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Re: A "heat" question



Each iteration of the heat, work, and energy discussions slowly
evolves my understanding. What a thorny topic. But worthy of
consideration, not just stubbornly clinging to current notions.

Anyhow, for what it's worth, here are some of my current thoughts on the topic.

Heat and work are forms of energy transfer, or if you prefer, ways of
changing the energy of a system. The energy of a system can be
divided into internal energy and (for want of a better name) bulk
energy.

Bulk energy certainly includes 0.5MV^2 where M = total mass and V =
velocity of center of mass. For a system divided into well-defined
macroscopic parts (blocks, springs, planets, etc) it can also include
the interactional potential energy between the parts (and probably
should if you want to make contact with the mechanical energy chapter
in intro texts).

Internal energy is the kinetic energy intrinsic to the parts (both
bulk rotational and microscopic translational) plus the potential
energy due to interactions between particles inside a part. It also
includes nonmechanical forms such as radiation, magnetic energy,
field energies, etc.

It may or may not be possible to divide energy transfer into heat and
work. Work is always defined as an integral of force dotted with
displacement, but there are many different kinds, depending on which
force and which displacement you consider: there is the work which
equals only the change in the bulk KE (usually called center-of-mass
or pseudo work), there is the kind which changes only the bulk
kinetic and potential energy (usually called nonconservative particle
work), there is the kind which appears in the first law of
thermodynamics, etc.

Heat is the nonadiabatic energy transfer. In reversible processes,
this corresponds to the energy transfer by virtue of the temperature
difference between two objects, usually divided into conduction,
convection, and blackbody radiation. (Note that ideal laser radiation
should properly be considered work not heat according to the
Caratheodory definition, ie. work is energy that can lift a weight
without other changes.)

For reversible processes, heat and work can always be distinguished
and it is often helpful to do so. For irreversible processes, this
distinction is not generally possible or useful. For example, when a
block slides against a rough surface, clearly there are frictional
forces doing work, but temperatures are changing leading to heat, and
it is not possible to distinguish work and heat from a thermodynamic
viewpoint. In that case, it may be helpful to lump both together and
talk about a particle-to-particle energy transfer which can be
described as mechanical work.

Similarly, it may or may not be possible and useful to divide
internal energy into thermal and nonthermal components. For example,
an ideal gas is all thermal. A rolling cylinder at constant
temperature has two obvious forms of internal energy (in additional
to the bulk translational energy): the nonthermal rotational energy
(some people prefer to count this as a form of bulk energy, but I
think this leads to problems - eg. what will you do about a flywheel
under the hood?) plus the thermal kinetic and potential energies of
the atoms and molecules making it a solid and giving rise to what we
call the temperature.

In contrast, if you consider a struck bell, obviously the energy
transferred during the strike is initially nonthermal (a collective
oscillation which is not thermalized) but eventually becomes entirely
thermal (including the energy lost to sound waves and thermalized in
the air). At some stage in between however, there is no way to neatly
separate the internal energy into thermal and nonthermal components.
Its reminiscent of trying to separate the energy transferred during
the sliding block into heat (via surface temperatures) and work (via
organized forces).

Well, these are thoughts I've gleaned from the list and my own
thinking. Don't jump all over me, but comments in public or private
always welcome. Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.