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>> From what I have heard on PHYS-L and elsewhere, I suggest the
>>or advanced undergraduate thermo and it is the view I prefer.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
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.