SECTION 7.6 • The Non-Isolated System—Conservation of Energy
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face of a heavy table and slowing down due to the friction force. Suppose the surface is
the system. Then the friction force from the sliding book does work on the surface.
The force on the surface is to the right and the displacement of the point of applica-
tion of the force is to the right—the work is positive. But the surface is not moving af-
ter the book has stopped. Positive work has been done on the surface, yet there is no
increase in the surface’s kinetic energy. Is this a violation of the work–kinetic energy
theorem?
It is not really a violation, because this situation does not fit the description of the
conditions given for the work–kinetic energy theorem. Work is done on the system of
the surface, but the result of that work is not an increase in kinetic energy. From your
everyday experience with sliding over surfaces with friction, you can probably guess
that the surface will be warmer after the book slides over it. (Rub your hands together
briskly to experience this!) Thus, the work that was done on the surface has gone into
warming the surface rather than increasing its speed. We call the energy associated
with an object’s temperature its
internal energy, symbolized E
int
. (We will define inter-
nal energy more generally in Chapter 20.) In this case, the work done on the surface
does indeed represent energy transferred into the system, but it appears in the system
as internal energy rather than kinetic energy.
We have now seen two methods of storing energy in a system—kinetic energy, re-
lated to motion of the system, and internal energy, related to its temperature. A third
method, which we cover in Chapter 8, is potential energy. This is energy related to the
configuration of a system in which the components of the system interact by forces. For
example, when a spring is stretched, elastic potential energy is stored in the spring due to
the force of interaction between the spring coils. Other types of potential energy in-
clude gravitational and electric.
We have seen only one way to transfer energy into a system so far—work. We men-
tion below a few other ways to transfer energy into or out of a system. The details of
these processes will be studied in other sections of the book. We illustrate these in
Figure 7.17 and summarize them as follows:
Work, as we have learned in this chapter, is a method of transferring energy to a
system by applying a force to the system and causing a displacement of the point of ap-
plication of the force (Fig. 7.17a).
Mechanical waves (Chapters 16–18) are a means of transferring energy by allow-
ing a disturbance to propagate through air or another medium. This is the method by
which energy (which you detect as sound) leaves your clock radio through the loud-
speaker and enters your ears to stimulate the hearing process (Fig. 7.17b). Other ex-
amples of mechanical waves are seismic waves and ocean waves.
Heat (Chapter 20) is a mechanism of energy transfer that is driven by a tempera-
ture difference between two regions in space. One clear example is thermal conduc-
tion, a mechanism of transferring energy by microscopic collisions. For example, a
metal spoon in a cup of coffee becomes hot because fast-moving electrons and atoms
in the submerged portion of the spoon bump into slower ones in the nearby part of
the handle (Fig. 7.17c). These particles move faster because of the collisions and bump
into the next group of slow particles. Thus, the internal energy of the spoon handle
rises from energy transfer due to this bumping process.
4
Matter transfer (Chapter 20) involves situations in which matter physically crosses
the boundary of a system, carrying energy with it. Examples include filling your auto-
mobile tank with gasoline (Fig. 7.17d), and carrying energy to the rooms of your home
by circulating warm air from the furnace, a process called convection.
4
The process we call heat can also proceed by convection and radiation, as well as conduction.
Convection and radiation, described in Chapter 20, overlap with other types of energy transfer in
our list of six.
▲
PITFALL PREVENTION
7.8 Heat is not a Form
of Energy
The word heat is one of the most
misused words in our popular
language. In this text, heat is a
method of transferring energy, not
a form of storing energy. Thus,
phrases such as “heat content,”
“the heat of the summer,” and
“the heat escaped” all represent
uses of this word that are incon-
sistent with our physics defini-
tion. See Chapter 20.