ABSTRACT
The assumption that a system is in equilibrium is very powerful. It allows a massive reduction of the mathematical complexity required to describe a system. It is also a very restrictive statement with needs that are never completely satisfied. This chapter outlines exactly where these assumptions play a role and how they are used to make more manageable problems apparently too complex. There are many definitions of what is balance. It is define as the simultaneous satisfaction of two assumptions. The first is that a balanced system is characterized by a unique set of variables, extensive and intensive, that does not change over time. The second is that after isolation of the system, all variables remain unchanged. A non-equilibrium system has no well-defined thermodynamic properties. Entropy, temperature, and energy-free are undefined for a general system.
