We know that some processes occur spontaneously but if we try to reverse the direction of process, the process do not occur spontaneously and further some external energy is required to move the given system away from state of equilibrium.

The question is that "why such reversed processes do not occur spontaneously?" could not be answered by first law because the total energy of system would remain constant in the reversed process as it did in the orginal path and ther is no voilation of first law. Therfore there must be some other natural principle in addition to first law which determines the direction in which a process can take place in an isolated system. This principle is "second law of thermodynamics" .

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__Second law of thermodynamics in terms of entropy__ "The entropy of an isolated system is fully conserved in every reversible process i.e. for every reversible process the sum of all changes in entropy taking place in an isolated system is zero. If the process is not a reversible one, then the sum of all changes in entropy taking place in an isolated system is greater than zero. In general we can say that in every process taking place in an isolated system the entropy of system either increases or remains constant."

__Condition for equilibrium of an isolated system__“If an isolated system is in such a state that its entropy is maximum, any change from that state would evidently lead to decrease in entropy and hence will not happen. Thus the necessary condition for equilibrium of an isolated system is that its "Entropy shall be maximum."

Other forms of second law of thermodynamics

**: It is impossible to construct a device which, operating in a cycle has the sole effect of extracting heat from a single reservoir and performing equivalent amount of work.**

__Kelvin-Planck statement__**: It is impossible for heat to flow from a cooler body to another hotter body without the aid of external energy.**

__Clausius statement__ "Study on heat engines is based on the above law"

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