Re: [Phys-L] heat content

[John Denker <jsd@av8n.com>]

On 02/10/2014 09:24 PM, Bob Sciamanda wrote:

> Is it not possible, and useful, to distinguish that part of the
> system energy which has been "thermalized" - that energy which
> directly affects the system temperature and phase?

That's an interesting point.  That probably explains why the
original article used the word "heat" ... and why the objections
to that word are invalid.

The article that provoked this discussion mentioned "ocean heat
energy".  The ocean does have some far-from-equilibrium "non-
thermalized" excitations, such as waves and huge currents.  The 
fluid dynamics is rather loosely coupled to the thermodynamics.
The vast majority of the energy stored in the Pacific ocean is
connected to the heat capacity, not to the macroscopic waves 
and currents.

I assume this is why the original article mentioned /heat/
energy, to exclude the macroscopic waves and currents.  Given 
the magnitude of the energies involved, the waves and currents
are not even worth mentioning, and word "heat" doesn't tell a 
scientist anything he didn't already know.  OTOH I reckon it 
is worthwhile sticking in a single short word to prevent non-
experts from making a wrong guess about where the dominant 
energy storage is.

More to the point, I cannot imagine how talking about "thermal
energy" or even "thermalized energy" is in any way clearer or
more correct than "heat energy".

It must be emphasized that 99% of the people who try to define
a notion of "heat content" aka "thermal energy content" are *not* 
talking about systems where the macroscopic waves and currents
are even worth mentioning, let alone relevant to the energy
budget.  They're trying to define a "Q" function such that "dQ" 
is equal to T dS.  They are never going to succeed.

Bottom line:  IMHO AFAICT the language in the original article 
was just fine.

=========

I'm still having a hard time seeing any issue of principle
that connects to "thermalized" energy.  Here's how far I've
managed to get:

We have a "system".  Within the system we have subsystem A,
which is "thermalized" i.e. it is in thermal equilibrium with 
itself.  It has some well-defined subsystem temperature.

Also within the system we have subsystem B.  It is not "thermalized"
i.e. not in thermal equilibrium with itself.  It is most definitely
not in thermal equilibrium with subsystem A.

At this point my train of thought goes off the rails.  How do
we even define a «system temperature and phase» for the whole
system?  I can see a temperature for subsystem A, but not for 
the whole system.

Also, what happens if after a few minutes subsystem B comes into
thermal equilibrium with itself?  It is still not in equilibrium
with subsystem A.  At this point, we have lost the distinction
between the "thermalized" energy and the other energy;  we just
have two subsystems.  It seems to me that the interesting physics
here is the weak coupling between the two subsystems.  Understanding
weakly coupled subsystems is essential to any serious thermodynamics.
For example, you can't have a heat engine without a hot reservoir
and a cold reservoir, and the two reservoirs must /not/ be in
equilibrium with each other.

Also I can't quite see how this rather advanced problem is relevant 
to the basic, fundamental issues that provoked this discussion.
People were suggesting that "heat energy" was nasty misinformation
whereas "thermal energy" was OK.

I don't see why it was necessary to object to "heat energy" in
the first place.  Secondly, I don't see why "thermal energy" or
"thermalized energy" is any kind of improvement.