Second law of thermodynamics
"It is impossible for a process to have as its sole result the transfer of heat from a cooler body to a hotter one."
Second law of thermodynamics is perhaps the most popular outside of the real of physics, because it is closely related to the concept of entropy, or the disorder created during a thermodynamic process. Reformulated as a statement regarding entropy, the second law reads:
"In any closed system, the entropy of the system will either remain constant or increase."In other words, each time a system goes through a thermodynamic process, the system can never completely return to precisely the same state it was in before. This is one definition used for the arrow of time, since entropy of the universe will always increase over time according to the second law of thermodynamics.
The change in entropy (ΔS) was originally defined for a thermodynamically reversible process as
- Which is found from the uniform temperature (T ) of a closed system dividing an incremental reversible transfer of heat into that system (dQ).
The above definition is sometimes called the macroscopic definition of
entropy because it can be used without regard to any microscopic picture
of the contents of a system. In thermodynamics, entropy has been found
to be more generally useful and it has several other formulations.
Entropy was discovered when it was noticed to be a quantity that behaves
as a function of state, as a consequence of the second law of
thermodynamics. Entropy is an extensive property, but the entropy of a
pure substance is usually given as an intensive property — either
specific entropy (entropy per unit mass) or molar entropy (entropy per
mole).
The absolute entropy (S rather than ΔS) was defined later, using either statistical mechanics or the third law of thermodynamics.
Entropy has the dimension of energy divided by temperature, which has a unit of joules per kelvin (J/K) in the International System of Units.
This summary covers the relevant relations concerning entropy generation , entropy evaluation and isentropic processes.
