Brian, I did not label the four-letter words "reactionary". What I am
trying to do is to trace the evolution of the meaning. I did not make up
my mind about using or not using them in teaching. Perhaps next time I
will say "the input of an engine is thermal energy while its output is
the degraded thermal energy plus mechanical energy. The energy was not
lost but it usefulness was."
Here are quotations from Modern College Physics of Richards, Sears, Wher
and Zemansky, the text I used when I started teaching 30 years ago. The book
is based on an earlier text by Sears and Zemansky; it had an impact on many
physics teachers of my generation. Mark Zemansky, by the way, was a research
thermodynamist, and a teacher.
I will use quotation marks to distinguish textbook sentences from my own.
1) Conservative and non-conservative (dissipative) forces are contrasted.
"The work of the friction force depends on the path; the longer the
path, the greater the work. There is no function the difference of which
equals the work of the friction force. When we slide an object on a rough
fixed surface back to its original position, the friction force reverses,
and instead of recovering the work done in the first displacement we must
again do work on the round trip. In other words, frictional work is not
completly recoverable." We know what they mean by "recovering the work
done"; but many can interpret this as 'work is energy'.
2) "When the friction acts alone, the total mechanical energy is not
conserved. ... The mechanical energy of a body is conserved only when
no dissipative forces act on it. We find that when friction forces act
on a moving body, heat is always developed. The more general principle
of conservation of energy includes this heat as another form of energy,
along with kinetic and potential energy, and when it is included the
total energy of any system remains constant." They clearly say that
heat is "another form of energy". A SLIP OF THE TANQUE?
3) The chapter on thermodynamic is preceeded by a chapter called "Heat
and Work". Work on compressing a gas is described in term of the area
under the p-V diagram. The authors show that the work done (in a
transition from a well defined initial point 1 to the well defined final
point 2 is path-dependent. Then they say that the state of water in a
container can be changed by either doing work on it (paddle wheel),
or by using a Bunzen burner, or by absorbing radiation from a light bulb.
In the last two processes "the state of the system is caused to change,
but in neither case can the agency for the change be described by
mechanical means. In these cases we say there is a flow of heat. The
flow of heat is a nonmechanical energy transfer brought about by the
temperature difference between two bodies." Some phys-L-ers wrote that
"flow of heat" is a dirty term to be avoided.
4) "We have seen that, in general, the work depends on the path by which
the system [gas] was brought from the initial to final state. Exactly
the same is true for the heat transferred to or from a body. ...It would
be just as incorrect to refer to the 'heat in the body' as it would be
to speak about the 'works in a body'". Can these undeniable assertions
be misinterpreted? Potential energy is also NOT in a body of an elevated
object. It is the property of the 'earth+body' system. Some would probably
argue that a large fraction of potential energy is in the empty space
bewtween the objects. But that is another matter. Or should I say another
field of physics? What is the Martian word for field?
The kinetic energy of a bullet is different in different inertial frames
of reference. How can we say it is IN the bullet? What I am trying to
say is that 'not being in a body' is not a unique attribute of heat and
work. Nothing profound; just trying to be picky. The rest of the Work
and Heat chapter is devoted to calorimetry, to phase transformations
and to mechanisms of "heat transfer". The concept of temperature was
introduced in the previous chapter.
5) Here are quotations from the chapter devoted to Laws of Thermodynamics.
"The transfer of heat and the performance of work constitute two methods
of adding energy to or subtracting energy from a system. They represent
energy in transit ... Once the transfer of energy is over, the body is
said to have undergone a change in internal energy. It is impossible to
separate internal energy into a mechanical and a thermal part. From the
molecular point of view, ..."
"Suppose a system is caused to change from state 1 to state 2 along a
definite path and that the heat absorbed, Q, and the work W is measured.
.... We may then calculate the difference Q-W. If now we do the same
thing over again for many different paths (between the same states 1
and 2), the important result is obtained that Q-W is the same for each
path ... But Q is the energy which has been added to the system by the
transfer of heat, and W is equal to the energy that has been extracted
from the system by performing work. ... A simple algebraic statement of
this fact in the form
Q = U2 - U1 + W
is known as the first law of thermodynamics." The authors know that
heat and works are distinct (path dependant) quantities but a student
is likely to reinforce the previously created belief that heat and
work are forms of energy. Later they write "The process of combustion
releases the internal energy and converts it into heat. In this form
the energy may be utilized for ... But to operate a machine, or to
propel a vehicule or a projectile, the heat must be converted to
mechanical energy ..." The well known heat engine diagram, a circle
with arrows for Q1, Q2 and W, and its verbal description, also treat
the three kinds of "joules" as forms of energies.
6) And here is what some would call the slip of the tangue made
by Paul Tipler.
"The purpose of a heat engine is to convert as much of the heat
input Qh into work as possible."
I found this sentence in his 1991 Physics for Scientists and Engineers
(p 566) and in his 1987 College Physics.
No, I am not trying to emitate Mario Iona. My point is that in the
context of thermal engines treating heat and work as if they were forms
of energy is useful to engineers. Nothing profound, just an observation.