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1.8 Second Law of Thermodynamics
All the relations developed in the preceding sections are used in making an energy
accounting for various types of systems. In addition to being able to write an energy
balance, an engineer must be able to predict how much of the heat transferred to a
system may be converted into useful work, or how much work is required to produce a
certain refrigerating effect, or whether or not a system will undergo a specified change.
These predictions cannot be made on the basis of the first law alone. For example, it is
impossible to predict from the first law alone how much of the heat transferred to a heat
engine may be converted into work. As far as the first law is concerned, all the heat
transferred could conceivably be converted into work. In order to obtain information
regarding these aspects of engineering analysis, the second law of thermodynamics
must be employed.
Limitation of the First Law. The First Law of Thermodynamics states that energy is
conserved. However, we can think of many thermodynamics process, which conserve
energy but which actually never occur. For example, when a hot body and a cold body
are put into contact it simply does not happen the hot body gets hotter and the cold body
gets colder. The Firs Law doesn’t restrict our ability to convert work into heat or heat
into work, except that energy must be conserved in the process. In practice we cannot
convert a given quantity of heat completely into work. The Second Law of
Thermodynamics deals with question of whether process does or doesn’t occur in
nature.
There have been many statements of the second law, each emphasizing another facet
of the law, but all are equivalent to one another.
Clausius’s statement: heat cannot flow by itself from a body at lower temperature to
another one at a higher temperature.
In a heat engine, heat flows in its natural direction — from a hot body to a cold one.
During this flow, a part of the heat is converted into the work. In a refrigerator, heat
flows in the reverse direction i.e., from a cold body to a hot one. But in this case, we
have to do some work for maintaining this natural flow of heat. This work is done on
the pump of the refrigerator without which the refrigerator cannot operate.
Carnot’s statement: the heat of a single reservoir cannot be converted into
mechanical work.
It is impossible to construct a heat engine, which can take heat from a single heat
reservoir and convert it completely into useful work. A heat engine must work between
two temperatures, so that it receives heat from a heat reservoir at a higher temperature
and rejects some of this heat to another heat reservoir at lower temperature. This
explains why we cannot utilize the immense heat energy stored in the seawater or in the
atmosphere.
The efficiency of all reversible engines operating between the same two temperatures
is the same, and no irreversible engines working between the same two temperatures
can have a greater efficiency than another one.

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