Definition 1.1 Thermodynamics is a phenomenological description of equilibrium properties of macroscopic systems.
Definition 1.2 As a phenomenological description, it is based on a number of empirical observations which are summarized by the laws of thermodynamics. A coherent logical and mathematical structure is then constructed on the basis of these observations, which leads to a variety of useful concepts, and to testable relationships among various quantities. The laws of thermodynamics can only be justified by a more fundamental(microscopic) theory of nature. For example, statistical mechanics attempts to obtain these laws starting from classical or quantum mechanical equations for the evolution of collections of particles.
Definition 1.3 A system under study is said to be in equilibrium when its properties do not change appreciably with time over the intervals of interest(observation times). The dependence on the observation time makes the concept of equilibrium subjective. For example, window glass is in equilibrium as a solid over many decades, but flows like a fluid over time scales of millennia. At the other extreme, it is perfectly legitimate to consider the equilibrium between matter and radiation in the early universe during the first minutes of the big bang.
Definition 1.4 The macroscopic system in equilibrium is characterized by a number of thermodynamic coordinates or state functions. Some common examples of such coordinates are pressure and volume (for a fluid), surface tension and area (for a film), tension and length (for a wire), Electric Field and Polarization(for a dielectric),  . A closed system is an idealization similar to a point particle in mechanics in that it is assumed to be completely isolated by adiabatic walls that don’t allow any exchange of heat with the surroundings. By contrast, diathermic walls allow heat exchange for an open system. In addition to the above mechanical coordinates, the laws of thermodynamics imply the existence of other equilibrium state functions.
References
This is a derivative work from [1] a Creative Commons Attribution-Noncommercial-Share Alike 3.0 work
[1] MIT OpenCourseWare, 8.333 Statistical Mechanics I: Statistical Mechanics of Particles, Fall 2007
| 