1.3.1 Processes in General

Actually, we are not much interested in systems that just sit there and don't do anything. We are interested in changes. We usually have a system in an initial state, constrained somehow to keep anything from happening. We then remove the constraint and the system changes, ending up in a final state.^1.3

An example would be a system containing two separate solutions, one of sodium chloride, the other of silver nitrate. That's the initial state. We now let the two solutions mix. A precipitate of silver chloride forms and settles to the bottom. When things come to rest we are in the final state.

In going from its initial state to its final state the system undergoes a process. By definition the initial state of the system is an equilibrium state. Otherwise, it would not have a state. The final state is also an equilibrium state for the same reason. As the system moves from its initial to final state it passes through a large number of intermediate situations. Usually none of these are equilibrium states.

This is important because the intensive variables of a system are not really defined when the system is not at equilibrium. The pressure and temperature, for example, may vary from point to point inside the system. The density may change from place to place as well. Indeed, it is fair to say that intensive variables don't even have values in a non-equilibrium system!

Extensive variables are different. We can imagine that no matter how disturbed a system is, it still has a total energy, a total volume, and a total mass. Of course these values may change from moment to moment, but at any moment, at least in principle, they do have values.

The sequence of situations the system goes through in passing from the initial state to the final state is called the path taken by the system.

Because the intensive variables often have no values during a process, it is usually not possible to exactly specify the path a process takes in terms of them.