The Second Law of Thermodynamics, Entropy
Many statements of the second law of thermodynamics have been proposed. Each of these can be called
a statement of the second law or a corollary of the second law since, if one is invalid, all are invalid.
In every instance where a consequence of the second law has been tested directly or indirectly by
experiment it has been verified. Accordingly, the basis of the second law, like every other physical law,
is experimental evidence.
Kelvin-Planck Statement
The Kelvin-Plank statement of the second law of thermodynamics refers to a thermal reservoir. A thermal
reservoir is a system that remains at a constant temperature even though energy is added or removed by
heat transfer. A reservoir is an idealization, of course, but such a system can be approximated in a number
of ways — by the Earth’s atmosphere, large bodies of water (lakes, oceans), and so on. Extensive
properties of thermal reservoirs, such as internal energy, can change in interactions with other systems
even though the reservoir temperature remains constant, however.
The Kelvin-Planck statement of the second law can be given as follows: It is impossible for any system
to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings
while receiving energy by heat transfer from a single thermal reservoir. In other words, a perpetualmotion
machine of the second kind is impossible.
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