[Phys-L] Re: entropy: increased by stirring, decreased by observation

[John Denker <jsd@AV8N.COM>]

On 06/14/05 07:01, Fayngold, Moses wrote:

>  Just because dQ is not a perfect differential does not mean that the=
> re is no such thing as dQ.

I disagree.  T dS is a non-exact differential.  dQ is, by
definition (!) the differential of Q.  d(anything) is
exact.  It's a theorem.  I have no problem with T dS
... you just can't set T dS equal to dQ.

>  I find the websites John referred me to very interesting, and some e=
> xamples just fascinating. I'll try to read more carefully both of the=
> m. However, here, too, I disagree with (or, may be, do not quite unde=
> rstand) some of the statements regarding the entropy. Here are some e=
> xcerpts:
>
> "(The Laws of Thermodynamics).
> The entropy of some things goes to zero as temperature goes to zero.
> This is true except when it's not true."
>
>   I wish I knew when it is not true.

Good question.  I just now added a section detailing this:
   http://www.av8n.com/physics/thermo-laws.htm#sec-s0t0
   "Low-Temperature Entropy (Alleged Third Law)"

You may need to hit "reload" on your browser.

> "(Sec. 4.3) When we want to quantify things, it is better to forget a=
> bout "heat" and just to quantify E and S, which are unambiguous and u=
> nproblematic."
>
>    This does not seat together with the statement that S is subjectiv=
> e characteristic depending on knowledge of the observer.

S would be *more* problematic if it did not take into
account information in the observer-system.

The "subjectivity" that seems to bother people becomes
less bothersome if you consider a complete closed _cycle_.
Then you see what matters is how the observer _obtains_
the information.  Measurement affects the entropy.
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