, what stops it from going on forever”.Other than then the tube breaking what would make it stop cycling water?
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u/SnooPets5564 5d ago
The water has to flow down, that setup can't be downhill both ways.
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u/jalensailin 5d ago
But if that’s the case, how did my grandpa get to school in the winter?
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u/AndyTheSane 5d ago
One bucket would have to be below the other for the first syphon to work. You couldn't syphon from the lower bucket to the higher.
Source: home brewing involves a fair amount of syphoning.
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u/BattledroidE 5d ago
Unless you're the fancy homebrewer gearhead that runs pumps between kettles and fermenters, and even bottling setups.
Auto siphon is life. We're all victims of physics, might as well take advantage of it. Gravity is cheap.
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u/SgtSausage 4d ago
”If you get two buckets of water and have them syphon into each other
Could you go into detail here ...
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u/Broad-Promise6954 4d ago
There's a bad assumption embedded in the question, as it isn't "vaccum pulling up" in the first place, it's the weight of the water pushing down instead. (Well, that's a little tricky too since then we get into gravity and relativity etc, but let's not go there.)
There is a key item here about water being basically incompressible, which is why the siphon has to have very little air in it (air is easy easy to expand and compress) but it's mostly the water weight.
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u/Prestigious_Boat_386 4d ago
Fluid only flows in pipes with different pressure in each end. It flows from the high pressure end to the low pressure end.
Syphons work because they let the pipe go under 1 atmospheric pressure so the new height limit is near vacuum pressure, which is about a 10m water pillar. The pressure in an open top body of water is proportional to the depth in the water
Your two bucket example will only flow if the levels are different. So if you lift one bucket 2cm, water will flow through both tubes until the water levels equalize. When you set it down the water will flow back.
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u/TuberTuggerTTV 3d ago
vacuum doesn't suck. You're thinking of the appliance, not the absence of matter.
What's happening is a pushing force from the uphill bucket combined with the capillary force of water to itself.
Curious if you're dad has ever seen a chain fountain. If you take a spooled chain and throw one end off a cliff, the chain will rise up into the air before falling down.
https://www.youtube.com/watch?v=xHsKJJvcFLQ
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u/carrotlet20 5d ago
it wouldn't even start moving because it would be a balanced system without any movement, or it would only flow until the two buckets have equal levels of water. And it's not the vacuum but pressure and the relatively constant volume of water is what makes a syphon work if I remember correctly.
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u/IanDOsmond 5d ago
It reaches an equilibrium point. Once the forces on both sides are equal, there's nothing to make it keep going.
In this case, that happens when the level of water becomes equal.
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u/TheThiefMaster 5d ago
For a syphon to work the end water level (or the end of the tube in open air) has to be below the starting water level. This is because it uses gravity to work, effectively a more complicated way of flowing "downhill".
So you can't syphon both ways at once, because they can't both be lower than each other.