1.3.4 Pathways

All processes can be put into one of three classes: quasi-static, reversible, and ordinary.

If the process is quasi-static or reversible, we can plot its progress on a suitable diagram such as a pressure-volume diagram. The line connecting the initial and the final state in such a diagram is called the path the process takes.

An ordinary process will not in general have a path that can be represented on a pressure-volume diagram. To specify it would require definition in far more microscopic detail than the others. In some extreme cases it might even be necessary to note the exact momenta and coordinates of all the particles in the system. And that could be a huge number.^1.4

In general, any line connecting the initial and final states on a pressure-volume diagram is a path, no matter how convoluted. In addition there are several special processes. Each of these is a particular way of doing a process. The resulting paths may be reversible or irreversible. All of these special paths depend on special properties of the walls of the system. These are not the only special paths possible. But the four below turn out to be both common and useful.

Isothermal Process

As the name implies, an isothermal process take place at constant temperature. Isothermal systems have walls that conduct heat and their surroundings have to be at a constant temperature.^1.5

Constant Volume Process

Constant volume Processes are obtained by having rigid walls around the system. The walls may or may not conduct heat.

Constant Pressure Process

Constant pressure processes take place in systems having flexible walls (think balloon) whose surroundings are at a constant pressure. A typical example is the path taken by a process that goes on in a flexibly-walled system surrounded by the atmosphere.

Adiabatic Processes

An adiabatic process takes place in a system whose walls are impermeable to heat. No heat passes into or out of the system. Typically an insulated bottle or vacuum bottle is used to carry out an adiabatic process.