ABSTRACT
The concept of entropy is of central importance in thermal physics. Historically, it was through the study of heat engine processes that the need for new concept was first appreciated. Heat engines operate by conversion of heat into useful work. A natural question that arises concerns the maximum possible efficiency of a heat engine. The expansion of an ideal gas, as used in the operation of an engine, may be carried out suddenly and irreversibly instead of reversibly. As an order of magnitude, the time should be shorter than the time it takes for a particle traveling at the average speed in the gas to traverse a distance comparable with the interparticle separation. The increase in entropy of the universe that occurs in irreversible processes corresponds to a loss of opportunity to obtain useful work from a system. An alternative fundamental approach uses three basic and reasonable postulates as a point of departure.
