Natural and reversible processes

Changes which take place in a system spontaneously and of their, own accord are called natural processes. Examples are the equalization of temperature between two pieces of metal, the mixing of two gases and all processes which can occur spontaneously within an adiabatic enclosure. From what has been said in the last section it seems that such changes can never be reversed in their entirety, for it is known from experience that the system in question can he restored to its original condition only by transferring a quantity of heat elsewhere. In this respect natural processes are said to be irreversible. In brief, a cycle of changes A- B- A on a particular 

83n3tem, where is a natural process, cannot be completed without 

It seems possible, however, that there may exist other types of change, involving a different pair of specified states, say, A and of the system under discussion, such that it is possible to carry out the cycle without leaving more than a negligible change 

A reversible process will therefore be defined as one which can be reversed without leaving more than a vanishingly small change in any other system. If the change A B involves a heat absorption q it must be possible to carry out the reverse change B - A rejecting an equal quantity of heat into the same heat bath. (The work effects on 

For the purpose of the detailed discussion of the second law in the next section it is necessary to postulate that reversibility can be approached, under limiting conditions, in real processes. We must now give some justification that such a state of affairs is a possible one. Consider the isothermal expansion of a fiuid in contact with a heat 

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