🚀 go-pugleaf

> So the liquid pressure of lesser density and at lesser depth fresh water
> will be lower than the water at higher density and further depth sea
> water on the other side of the membrane. Then the difference in
> pressures at the depth of the membrane is proportional to the difference
> in water densities. But the power needed to get a flow going in a full
> pipe is proportional not only to the pressure differential, but also the
> force of gravity over the distance of the flow (not to mention other
> factors such as average velocity). So even though there is an upward
> pressure due to the density difference, wouldn't there still need to be
> a differential greater than 9800 Newtons per meter^3 x the depth the
> fresh water would have to travel to reach the exit of the tube, in order
> to force the water out? (Density of fresh water is 1000 kg/m^3.)
> 
> -lozang trinlae

The pressure on the water at the bottom of the tube owing to the volume
of water above it would be equal to that on the water outside the tube
at the same depth, (imagine the water without the tube being there), the
only difference to this equilibrium is the additional pressure placed on
the pure water due to osmosis, this extra pressure forces the water up
out of the tube.
To answer another query on this thread the energy is actually coming
from a reduction in the potential energy of the ocean as a whole as
pushing the water out will of course reduce the overall sealevel.

Peter Connor