Entropy: What does it really mean?

This aspect of the entropy concept — also called physical entropy — has traditionally been constructed so as to refer exclusively to equilibrium conditions in isolated systems. Only in those conditions might it be estimated, from the increase in ambient temperature after work has been done. Hence one hears that entropy is a state function defined only for equilibrium conditions in isolated systems. This stricture follows from entropie’s technical definition as
a change in heat from one state to another in an isolated system when the change in state is reversible (i.e., at equilibrium, irrespective of the path taken to make the change).

The present approach takes this as an historico- pragmatic constraint on the physical entropy concept making it applicable to engineering systems, and therefore, from the point of view of seeking the broader meanings of the concept, merely one attempt among others. One point to note about the engineering version is that entropy is therein measured as a change in temperature divided by the average temperature of the system prior to the new
equilibrium. This means that a given production of heat in a warmer system would represent less entropy increase than the same amount produced in a cooler system.

Thus, while hotter systems are greater generators of entropy, cooler ones are more receptive to its accumulation. Since the production of entropy during work can theoretically be reduced arbitrarily near to almost none by doing the work increasingly slowly (up to the point where it becomes reversible), physical entropy is the aspect of the entropy concept evoked by the observation that haste makes waste. Of course, it is also the aspect of the concept evoked by the fact that all efforts of any kind in the material world meet resistance, (which generates friction, which dissipates available energy as heat).