What is wroong with this scenario?

[elpenmaster]

Say i take two tubes, and put them next to each other. at the bottom between them, i put a semipermeable barrier, that water can pass throught butt that substance x cannot. now, i put water halfway up one andsubstance x inthe other tube. by osmosis, the water goes from its tube to the tube of substance x. and since substance x is denser than water, the water rises to the top. then i put a semipermeable barrier at the top. so only water can get through. then the water goes away from substance x and back into its own tube. then the process repeats. would not this be a perpetual motion machine? :?

[jinydu]

The water wouldn't go back to its original tube, due to higher osmotic pressure in the original tube.

[monkeymeister]

how would the water get back to the top of the tube?

[elpenmaster]

well, the tube with the water in it would be half full. the semipermeable barrier there is just to stop substance x from going into the tubeof water. but osmotic pressure wouldnt apply because the water would be falling into air, and then into the water, which only comes halfway up the container

the water would get back to the top of the tube because it is less dense than liquid x, so it would go to the top

[PWrong]

What supplies the osmotic pressure? Wouldn't you need extra energy for it?

[elpenmaster]

I belive that osmosis is due to the randomocity of water molecules. the water on the all water side can go through to the other side, but on the other side not all the water can go throught the semipermeable since it is blocked by the other molecules. this continues until the normal pressure in the not-all-water container equals the osmotic pressure. this effortlessly raises the level in one container and lowers it in the other. then you can let gravity equalize these two and the process can repeat. can this be used to supply energy as a perpetual motion machine?

[RQ]

The kind of osmosis you are talking about is nonexistent. There's no reason for the water to go back up the semipermeable membrane, against gravity, and especially not if there is a denser fluid under it, which disallows it to move freely and actually hit the semipermeable membrane and pass it. Furthermore, if gravity can initially make the water pass through the membrane down in the first place, then why is there a greater opposing force on the bottom as soon as the water goes in so it can go back again? The only way you can do this is if you change something which would require energy.

The only perpetual motion device that I could think of using these settings is if the tube was spinning, but then again you need energy to make it spin, and substance x is useless in my opinion.

Edit: Also, on impact with the fluid or container, the water loses energy and thus speed and momentum so after a while any process would stop that is not being fed energy.

[elpenmaster]

okay, lets try to clear up the misunderstanding here:

1. if i have two containers with a semipermeable barrier between them, and water in one and substance x (which is denser than water) in the other, would the water move, by osmotic pressure, into the other container?
2. If yes, then would the water rise to the top of the container with substance x in it?
3. If yes, then lets put a semipermeable barrier around the top of this container. that way, substance x cant get out, but when the water gets to the top, it will overflow through the semipermeable barrier and back into the first container that has only water in it, right?
4. If yes, then the water would fall down to the water level in the first container, and if a waterwheel was put in the container, the falling water would turn the waterwheel and create energy, right?
5. If yes, then the water goes back to the first container and can do this over and over again creating a perpetual motion machine, right?

obviously, one of these steps is probably wrong. which one?

[RQ]

First there are models of perpetual motion, but that's completely different from an infinite drive.

Initially the water may be able to do both, if say substance x is very hard, and the water bounces somehow off of it and back up the semipermeable membrane, and then down again due to gravity, but eventually it would lose energy.

Assuming such consequences are true, then if the water has gotten through substance X's resistance due to gravity, then it shouldn't be able to go back up.

If substance X is not filling up all of the other half of the container, but is say half full, if the water coming is greater than the volume it is given, then it will be forced back up the membrane since that would be down its concentration gradient and it might do this for a bit until it settles in equilibrium wherever that may be.

Again if you want case 2 to continue then you need to supply it energy which contradicts the method being an energiless source of energy. If you think about it, it defies the first law of thermodynamics.

So it's not the steps that are wrong, it's the initial assumptions.

Also, doesn't gravity require energy, or isn't it energy?

[houserichichi]

Gravity is curvature of spacetime - no energy required (it's a force). Gravitational potential energy is energy though

[elpenmaster]

here is a picture of what i mean:

|__
| |
| |
|_|

the column on the left is the thing with the water, the one on the right is the substance x, and the two horizontal bars are the semipermeable barriers. the water level sarts out at the second bar, and the substance x goes up to the top of the third :wink:

[RQ]

You're gonna have to make a better picture than that. This is what I visualized as your experiment:

____
l.......l
l~~~l
l____l
l------l
l~~~l
l____l

The top half contains water, the middle is the semipermeable membrane, bottom is substance X.

[elpenmaster]

well, no, something more like this:

|....|\_
|....|---\....|
|....|---|ww|
|....|---|....|
|....|---|....|
|__|~~|__|

where the container on the left is substance x, the container on the right is water, the ~~ is the semipermeable barrier, ww is the original water level, and the \_ thingy is a path for water to get back into the original container.

would the water go from its original container to substance x through the semipermeable barrier due to osmotic pressure, rise to the top of the denser substance x, and then overflow the x container and go back into its original container?

[jinydu]

Why don't you settle this question using an actual experiment? I think just about all of the materials needed are household items.

[PWrong]

I belive that osmosis is due to the randomocity of water molecules.

I think I get it now. I think I can explain why it doesn't work though.

In a normal situation, without the barrier, the water move underneath substance x, then then substance x would fall through it, pushing the water to the top.

But substance x has to actually fall for it to work. With the barrier in place, substance x doesn't move at all, so there's no reason for the water to move up.

[RQ]

This can easily be avoided, PWrong, if substance X had a very low viscosity so the water can easily go through, yet denser than water or 1 ml/cm^3. However, the reason that this is not an infinite drive, is because when the water is on its way up, it will bump into the molecules of substance x or the container, and will loser energy in the form of heat, sound and etc.

However if the water had good momentum to begin with, this might go on for a while seemingly endlessly. Might've fooled people in the middle ages. You could have been rich you know, or you could just swindle a kid into buying it.

[PWrong]

The viscosity of substance X doesn't prevent the water from going up. In fact nothing prevents it from rising. The only problem is that nothing causes it to rise either.

If the substance was falling, it would exert a force on the water. The water would react with an equal & opposite force, and push through the substance up to the top of the tube.

But because the substance is held in place by the barrier, there is no force on the water, so the water just sits still.

[Wiarton WIlly]

If water and X do not mix then there is no increase in entropy and there is no osmotic force.

[RQ]

The viscosity of substance X doesn't prevent the water from going up. In fact nothing prevents it from rising.

Oh shoot, you're right, I thought if substance X was very thick it would make some kind of an opposition to the rising water, but it'll probably have to be a solid block to stop the water.

[elpenmaster]

what am i missing here?

will the water go through the semipermeable barrier and arrive underneath substance x?
if so, will it rise to the top of substance x?
if so, then it will fall into the first container and do this all over again

is this so?

[Wiarton WIlly]

what am i missing here?

If water and X do not mix then there is no increase in entropy and there is no osmotic force.

Nothing will happen. Osmosis will only work if the two substances will mix.

[elpenmaster]

what if water and substance x do mix? can they mix, and then water, being less dense than substance x, would rise to the top of the container, spill over, etc? :?

[jinydu]

In that case, some of the substance X would come along for the ride. Eventually, there would be equal concentrations of substance X on both sides, causing the process to halt, if something else doesn't cause it to halt first.

[Wiarton WIlly]

what if water and substance x do mix? can they mix, and then water, being less dense than substance x, would rise to the top of the container, spill over, etc? :?

If they mix to provide osmotic flow they won't conveniently unmix to produce the elusive perpetual motion machine. Either they mix or they don't.

[elpenmaster]

well the trick there is to put another semipermeable barrier at the top of this container. that way, although both water and substance x rise to the top of the container, only water can get back through to the original side. that way, all of substance x stays in its own container

then, the water could go through this second semipermeable barrier, fall to the water level of the first container, and get right back to where it started

[Wiarton WIlly]

That's what they call reverse osmosis and it takes energy. Good RO systems need to be plugged-in while cheaper ones will run off of your city's water pressure but get clogged-up easier. You could probably even make a gravity-feed, RO water system but it would still be consuming energy, from a thermodynamic standpoint.

The search for the elusive Perpetual Motion Machine continues....

[elpenmaster]

why does reverse osmosis require energy? :?

[RQ]

Because to reverse direction you need energy. :?

[elpenmaster]

Because to reverse direction you need energy. :?

why is this and what does it mean?

basically we have water and substance x in a container with a semipermeable barrier leading to air. will the water go through the semipermeable barrier?

[Watters]

what am i missing here?

will the water go through the semipermeable barrier and arrive underneath substance x?
if so, will it rise to the top of substance x?

is this so?

the problem is the pressurse would equal out and there would not be enough preasure on the left hand side to push the water into the thicker substance. Remember that both sides would ahve to be filled to an equal level or again they would balance and you would not have the falling water, also that this thicker substance weights more then the water so as this diagram hopeuly shows there will be two forces on each tube and because we are dealing with the preasure of all the water on top on all the water on the bottem this can be displayed as one force....same for the other side.

| force in tube left |
| | force tube right
| |
| |
\/ \/


but again remember that the left side is stronger due to it's higher density and they both have the same volume.

and because this force is stronger that water at the bottem of tube left
would not be able to have the force to move in to the thicker substance
------->

it would end up that all the water would be in the left hand side and all the thicker substance in the right... if you were to add more water it would balance out that the levels in the two tubes would be the same with all water on the left and all the thicker substance in the right with a bit of water at the same level on the right.