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Viewpoints

From the method of expression of energy transfer and some notion of absolute temperature. we can form a definition of entropy, S:

From the one invariable fact that heat always flows from hot to cold, "dS > 0" is a statement of irreversible change, i.e. of the direction of time.

Entropy gives a means of thinking about those changes that take place which are ignored in mechanics: what happens to energy dissipated in friction? What is the maximum useful work we can get out of a heat engine? We can say that the greater the entropy of a system, the more the energy that has gone out of reach from the point of view of doing useful work. Where has it gone? It has been lost within the system and is no longer available by any means whatsoever except as heat.

But how can this energy be "lost" in this way? Reflect a moment on the nature of "useful work". This entails operations that we can do by using systems. These operations are macroscopic in character. That which belongs exclusively to the microscopic molecular world is out of reach of our operations. That is why we relate entropy to the concept of disorder, meaning complexity out of control. What appears chaotic and uncertain on the molecular level, appears as uniformity on the level of the system as a whole. "Within" the tranquillity of a gas in thermal equilibrium, there is a directionless agitation that only a study of molecular behaviour can help us to understand. Thus, the concept of entropy bridges two worlds (this idea we take up in Study Unit 8).