Page 13 - Entrophy Analysis in Thermal Engineering Systems

Fundamental concepts 3

1.3 Conservation of energy

The principle of energy conservation states that energy is neither cre-
ated nor destroyed. It may transform from one type to another. Like the mass
conservation principle, the validity of the conservation of energy relies on
experimental observations; thus, it is an empirical law. No experiment
has violated the principle of energy conservation yet. The common forms
of energy include thermal, electrical, chemical, mechanical, kinetic, and
potential. It may also be stated that the sum of all kinds of energy is constant.

X
E k ¼ constant (1.3)
k
where E denotes energy and subscript k refers to the different types of energy.
Many engineering applications involve transformation of energy
between two or three types only. For instance, in dynamics problems, the
conservation of energy accounts for two types of energy, i.e., kinetic and
potential (in some cases frictional work), neglecting the effect of other forms
like chemical, thermal, or electrical. In chemical reactions, the conservation
of energy includes thermal and chemical energies, and the effect of other
forms of energy is ignored. In most thermodynamic problems, the principle
of energy conservation applied to nonreactive systems accounts for thermal
and mechanical energies.

1.4 First law of thermodynamics

The two main laws of thermodynamics formulated in 19th century
were first introduced as the main principles of the Mechanical Theory of
Heat. The first law is indeed an expression of the energy conservation prin-
ciple that accounts for transformation of energy in the form of heat and
work. The first law, in the words of Clausius [3], is the principle of the
equivalence of heat and work.
The mathematical expression of the first law applied to a closed system
with a fixed quantity of mass can be written as follows.

(1.4)
ΔU ¼ Q W
where U is the internal energy, Q is the amount of heat transferred to the
system, and W denotes the work done by the system on its surrounding.