Re: [Phys-L] heat content

["Rauber, Joel" <Joel.Rauber@SDSTATE.EDU>]

Relevant cartoon to the discussion

http://xkcd.com/1322/


-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
Sent: Tuesday, February 11, 2014 1:31 AM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] heat content

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.
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