🚀 go-pugleaf

On Thu, 23 Oct 1997 23:10:32 +0800, Lozang Trinlae
<lozang@mindless.com> wrote:

>John Cook wrote:
>
>> Here is the plan...
>> > 
>> > Imagine a long tube reaching from the surface to the bottom of the japan
>> > trench.  It's bottom end is a reverse osmosis filter unit.  It is filled with
>> > fresh water.
>> > The water outside the tube is salty and weighs around 1028 kg/cu mt - the
>> > water inside the tube is fresh and weighs 1000 kg/cu mt.  This causes a
>> > pressure diferential across the osmotic membrane of about 2 to 3% of the
>> > external water pressure.
>> > 
>> > Thats it.  Sufficient depth of water WILL cause a flow through the membrane
>> > and up the tube.
>> > 
>> > Such a device will sit there for'ever' emiting a fountain of fresh water.
>> > 
>> > Kewl?
>> > 
>> > I think the energy comes from slightly lowering the potential energy of the
>> > salt in the ocean. Am I right about this?
>> > 
>
>I don't know what you mean by the 'potential energy of the salt'.
>Basically what you have here is a type of barometer. By reducing the
>density of the water (by filtering out the salt) in the tube that is
>placed in the ocean, and atmospheric pressure would push the liquid up
>into the reduced-pressure region, like the way a drinking straw works.
>That is mechanical in nature; i.e., a mechanical potential difference
>would occur across the filter membrane boundary. But if you look at
>Boyle's law, P.1*V.1=P.2*V.2, you can see that V.2 (the volume of water
>in the tube) = (P.1*V.1)/P.2. Any differences in P.1/P.2 look like they
>will get washed out (no pun intended), by the relatively infinite value
>of V.1 (the volume of the water in the ocean).
>
>However this kind of thing looks like it could be experimentally tried
>easily at home, given the suitable filter. Go for it and let us know
>your results!
>
>-Lozang Trinlae
>
>
I think you have the wrong law and the wrong reasoning here.  Boyle's
Law does not apply to water and solutions.  The volume of the ocean
has no bearing on the issue, only the depth of the membrane below the
surface.  The pressure at this depth is a function of the depth and
the density of the sea water.  That pressure will support a greater
depth of fresh water inside the tube, therefore the water would tend
to rise above the level of the ocean.