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USING ENERGY AND THE FIRST
LAW OF THERMODYNAMICS
3
Introduction…
Energy is a fundamental concept of thermod
l
ynamics and one of the most
significant aspects of engineering analysis. In this chapter we discuss energy
and develop equations for applying the principle of conservation of energy. The
current presentation is limited to closed systems. In
Chap. 5
the discussion is
extended to control volumes.
Energy is a familiar notion, and you already know a great deal about it. In the
present chapter several important aspects of the energy concept are developed.
Some of these we have encountered in
Chap. 1.
A basic idea is that energy can
be
stored
within systems in various forms. Energy also can be
converted
from one
form to another and
transferred
between systems. For closed systems, energy can
be transferred by
work
and
heat transfer.
The total amount of energy is
conserved
in all transformations and transfers.
The
objective
of this chapter is to organize these ideas about energy into forms
suitable for engineering analysis. The presentation begins with a review of energy
concepts from mechanics. The thermodynamic concept of energy is then intro
duced as an extension of the concept of energy in mechanics.
Reviewing Mechanical Concepts of Energy
Building on the contributions of Galileo and others, Newton formulated a general descrip
tion of the motions of objects under the influence of applied forces. Newton’s laws of motion,
which provide the basis for classical mechanics, led to the concepts of
work, kinetic energy,
and
potential energy,
and these led eventually to a broadened concept of energy. In the pres
ent section, we review mechanical concepts of energy.
3.1.1
Kinetic and Potential Energy
Consider a body of mass
m
that moves from a position where the magnitude of its ve
locity is V
1
and its elevation is
z
1
to another where its velocity is V
2
and elevation is
z
2
,
each relative to a specified coordinate frame such as the surface of the earth. The quan
tity
1
/
2
m
V
2
is the
kinetic energy,
KE, of the body. The
change
in kinetic energy,
±²³
,of
the body is
(3.1)
Kinetic energy can be assigned a value knowing only the mass of the body and the mag
nitude of its instantaneous velocity relative to a specified coordinate frame, without regard
¢
KE
´
KE
2
µ
KE
1
´
1
2
m
1
V
2
2
µ
V
1
2
2
3.1
31
chapter objective
z
m
g
1
2
kinetic energy
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Chapter 3. Using Energy and the First Law of Thermodynamics
for how this velocity was attained. Hence,
kinetic energy is a property
of the body. Since
kinetic energy is associated with the body as a whole, it is an
extensive
property.
The quantity
mgz
is the
gravitational potential energy,
PE. The
change
in gravitational
potential energy,
±
PE, is
(3.2)
Potential energy is associated with the force of gravity (
Sec. 2.3
) and is therefore an attrib
ute of a system consisting of the body and the earth together. However, evaluating the force
of gravity as
mg
enables the gravitational potential energy to be determined for a specified
value of
g
knowing only the mass of the body and its elevation. With this view, potential en
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 Fall '08
 Lee

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