Solution:

• Second law of thermodynamics: In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases.
• Equivalently, perpetual motion machines of the second kind (machines that spontaneously convert thermal energy into mechanical work) are impossible.
• The second law is applicable to a wide variety of processes, reversible and irreversible.
• All natural processes are irreversible.
• Reversible processes are a useful and convenient theoretical fiction, but do not occur in nature.
• A prime example of irreversibility is in the transfer of heat by conduction or radiation.
• It was known long before the discovery of the notion of entropy that when two bodies initially of different temperatures come into thermal connection, then heat always flows from the hotter body to the colder one.
• The second law tells also about kinds of irreversibility other than heat transfer, for example those of friction and viscosity, and those of chemical reactions.
• The notion of entropy is needed to provide that wider scope of the law.