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I am Snake
10-01-2011, 11:48 PM
I've been wondering about this for a while now. If a siphon works by having a greater sum force of gravity on the water in one end of the tube than the other, then couldn't you create a never-ending siphon by having one end of the tube going straight up out of the water, then straight over, then spiraling down (this makes more water be on one side than the other), then stopping above the surface of the water? For example, the water would travel up for one foot, then travel sideways for one foot, then spiral down for 4 feet of tube, but only 10 inches of height. In my way of thinking, the water would run into a place higher than its starting point, so if it was running into the same place it was coming from, it would never stop, unless it evaporated.

Is there some flaw in my thinking? I would imagine there must be, considering this could be a source of infinite (although small, even if made large scale) energy if combined with a turbine.

NinetyWt
10-01-2011, 11:53 PM
There has to be a higher head pressure on one side of the siphon than on the other. In other words, if you are trying to use gravity, the downstream end of the siphon tube must be at a lower elevation than the upstream end. The tube in your example wouldn't fill up because there's nothing pushing or pulling the water into the tube.

I am Snake
10-01-2011, 11:57 PM
Well to start the siphon, you would have to fill the whole tube with water. But after that, it should continuously siphon. The sum of the gravity on the 4 foot side would be pulling the water in the one foot side, because there would be less force on it.

TriPolar
10-02-2011, 12:29 AM
Well to start the siphon, you would have to fill the whole tube with water. But after that, it should continuously siphon. The sum of the gravity on the 4 foot side would be pulling the water in the one foot side, because there would be less force on it.

But it doesn't work. In the case described, air would enter the end of the tube out of the water instead of water being pulled in from the end of the tube under water. When enough air gets in, the rest of the water drains down through the end of the tube in the water, and you end up with an empty tube except for the part under water.

NinetyWt
10-02-2011, 12:33 AM
nm

JWT Kottekoe
10-02-2011, 12:39 AM
The greater weight of water in the spiraling tube is compensated for by the upward force of the walls of the spiral tube on the water. The walls of the vertical tube exert no upward force, but since the walls of the spiral tube must have a horizontal component, they do exert an upward force.

If you just worry about the water pressure, instead of the weight, it neatly takes account of all these factors. This leads to the result often summarized as "water seeks its own level."

TriPolar
10-02-2011, 12:43 AM
But is it possible to make a perpetual siphon that combines gravity and (surface tension?). If you suspend a strip of cloth with one end in the water, and the other end above the water, the cloth will absorb water until it is saturated. Would water drip off the end out of the water? If so, why isn't that making a perpetual motion machine? I presume for some reason the water won't drip out. I know it wouldn't be the 'until the end of time' type of perpetual, but if does seem to defy logic.

Malacandra
10-02-2011, 12:54 AM
The cloth won't absorb water until it is saturated - it will absorb a certain amount of water depending on one or two factors I shan't go into at this hour of the morning, then stop. You can get continuous wicking if you're able to remove the absorbed liquid by some means (evaporation, for instance) but you need an energy input to do this. See here (http://en.wikipedia.org/wiki/Capillary_action).

TriPolar
10-02-2011, 01:10 AM
The cloth won't absorb water until it is saturated - it will absorb a certain amount of water depending on one or two factors I shan't go into at this hour of the morning, then stop. You can get continuous wicking if you're able to remove the absorbed liquid by some means (evaporation, for instance) but you need an energy input to do this. See here (http://en.wikipedia.org/wiki/Capillary_action).

That would be the dipping bird type of motion. It didn't seem it would be possible based simply on gravity, and anything else would seem to reach equilibrium in a closed system.

beowulff
10-02-2011, 09:15 AM
Well to start the siphon, you would have to fill the whole tube with water. But after that, it should continuously siphon. The sum of the gravity on the 4 foot side would be pulling the water in the one foot side, because there would be less force on it.

You are misunderstanding how syphons work. "Gravity" doesn't have very much to do with them. Syphons work on the air pressure difference between one end and the other. That means three things:
1) Syphons don't work in a vacuum - at all.
2) The maximum vertical distance that a syphon can extend above the intake is determined by atmospheric pressure, and is around 32 feet, but probably less in practice. 3) The maximum vertical distance that the syphon can rise above the intake is also determined by the density of the liquid - it's only around a yard for Mercury, which is much denser than water.

The Librarian
10-02-2011, 09:34 AM

- Buy 2 buckets and 6 foot of gardenhose
- Fill one bucket with water.
- See for yourself how this stuff works.

If you found perpetual motion, you keep the profit.
If you learn something else wire me \$20.

Malacandra
10-02-2011, 09:36 AM
Now what I want to shoot for is an inverted J-shaped tube less than the maximum capillary length for its diameter. Dip the bottom of the long arm in water, and obviously water will rise up the tube, round the bend, and drip down again under gravity. Profit! :dubious:

JWT Kottekoe
10-02-2011, 09:49 AM
Once the capillary walls have been wetted, they have no more pull. Like all perpetual motion machines, this is doomed to failure.

CalMeacham
10-02-2011, 10:42 AM
You are misunderstanding how syphons work. "Gravity" doesn't have very much to do with them. Syphons work on the air pressure difference between one end and the other. That means three things:
1) Syphons don't work in a vacuum - at all.
2) The maximum vertical distance that a syphon can extend above the intake is determined by atmospheric pressure, and is around 32 feet, but probably less in practice. 3) The maximum vertical distance that the syphon can rise above the intake is also determined by the density of the liquid - it's only around a yard for Mercury, which is much denser than water.

I'm afraid you're incorrect. Jearl Walker wrote about siphons in vacuum in his Flying Circus of Physics. And from Wikipedia (with four cites)

In practical siphons, atmospheric pressure pushes the liquid up the tube into the region of reduced pressure at the top of the tube. The reduced pressure is caused by liquid falling on the exit side. In the laboratory, some siphons have been demonstrated to work in a vacuum,[1][2][3][4] indicating the tensile strength of the liquid is contributing to the operation of siphons at very low pressures.

http://en.wikipedia.org/wiki/Siphon

CalMeacham
10-02-2011, 10:55 AM
By the way, people have tried to build perpetual motion machines using siphons, surface tension, and gravity. None of them worked. My favorite one was two finely-supported circuklar plates that pivoted amost frictioonlessly about their centers. They were placed with their centers far apart, but with sthe surfaces of the plates very close together, then in a bath of water. The idea was that surface tension would make the water climb the gap between the plates, which would make that side heavier, so the plates would turn in that direction (one clockwise, one counterclockwise), which starts the process over, so that you get perpetually counter-rotating plates.

Didn't work

http://www.amazon.com/Perpetual-Motion-Obsession-Arthur-Ord-Hume/dp/1931882517/ref=sr_1_1?s=books&ie=UTF8&qid=1317570692&sr=1-1

JWT Kottekoe
10-02-2011, 11:27 AM
I'm afraid you're incorrect. Jearl Walker wrote about siphons in vacuum in his Flying Circus of Physics

Yes, gravity is the key to the operation of a siphon. Short siphons can work in a vacuum, but no siphon can work in zero gravity. Atmospheric pressure is very useful, because water has very little tensile strength. Atmospheric pressure guarantees that the water is always in compression.

For the master's take on this see:

TriPolar
10-02-2011, 12:39 PM
Now what I want to shoot for is an inverted J-shaped tube less than the maximum capillary length for its diameter. Dip the bottom of the long arm in water, and obviously water will rise up the tube, round the bend, and drip down again under gravity. Profit! :dubious:

How about filling us in the limits of capillary action now?

Chronos
10-02-2011, 02:12 PM
If the capillary action is strong enough to pull in water against the pull of gravity on one end, it'll also be strong enough to hold it in against the pull of gravity on the other. So it'll go in, but it won't drip out.

Mangetout
10-02-2011, 03:12 PM
If this endless siphon was possible, it would also be possible to set up a funnel feeding a thin tube that looped back up into the top of the funnel - the greater weight of water in the large funnel forcing the small amount of water in the thin tube around the circuit.

But of course that doesn't work. If it did, the other thing that would happen is that the oceans would empty themselves out via the smaller bodies of water attached to them (rivers, etc)

Malacandra
10-02-2011, 05:19 PM
If the capillary action is strong enough to pull in water against the pull of gravity on one end, it'll also be strong enough to hold it in against the pull of gravity on the other. So it'll go in, but it won't drip out.

Yeah, I know. ;) :D

Princhester
10-02-2011, 06:22 PM
You are misunderstanding how syphons work. "Gravity" doesn't have very much to do with them.

Other than cause the air pressure that drives them, you mean? ;)

Napier
10-02-2011, 06:29 PM
My favorite one was two finely-supported circuklar plates that pivoted amost frictioonlessly about their centers. They were placed with their centers far apart, but with sthe surfaces of the plates very close together, then in a bath of water. The idea was that surface tension would make the water climb the gap between the plates, which would make that side heavier, so the plates would turn in that direction (one clockwise, one counterclockwise), which starts the process over, so that you get perpetually counter-rotating plates.

Well, I can see one reason why this might be your favorite - it sounds like it should work!

Not that I seriously think somebody has thought up a real free energy machine and then everybody who has investigated it has made mistakes and come to the incorrect conclusion it doesn't work. I think the underlying problem is my insight. But - can you help me see why this doesn't work? The comments about surface tension at the lower end of the gap should not apply, as there are no gas interfaces down there. But I don't get what else would explain it...

Chronos
10-02-2011, 07:05 PM
The surface tension which is drawing up the water in the first place also acts as a "glue" that holds the wheels together and prevents them from turning.

Napier
10-03-2011, 05:49 AM
Well, I can see one reason why this might be your favorite - it sounds like it should work!

Not that I seriously think somebody has thought up a real free energy machine and then everybody who has investigated it has made mistakes and come to the incorrect conclusion it doesn't work. I think the underlying problem is my insight. But - can you help me see why this doesn't work? The comments about surface tension at the lower end of the gap should not apply, as there are no gas interfaces down there. But I don't get what else would explain it...

On further thought, a better way of looking at this might be that the force per length on the wheel surface is the same anyplace the wheel passes between water and air. It makes a capillary column rise where the column is narrow because that same force per length acts on a narrower column, which weighs less per that same length. But the wheel doesn't experience an imbalance.

Chronos, how close is that to your meaning?

Cheesesteak
10-03-2011, 10:27 AM
The sum of the gravity on the 4 foot side would be pulling the water in the one foot side, because there would be less force on it.The spiral side has 4x the water as the straight side. Each new foot of water you siphon up moves 4 feet of water on the spiral side 1 linear foot, but only 1/4 of a foot vertically. So the energy you get back from the falling water is exactly the same as the energy you put into the rising water.

This, of course, assumes no friction losses of any kind. Which, in an "perpetual motion" discussion is kind of silly since anything can be a perpetual motion machine if you assume no losses.

Rhythmdvl
10-03-2011, 10:43 AM
On this Board, we obey the laws of thermodynamics (http://www.youtube.com/watch?v=Xy0UBpagsu8&feature=player_detailpage#t=7s)!

TriPolar
10-03-2011, 11:07 AM
On further thought, a better way of looking at this might be that the force per length on the wheel surface is the same anyplace the wheel passes between water and air. It makes a capillary column rise where the column is narrow because that same force per length acts on a narrower column, which weighs less per that same length. But the wheel doesn't experience an imbalance.

Chronos, how close is that to your meaning?

I was thinking it had to be something like that. The same thing could be done with two almost parallel, slightly conical disks (like cymbals), so they can be close on one side, and apart on the other, and there doesn't have to be counter-rotation or gluing effect. But of course, for some reason it still won't work.

Now writing this, I go back to Chronos' earlier comments on the wet cloth, and I'm thinking somehow that the forces that draw water into the close gap through capillary action are also resisting having that side sink into the water, through surface tension maybe. Maybe gravity is just fighting capillary effect and reaching an equilibrium at the close gap?

DHMO
10-03-2011, 12:50 PM
A Water Treatment professional weighing in here:

The pressure at the bottom of a column of Water is a function of the height of the column, alone. The diameter of the container, and its shape, have no bearing on the pressure. A tube with a cross-section area of one square inch and 33 feet high will have a pressure at the base of 14.7 pounds per square inch. A tank with a cross-section area of 100 square feet and 33 feet high will have a pressure at the base of 14.7 pounds per square inch. You may curl and twist the tube all you like, but the pressure at the base will be the same, regardless of the shape or volume of the container.

The premise of the OP is that there is a mostly U-shaped tube, with one leg slightly shorter than the other. The longer leg is submerged in the vessel of water, and the shorter leg is slightly above the surface of the water in the vessel. The tube reaches some maximum height above the surface, which is common to both legs. The pressure at the base of the leg which does not quite reach the surface will be less than the leg immersed in the water, no matter how much volume, or weight, is in the shorter leg.

Water will flow out the longer leg, into the vessel, and suck the water up the shorter leg, to be replaced by air drawn into the tube through the small gap between the free end and the water's surface.

The discussion on capillary action, likewise, is flawed as a method to cause a perpetual siphon. The force of attraction between the water and the walls of the capillary tube are determined by the diameter of the tube, and the wetting factor for the particular variety of glass with water. The narrower the tube, the stronger the force drawing water into the submerged end. It is capillary action which draws water from the roots to the highest branches in the crown of the tallest trees, after all. But, in the case of trees (and plants, generally) evaporation and transpiration take water out at the leaves, which drives the capillary "pump."

The problem here is that, however you arrange the "perpetual siphon," the "free" end where the water is supposed to come out experiences the same force drawing water in as the end immersed in the vessel. And, since by the definition of the problem, the lower outlet of the free end is higher than the immersed end, any flow would be reverse what is required.