Subsections
6.8 Some Overall Comments on Entropy, Reversible and Irreversible
Processes
[Mainly excerpted (with some alterations) from: Engineering
Thermodynamics, William C. Reynolds and Henry C. Perkins,
McGraw-Hill Book Company, 1977.]
- Entropy is a thermodynamic property that measures the degree of
randomization or disorder at the microscopic level. The
natural state of affairs is for entropy to be produced by all
processes.
- A macroscopic feature which is associated with entropy production is a
loss of ability to do useful work. Energy is degraded to a less
useful form, and it is sometimes said that there is a decrease in
the availability of energy.
- Entropy is an extensive thermodynamic
property. In other words, the entropy of a complex system is the sum
of the entropies of its parts.
- The notion that entropy can be
produced, but never destroyed, is the second law of
thermodynamics.
6.8.2 Reversible and Irreversible Processes
Processes can be classed as reversible or irreversible. The concept
of a reversible process is an important one which directly relates
to our ability to recognize, evaluate, and reduce irreversibilities
in practical engineering processes.
Consider an isolated system. The second law says that any process
that would reduce the entropy of the isolated system is impossible.
Suppose a process takes place within the isolated system in what we
shall call the forward direction. If the change in state of the
system is such that the entropy increases for the forward process,
then for the backward process (that is, for the reverse change in
state) the entropy would decrease. The backward process is therefore
impossible, and we therefore say that the forward process is
irreversible.
If a process occurs, however, in which the entropy is
unchanged by the forward process, then it would also be
unchanged by the reverse process. Such a process could go in either
direction without contradicting the second law. Processes of this
latter type are called reversible.
The key idea of a reversible process is that it does not produce any
entropy.
Entropy is produced in irreversible processes. All real processes
(with the possible exception of superconducting current flows) are
in some measure irreversible, though many processes can be analyzed
quite adequately by assuming that they are reversible. Some
processes that are clearly irreversible include: mixing of two
gases, spontaneous combustion, friction, and the transfer of energy
as heat from a body at high temperature to a body at low
temperature.
Recognition of the irreversibilities in a real process is especially
important in engineering. Irreversibility, or departure from the
ideal condition of reversibility, reflects an increase in the amount
of disorganized energy at the expense of organized energy. The
organized energy (such as that of a raised weight) is easily put to
practical use; disorganized energy (such as the random motions of
the molecules in a gas) requires ``straightening out'' before it can
be used effectively. Further, since we are always somewhat uncertain
about the microscopic state, this straightening can never be
perfect. Consequently the engineer is constantly striving to reduce
irreversibilities in systems, in order to obtain better performance.
6.8.3 Examples of Reversible and Irreversible Processes
Processes that are usually idealized as reversible include:
- Frictionless movement
- Restrained compression or expansion
- Energy
transfer as heat due to infinitesimal temperature nonuniformity
- Electric current flow through a zero resistance
- Restrained chemical reaction
- Mixing of two samples of the same substance at the same
state.
Processes that are irreversible include:
- Movement with
friction
- Unrestrained expansion
- Energy transfer as heat due to large
temperature non uniformities
- Electric current flow through a non
zero resistance
- Spontaneous chemical reaction
- Mixing of matter of
different composition or state.
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