SECOND LAW OF THERMODYNAMICS
Thermodynamics
With the Second Law of Thermodynamics, the limitations imposed on any process can be studied
to determine the maximum possible efficiencies of such a process and then a comparison can be
made between the maximum possible efficiency and the actual efficiency achieved. One of the
areas of application of the second law is the study of energy-conversion systems. For example,
it is not possible to convert all the energy obtained from a nuclear reactor into electrical energy.
There must be losses in the conversion process. The second law can be used to derive an
expression for the maximum possible energy conversion efficiency taking those losses into
account. Therefore, the second law denies the possibility of completely converting into work all
of the heat supplied to a system operating in a cycle, no matter how perfectly designed the
system may be. The concept of the second law is best stated using Max Planck’s description:
It is impossible to construct an engine that will work in a complete cycle and
produce no other effect except the raising of a weight and the cooling of a heat
reservoir.
The Second Law of Thermodynamics is needed because the First Law of Thermodynamics does
not define the energy conversion process completely. The first law is used to relate and to
evaluate the various energies involved in a process. However, no information about the direction
of the process can be obtained by the application of the first law. Early in the development of
the science of thermodynamics, investigators noted that while work could be converted
completely into heat, the converse was never true for a cyclic process. Certain natural processes
were also observed always to proceed in a certain direction (e.g., heat transfer occurs from a hot
to a cold body). The second law was developed as an explanation of these natural phenomena.
Entropy
One consequence of the second law is the development of the physical property of matter termed
entropy (S). Entropy was introduced to help explain the Second Law of Thermodynamics. The
change in this property is used to determine the direction in which a given process will proceed.
Entropy can also be explained as a measure of the unavailability of heat to perform work in a
cycle. This relates to the second law since the second law predicts that not all heat provided to
a cycle can be transformed into an equal amount of work, some heat rejection must take place.
The change in entropy is defined as the ratio of heat transferred during a reversible process to
the absolute temperature of the system.
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