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# Second Law of Thermodynamics

The first law of thermodynamics is a law of equivalence and tells us the inter-convertibility of heat and mechanical work. Now we can ask some questions.
• Why does heat always flow from an object at a higher temperature to an object at a lower temperature and not from the latter to former?
• Why are we not able to convert the whole amount of available heat into work?
The second law of thermodynamics gives the answer for these questions and gives a fundamental limitation to the efficiency of a heat engines and the coefficient of performance of a refrigerator.
Simply put, the efficiency of any heat engine can never be equal to unity. This implies that heat released to the cold reservoir can never be made zero. For a refrigerator the coefficient of performance can never be made to infinite.
There are two types of statement of second law of thermodynamics which denies the possibility of perfect heat engine and perfect refrigerator.

Kelvin-Planck Statement
No process is possible whose sole result is the absorption of heat from a reservoir and the complete conversion of the heat into work.

Clausius Statement
No process is possible whose sole result is the transfer of heat from a colder object to a hotter object.

Reversible and Irreversible processes
An important concept in the subject of thermodynamics is that of a reversible process. Suppose a system changes from state A to state B after undergoing a series of transformations. During that process, the system may do work (or work may be done on it) or it may absorb heat from (or reject heat to) its surroundings at various stages.

We now force the system in the reverse direction. For example, in a direct process, a gas enclosed in a cylinder may be compressed by a piston and, in the reverse process, the gas is expanded by moving the piston in the opposite direction.
If, in the reverse process, the system completely retraces its path in the reverse order, the process is called reversible. It means that if some heat is absorbed from the surroundings in the direct process, the same amount of heat is given out to the surroundings in the reverse process, and if some work is done on the system in the direct process then some work is done to the system in the reverse process.

Thus a reversible process is one that is performed in such a way that, at the conclusion of the process, both the system and its surroundings are restored to their initial states, without producing any change in the rest of the universe. A process that does not fulfill this stringent requirement is said to be irreversible.

Conditions Necessary for a Reversible Process
• The temperature and pressure of the system undergoing a reversible change must not be very different from those of the surroundings, so that the two are always in thermal equilibrium.
• The process must be extremely slow.
• Working parts must be free from friction.
• There must be no loss of heat due to conduction or radiation.
In nature, all processes are irreversible because no natural process can fulfill the stringent requirements of a reversible process. The reversible process is an idealization and its importance lies in the fact that it can be analyzed and yields useful information on the departures from reversibility.

An irreversible process is one in which, at the conclusion of the process, the system cannot be restored to its initial state when the process is reversed. A permanent change is left somewhere.

A few common examples of an irreversible process are:
• Gas in a vessel at a higher pressure moves to a vessel at a lower pressure until the pressures are equalized. The process is unidirectional.
• Two solutions having different concentrations are mixed and the mixture acquires a uniform concentration due to diffusion. The reverse is not possible.
• Chemical reactions are irreversible.
• Heat loss due to friction is irreversible.
• Exchange of heat due to conduction or radiation is irreversible.