Re: ENERGY WITH Q PLEASE VOTE

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Larry's call for a vote:

> At 3:44 PM -0500 11/9/01, Carl E. Mungan wrote:
> > From what I have heard on PHYS-L and elsewhere, I suggest the
> > following basic definitions of heat:
> >
> > 1. Heat is the energy transferred between two bodies owing to their
> > difference in temperatures. This energy can be transferred by
> > conduction, convection, or radiation. The canonical example is a
> > hot plate warming up a gas, where the system is the gas. The heat
> > transfer can be either reversible or irreversible. If reversible,
> > it equals the integral of TdS. If irreversible, it is possible to
> > construct an equivalent hypothetical reversible path between the
> > same initial and final states of the system, such that the integral
> > of dQ/T for that process equals the entropy change. At the risk of
> > agitating some members of this list, I believe this is the
> > conventional definition found in most texts of either intro physics
> > or advanced undergraduate thermo and it is the view I prefer.

Unfortunately for this definition--even in the supposedly
'irreversible' case--it requires that the initial and final
macrostates be in equilibrium so that there is an initial and final
temperature defined before and after the heating process.
Unfortunately, a heating process can exist even when a temperature
does not.

> > 2. Heat is internal energy, or possibly just certain forms of
> > internal energy called thermal energy. Heat therefore does not
> > necessarily get transferred from a hot body. For example, the
> > adiabatic compression of an ideal gas produces heat in the gas
> > because the gas warms up. For more general materials, heat is also
> > produced during phase changes. The heat of an isolated body is time
> > dependent in general - for example, a rotating fluid initially
> > possesses mechanical energy but viscosity slowly transforms that
> > into heat. Heat is the integral of TdS regardless of whether the
> > process is reversible or irreversible. Hence, in a free expansion
> > of an ideal gas, heat is positive since the entropy increases,
> > despite the fact that the gas temperature is constant and the
> > system is isolated. This appears to be Ludwik's view in at least
> > some parts of his document.

This is even more ambiguous than the previous definition.  Just how
thermalized is energy supposed to be as it is distributed over the
system's internal degrees of freedom before it counts as heat?  Which
of the internal energies and/or enthalpy-like quantities are
supposed to have this designation & why?  Why not call such a
quantity by its actual name, such an internal energy, thermalized
energy, enthalpy or whatever particular kind of energy one wants to
discuss?  Also since this definition strongly violates the Q+W
partition of energy changes entailed in many formulations of the
first law this leave the Q term without a name.  I guess this doesn't
matter for those that don't like that particular partition in the
first place.  I am not one of those, and am relatively happy with
such a partition, and think the Q term ought to have a name.

> > 3. There is no such thing as heat. I know what energy is. I know
> > what work is. Heat is just a special kind of work applicable to
> > specially contrived problems. So who needs it? Certain list members
> > seems to hold this view. But they can and have spoken for
> > themselves.

I'm opposed to this view unless one wishes to consider all energy
changes from the purely microscopic viewpoint.  But at the
macroscopic level such a view sloughs off real distinctions between Q
and W.

> If we were to take a vote could everyone vote for one of these
> choices?

No. I'm for none of the above.

> Has Carl stated them satisfactorily?  Are there other candidates?

Yes, there are other candidates.

4. I'm for the standard definition of heat given in statistical
mechanics as the integral of the infinitesimal contribution to the
differential change in the macroscopic energy expectation due to a
change in the probability distribution for the system occupying its
various microscopic states.  Such a change in the macroscopic energy
expectation will typically be associated with a change in the
system's entropy resulting from a change in the system's distribution
of microstates which are accessible to the system's microscopic
dynamics over the time interval that the changes occur.

This definition does not invoke the concept of temperature, which is
only really well-defined for an equilibrium situation anyway, and
thus doesn't suffer from the problem that def. 1. has (which doesn't
work too well for a situation involving heating processes that
connect nonequilibrium macrostates where temperature cannot even be
defined).

> Let's take the vote.
>
> How many think 1) heat is energy transfer due to temperature
> difference?
>
> How many vote for 2) heat is internal energy or thermal energy?
>
> How many vote for 3) heat is not a noun?

Let's add definition 4) as a choice before the voting commences.

> I think we're pretty sure Jim Green, Leigh Palmer, and the editor of
> AJP vote for #3.
>
> Lots of freshman textbooks seem to be in the #1 camp.

This is probably by default because they do not wish to introduce and
discuss the concepts of microscopic states (as distinct from the
macrostate), their huge number, their various relative probabilities
and their accessibility, the real meaning of entropy, etc., and thus
have no way to implement definition 4.  Definition 1 is probably the
best they can do without introducing the necessary concepts to
understand definition 4.

> I don't necessarily think physics can be decided by democratic vote
> ....

Good.

> To those who disagree with Jim Green: do you disagree that #3 is
> correct,

I do disagree about the non-noun status of heat, but I also think it
is very useful to discipline ourselves to try to use it in its verb
form as much a possible to help avoid the pedagogical pitfall for the
students who may otherwise think of it as a kind of stuff, fluid or
energy rather than the amount of a contribution to an energy change
due to a particular kind of process.

> preferable, or more clear (please read his web page about the first
> law at <http://users.sisna.com/jmgreen/>); or do you just not think
> it worth the effort to change even though he be right?

I don't think he is technically right, but he has a point that is
useful for teaching about heat in a way that may hopefully, prevent
or head off more student misunderstanding than is absolutely
necessary for the subject.

David Bowman