Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Now if asked to name the energy transformations associated with the
operation of a refrigerator, we may still have a hard time answering because
all we see is electrical energy entering the refrigerator and heat dissipated
from the refrigerator to the room air. Obviously there is need to study the
various forms of energy first, and this is exactly what we do next, followed
by a study of the mechanisms of energy transfer.

2–2 ■ FORMS OF ENERGY

Energy can exist in numerous forms such as thermal, mechanical, kinetic,
potential, electric, magnetic, chemical, and nuclear, and their sum consti-
tutes the total energyEof a system. The total energy of a system on a unit
massbasis is denoted by eand is expressed as

(2–1)

Thermodynamics provides no information about the absolute value of the
total energy. It deals only with the changeof the total energy, which is what
matters in engineering problems. Thus the total energy of a system can be
assigned a value of zero (E
0) at some convenient reference point. The
change in total energy of a system is independent of the reference point
selected. The decrease in the potential energy of a falling rock, for example,
depends on only the elevation difference and not the reference level
selected.
In thermodynamic analysis, it is often helpful to consider the various
forms of energy that make up the total energy of a system in two groups:
macroscopicand microscopic. The macroscopicforms of energy are those a
system possesses as a whole with respect to some outside reference frame,
such as kinetic and potential energies (Fig. 2–3). The microscopicforms of
energy are those related to the molecular structure of a system and the
degree of the molecular activity, and they are independent of outside refer-
ence frames. The sum of all the microscopic forms of energy is called the
internal energyof a system and is denoted by U.
The term energywas coined in 1807 by Thomas Young, and its use in
thermodynamics was proposed in 1852 by Lord Kelvin. The term internal
energyand its symbol Ufirst appeared in the works of Rudolph Clausius
and William Rankine in the second half of the nineteenth century, and it
eventually replaced the alternative terms inner work, internal work, and
intrinsic energycommonly used at the time.
The macroscopic energy of a system is related to motion and the influ-
ence of some external effects such as gravity, magnetism, electricity, and
surface tension. The energy that a system possesses as a result of its motion
relative to some reference frame is called kinetic energy(KE). When all
parts of a system move with the same velocity, the kinetic energy is
expressed as

KE
m (2–2)

V^2

2

¬¬ 1 kJ 2

e

E

m

¬¬ 1 kJ>kg 2

Chapter 2 | 53

FIGURE 2–3

The macroscopic energy of an object

changes with velocity and elevation.

SEE TUTORIAL CH. 2, SEC. 2 ON THE DVD.

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