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On Saturday, May 31, 2003, at 14:51 US/Eastern, Brian Whatcott wrote:
At 11:45 AM 5/31/2003 -0400, you wrote:
I have seen capillary tubes narrow enough to draw water
to a hight above 1 cm. Suppose that such tube is inserted
into water vertically and that only 0.5 cm is sticking out.
One might imagine that water flows continuously upward
in such tube. One can also imagine a tiny water wheel
working continuously, next to the tube.
Would this be a perpetual motion machine of the first kind
or of the second kind? I know that this will not happen. But
why not? Suppose that the easy answer "this would conflict
with laws of thermodynamics" is not acceptable.
Ludwik Kowalski
A fair question: why not?
I will attempt an answer. Suppose that water rises at a
glass surface whose curvature is sufficiently pronounced
until the capillary force lifting the water thread is
balanced by the weight of the excess water column.
If this curvature diminishes while there is still an
unbalanced lifting force, so does the motive force.
And at a top or side aperture, the curvature of which I speak
falls to planarity. The water surface goes no further.
If one substitutes the concept of electric force, for capillary
force in this paragraph, then it might serve to explain the
reason that water dielectric shrinks from edges and holes
though otherwise there would be sufficient force to motivate
further water lift.
Thank you Brian:
You just reminded me that the upward force is a net of two forces:
weight and attraction of water molecules by molecules on the dry
glass surface. The net force vanishes when the inner surface
becomes wet up to 0.5 cm, in my example.
Let me mention an interesting capillary effect worth sharing. I took
a very thin wire and inserted it into a capillary tube. The wire ended
at the same level as the mouth of the tube. I inserted the tube into a
conducting liquid -- a solution uranium nitrate. Then I removed the
loaded tube from the liquid and placed it (with the wire and some
liquid inside) above a metallic foil. The distance between the mouth
of the tube and the foil was about 2 cm in air.
The difference of potential of several thousand volts was applied
between the wire and the foil. This produced a shower of tiny
droplets; the tube emptied after about one minute. A thin layer of
uranium nitrate was deposited on the foil. That is how thin uranium
targets were prepared for my doctoral work. The central areas
covered with uranium were usually sufficiently uniform in
thickness. This method of thin targets preparation was invented
by two researchers from Harwell Laboratory in England.
Ludwik Kowalski