The Conceptual Meaning of Thermodynamic Entropy in the 21st Century1

Although entropy change – the process described by Clausius and characterized as the second law of thermodynamics – involves only two terms, energy change and temperature, neither he nor distinguished scientists of the past century focused on that fact. The novel word “entropy” apparently drew them to concentrate on its ‘meaning’ rather than on dq, the energy becoming spread out or dispersed in phase space in any spontaneous process. Today, beginners and experts can readily see that process as fundamental in thermodynamic change.