Modern Engineering Thermodynamics

7.4 Carnot’s Heat Engine and the Second Law of Thermodynamics 211
mechanical friction fluid viscosity electrical resistance
shock waves mixing chemical reactions
heat transfer plastic deformation hysteresis
Thus, it is easy to see that all engineering processes of interest are really irreversible processes, and aside from a
few mathematical formalities, we have very little need for the reversibility concept.However,wearenotyet
capable of analyzing all the irreversibilities that occur in the complex real world, so we indeed need to model
our device as reversible during the design stage, then build and test a prototype of it to determine the effect of
KELVIN-PLANCK STATEMENT OF THE SECOND LAW
OF THERMODYNAMICS 4
You cannot make a continuously operating heat engine that converts all of its heat input directly into work output (Figure 7.4).
High-temperature reservoir
Heat in
Cyclic heat
engine
Work out = Heat in
Impossible!
No heat out!
Low-temperature reservoir
FIGURE 7.4
Kelvin-Planck statement.
4 The Kelvin-Planck statement has many different written forms. For example, it can be expressed as “It is impossible to build a continuously
operating device that produces a work output while absorbing heat from a single thermal reservoir,” and as “No heat engine can be more
than 100% efficient.” Another form of the Kelvin-Planck statement is “It is impossible to build a continuously operating device that produces a
work output while absorbing heat energy from a single thermal reservoir.”
WHAT DOES THE WORD ENTROPY MEAN?
Rudolph Clausius was German, and the word he chose for Carnot’s energy transformation concept was verwandlungsinhalt,
meaning “transformation content.” Fortunately, in 1865, he chose to rename this concept by choosing the Greek word
entropy, meaning simply “to change, or transform.” Later, there was an unsuccessful attempt to name an entropy unit the
clausius, Cl, after him. It was defined as 1 Cl = 1 kcal/K = 4.186 J/K, but it was not universally accepted.
REVERSIBLE PROCESSES: CLASSICAL DEFINITION
A process is called reversible if, at any time, both the system and the environment can be returned to their original states.
REVERSIBLE PROCESS: MODERN DEFINITION
A reversible process is defined to be any process for which the entropy production or the entropy production rate for the
process is zero (i.e., S P = Ṡ P = 0).