# Re: Information: a basic physical quantity or rather emergence/supervenience phenomenon

```On Fri, Feb 03, 2012 at 08:50:40PM +0100, Evgenii Rudnyi wrote:
>
> I guess that you have never done a lab in experimental
> thermodynamics. There are classical experiment where people measure
> heat of combustion, heat capacity, equilibrium pressure, equilibrium
> constants and then determine the entropy. If you do it, you see that
> you can measure the entropy the same way as other properties, there
> is no difference. A good example to this end is JANAF Thermochemical
> Tables (Joint Army-Naval-Air Force Thermochemical Tables). You will
> find a pdf here
>
> http://www.nist.gov/data/PDFfiles/jpcrdM9.pdf
>
> open it and explain what is the difference between the tabulated
> entropy and other properties there. How your personal viewpoint on a
> thermodynamic system will influence numerical values of the entropy
> tabulated in JANAF? What is the difference with the mass or length?
> I do not see it.
>
> You see, the JANAF Tables has started by military. They needed it to
> compute for example the combustion process in rockets and they have
> been successful. What part then in a rocket is context dependent?
>
> This is the main problem with the books on entropy and information.
> They do not consider thermodynamic tables, they do not work out
> simple thermodynamic examples. For example let us consider the next
> problem:
>
> -----------------------------------------------
> Problem. Given temperature, pressure, and initial number of moles of
> NH3, N2 and H2, compute the equilibrium composition.
>
> To solve the problem one should find thermodynamic properties of
> NH3, N2 and H2 for example in the JANAF Tables and then compute the
> equilibrium constant.
>
> From thermodynamics tables (all values are molar values for the
> standard pressure 1 bar, I have omitted the symbol o for simplicity but
> it is very important not to forget it):
>
> Del_f_H_298(NH3), S_298(NH3), Cp(NH3), Del_f_H_298(N2), S_298(N2),
> Cp(N2), Del_f_H_298(H2), S_298(H2), Cp(H2)
>
> 2NH3 = N2 + 3H2
>
> Del_H_r_298 = Del_f_H_298(N2) + 3 Del_f_H_298(H2) - 2 Del_f_H_298(NH3)
>
> Del_S_r_298 = S_298(N2) + 3 S_298(H2) - 2 S_298(NH3)
>
> Del_Cp_r = Cp(N2) + 3 Cp(H2) - 2 Cp(NH3)
>
> To make life simple, I will assume below that Del_Cp_r = 0, but it is
> not a big deal to extend the equations to include heat capacities as well.
>
> Del_G_r_T = Del_H_r_298 - T Del_S_r_298
>
> Del_G_r_T = - R T ln Kp
>
> When Kp, total pressure and the initial number of moles are given,
> it is rather straightforward to compute equilibrium composition. If
> you need help, please just let me know.
> -----------------------------------------------
>
> So, the entropy is there. What is context dependent here? Where is
> the difference with mass and length?
>
> Evgenii
> ```
```
The context is there - you will just have to look for it. I rather
suspect that use of these tables refers to homogenous bulk samples of
the material, in thermal equilibrium with a heat bath at some given
temperature.

If we were to take you at face value, we would have to conclude that
entropy is ill-defined in nonequlibrium systems.

More to the point - consider milling whatever material you have chosen
into small particles. Then consider what happens to a container of the
stuff in the Earth's gravity well, compared with the microgravity
situation on the ISS. In the former, the stuff forms a pile on the
bottom of the container - in the latter, the stuff will be more or
less uniformly distributed throughout the containers volume. In the
former case, shaking the container will flatten the pile - but at all
stages the material is in thermal equilibrium.

In your "thermodynamic context", the entropy is the same
throughout. It only depends on bulk material properties, and
temperature. But most physicists would say that the milled material is
in a higher entropy state in microgravity, and that shaking the pile
in Earth's gravity raises the entropy.

Furthermore, lets assume that the particles are milled in the form of
tiny "Penrose replicators" (named after Lionel Penrose, Roger's
dad). When shaken, these particles stick together, forming quite
specific structures that replicate, entraining all the replicators in
the material. (http://docs.huihoo.com/reprap/Revolutionary.pdf).

Most physicists would say that shaking a container of Penrose
replicators actually reduces the system's entropy. Yet, the
thermodynamic entropy of the JNAF context does not change, as that
only depends on bulk material properties.

We can follow your line of thinking, and have a word entropy that is
only useful in certain contexts, then we'll need to make up a
different word for other contexts.  Alternatively, we can have a word
that applies over all macroscopic contexts, and explicitly qualify
what that context is. The underlying concept is the same in all cases
though. It appears to me, that standard scientific usage has become to
use the same word for that concept, rather than coin different words
to describe the same concept in all the possible different contexts
that there are.

--

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Prof Russell Standish                  Phone 0425 253119 (mobile)
Principal, High Performance Coders
Visiting Professor of Mathematics      hpco...@hpcoders.com.au
University of New South Wales          http://www.hpcoders.com.au
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