State the second law of thermodynamics and explain it.

Explanation

Second law of thermodynamics may be stated as
Heat cannot flow from a colder body to a hotter body on its own.
(or)
Heat cannot be converted into work completely without causing some permanent changes in the system involved or in the surroundings.
(or)
All spontaneous processes are thermodynamically irreversible and entropy of the system increases in all spontaneous processes.
( or )
It is impossible to construct a machine which is working in cycles that can transform heat from lower temperature to higher temperature without the help of an external agency.
Second law of thermodynamics is useful in predicting
1. Whether a process occurs in a specified direction or not on its own without the intervention of any external agency i.e., whether a process is spontaneous or not in the specified direction.
2. If a transformation or a process occurs, what fraction of one form of energy is converted into another form of energy in this transformation or process.
3. A machine which transfers heat from lower temperature to higher temperature on its own is called perpetual motion machine of second kind. Second law of thermodynamics predicts that perpetual motion machine is not possible.