17
Work, Heat, and the First Law of
Thermodynamics
Recommended class days:
2
Background Information
Chapter 16 introduced the state variables needed to characterize a macroscopic system. Now we
begin the process of looking at how those state variables change as a consequence of energy
transferred into or out of the system.
Chapter 11 developed the workkinetic energy theorem in the form
∆
K
=
W
=
W
c
+
W
nc
+
W
ext
where
W
c
is the work done by conservative forces,
W
nc
is the work done by nonconservative forces
within the system (e.g., friction), and
W
ext
is the work done by external forces. Dividing the forces
into these three groups is important. Conservative forces are associated with potential energies via
∆
U
=

W
c
, and the dissipative nonconservative forces are associated with increasing the thermal
energy of the system via
∆
E
th
=

W
nc
. This leads to
∆
E
mech
+
∆
E
th
=
W
ext
where
E
mech
=
K
+
U
.
The systems we want to study in thermodynamics are
stationary
containers of gases or liquids
whose centerofmass mechanical energy does not change. Thus
∆
E
mech
=
0 and
∆
E
th
=
W
ext
Here
W
ext
is the work done by piston rods or by other forces that compress or expand the gas.
These are forces of a very different nature from the friction forces we’ve called nonconservative
forces. Since
all
the forces in thermodynamics that do work are external forces, we can drop the
subscript. Then, after noting that work is one way to transfer energy to the system, but not the only
way, we can define heat
Q
as a nonmechanical energy transfer due to a temperature difference.
Heat enters on an equal footing with work, with both being mechanisms for transferring energy.
With heat included, we have
∆
E
th
=
W
+
Q
This is, of course, the first law of thermodynamics. We have continued to modify and enlarge
the concept of energy, but there has been no discontinuity of ideas in moving from mechanics to
thermodynamics. You should encourage students to review the development that has taken us from
kinetic energy to the first law, and you can ask students to articulate what new ideas were added at
each step. These ideas are summarized in graphical form in the “thermodynamics energy model” of
Figure 17.12. I urge you to explicitly call this figure to your students’ attention.
The major new concept is
heat
. It’s well known that students confuse heat with thermal energy.
Most students think that heat is what a thermometer measures—not a surprising belief, considering
the everyday use of the word
heat
. It is important at every opportunity to emphasize that heat and
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Instructor’s Guide
work are energy that is
transferred
between the system and the environment. Neither heat nor work
is a property
of
the system. Because temperature measures a property of the system, temperature
must be measuring something other than heat.
Our own language can mislead students if we’re not careful. I’ve caught myself saying, “I’m
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 Spring '10
 kant
 Thermodynamics, Heat

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