Here's a little note on thermodynamics I saw today. (Some of you have heard my stance on the subject.... Essentially, I think thermodynamics needs to be reformulated.)
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 598 July 17, 2002
by Phillip F. Schewe, Ben Stein, and James Riordon
PUSHING THE SECOND LAW TO THE LIMIT.
Australian
researchers have experimentally shown that microscopic systems
(a nano-machine) may spontaneously become more orderly for
short periods of time - a development that would be tantamount to
violating the second law of thermodynamics, if it happened in a
larger system. Don't worry, nature still rigorously enforces the
venerable second law in macroscopic systems, but engineers will
want to keep limits to the second law in mind when designing
nanoscale machines. The new experiment also potentially has
important ramifications for an understanding of the mechanics of
life on the scale of microbes and cells.
There are numerous ways to summarize the second law of
thermodynamics. One of the simplest is to note that it's impossible
simply to extract the heat energy from some reservoir and use it to
do work. Otherwise, machines could run on the energy in a glass
of water, for example, by extracting heat and leaving behind a
lump of ice. If this were possible, refrigerators and freezers could
create electrical power rather that consuming it. The second law
typically concerns collections of many trillions of particles - such
as the molecules in an iron rod, or a cup of tea, or a helium balloon
- and it works well because it is essentially a statistical statement
about the collective behavior of countless particles we could never
hope to track individually. In systems of only a few particles, the
statistics are grainier, and circumstances may arise that would be
highly improbable in large systems. Therefore, the second law of
thermodynamics is not generally applied to small collections of
particles.
The experiment at the Australian National University in
Canberra and Griffith University in Brisbane (Edith Sevick,
[email protected], 011+61-2-6125-0508) looks at aspects of
thermodynamics in the hazy middle ground between very small
and very large systems. The researchers used optical tweezers to
grab hold of a micron-sized bead and drag it through water. By
measuring the motion of the bead and calculating the minuscule
forces on it, the researchers were able to show that the bead was
sometimes kicked by the water molecules in such a way that
energy was transferred from the water to the bead. In effect, heat
energy was extracted from the reservoir and used to do work
(helping to move the bead) in apparent violation of the second law.
As it turns out, when the bead was briefly moved over short
distances, it was almost as likely to extract energy from the water
as it was to add energy to the water. But when the bead was moved
for more than about 2 seconds at a time, the second law took over
again and no useful energy could be extracted from the motion of
the water molecules, eliminating the possibility of micron-sized
perpetual motion machines that run for more than a few seconds.
Nevertheless, many physicists will be surprised to learn that the
second law is not entirely valid for systems as large as the bead-
and-water experiment, and for periods on the order of seconds.
After all, even a cubic micron of water contains about thirty billion
molecules. While it's still not possible to do useful work by turning
water into ice, the experiment suggests that nanoscale machines
may have to deal with phenomena that are more bizarre than most
engineers realize. Such tiny devices may even end up running
backwards for brief periods due to the counterintuitive energy
flow. The research may also be important to biologists because
many of the cells and microbes they study comprise systems
comparable in size to the bead-and-water experiment.(G.M.Wang
et al., Physical Review Letters, 29 July 2002)
10:06 - 2002-07-18
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