On 3/25/2012 6:44 AM, Evgenii Rudnyi wrote:
On 14.03.2012 19:58 meekerdb said the following:
On 3/14/2012 11:51 AM, Evgenii Rudnyi wrote:
...
Then the thermodynamic entropy is subjective. Try to convince in this
engineers who develop engines, or chemists who compute equilibria, and
see what happens.
It is relative not just to the information but the use of that
information. Even if you told an engineer designing a steam turbine the
position and momentum of each molecule of steam he would ignore it
because he has no practical way of using it to take advantage of the
lower entropy that is in principle available. He has no way to flex and
deform the turbine blades billions of times per second in order to get
more power from the steam. The experiment I linked to is extremely
simple so that it is possible to use the information.
Brent
I have looked the paper that you have linked
On 13.03.2012 20:09 meekerdb said the following:
> On 3/13/2012 10:28 AM, Evgenii Rudnyi wrote:
...
>> Could you please give one example from physics (yet please not a
>> thought experiment) where information allows us to reduce entropy?
>
> http://www.nature.com/news/2010/101114/full/news.2010.606.html
>
Experimental demonstration of information-to-energy conversion and validation of the
generalized Jarzynski equality
Shoichi Toyabe, Takahiro Sagawa, Masahito Ueda, Eiro Muneyuki & Masaki Sano
Nature Physics, Volume: 6, Pages: 988–992 (2010)
I should say that I am not impressed. One can make a feedback mechanism indeed (by the
way, it is quite common in engineering), but then in my view we should consider the
whole system at once. What is the information then and what is its relationship with the
entropy of the whole system?
What you asked for was an example of using information to reduce entropy: not obtaining
information AND using it to reduce entropy.
"The experiment does not actually violate the second law of thermodynamics, because in the
system as a whole, energy must be consumed by the equipment — and the experimenters — to
monitor the bead and switch the voltage as needed."
By the way the information about the position of the bead have nothing to do with its
entropy.
It has to do with the entropy of the system of bead plus medium. The rotating bead could
be used to do mechanical work via energy which was extracted from the random motion of the
molecules in the medium. This is Gibbs free energy, so the bead plus medium plus
information has a lower entropy that just the bead plus medium.
This is exactly what happens in any feedback systems. One can introduce information,
especially with digital control, but it has nothing to do with the thermodynamic entropy.
Because it is not extracting energy from random molecular motion, aka "heat".
Then I like
"In microscopic systems, thermodynamic quantities such as work, heat and internal energy
do not remain constant".
The authors seem to forget that work and heat are not state functions. How work and heat
could remain constant even in a macroscopic systems?
They don't remain constant, but their statistical fluctuations are very small compared to
their absolute value. Of course if you had information about these fluctuations you could
use it to extract energy and decrease the entropy of the system.
I also find the assumption at the beginning of the paper
"Note that, in the ideal case, energy to place the block can be negligible; this implies
that the particle can obtain free energy
without any direct energy injection."
funny. After block is there, the particle will jump in the direction of the block and it
will interact with the block. This interaction will force the particle to jump in the
other direction
The molecular motion of the medium forces it to jump one way or the other at random, the
information is used to keep it from jumping back. So the work is extracted from the heat
energy of the medium, not from the interaction with the blocks.
and I would say the energy is there. The authors should have defined better what they
mean by direct energy injection.
In essence, in my view the title "information-to-energy conversion" is some word game.
It could work when instead of considering the whole system in question, one concentrates
on a small subsystem.
Any demonstration of the principle is going to concentrate on a small system because it is
impossible to use information about 1e26 molecules. And of course it will be a
"subsystem" in the sense that some other device has to be used to get the information and
if that device in included as part of a closed system, then the 2nd law will apply - since
it applies to closed systems.
You seem to be arguing against claims that were not made by saying a laboratory
demonstration isn't a practical application.
Brent
Say if I consider a thermostat then I could also say that information about the current
temperature is transformed to the heater and thus to energy. I am not sure if this makes
sense though.
Evgenii
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