Description
The first law of thermodynamics provides the basic definition of thermodynamic energy, also called internal energy, associated with all thermodynamic systems, but unknown in mechanics, and states the rule of conservation of energy in nature.
However, the concept of energy in the first law does not account for the observation that natural processes have a preferred direction of progress. For example, heat always flows spontaneously from regions of higher temperature to regions of lower temperature, and never the reverse, unless external work is performed on the system. The first law is completely symmetrical with respect to the initial and final states of an evolving system. The key concept for the explanation of this phenomenon through the second law of thermodynamics is the definition of a new physical property, the entropy.
A change in the entropy (dS) of a system is the infinitesimal transfer of heat (δQ) to a closed system driving a reversible process, divided by the equilibrium temperature (T) of the system.
The entropy of an isolated system that is in equilibrium is constant and has reached its maximum value.
Empirical temperature and its scale is usually defined on the principles of thermodynamics equilibrium by the zeroth law of thermodynamics. However, based on the entropy, the second law permits a definition of the absolute, thermodynamic temperature, which has its null point at absolute zero.
The second law of thermodynamics may be expressed in many specific ways, the most prominent classical statements being the statement by Rudolph Clausius (1854), the statement by Lord Kelvin (1851), and the statement in axiomatic thermodynamics by Constantin Carathéodory (1909). These statements cast the law in general physical terms citing the impossibility of certain processes. They have been shown to be equivalent.
Read more about this topic: Second Law Of Thermodynamics
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