Thermal Processes:
* When a substance changes from on state of equilibrium to another state of equilibrium, the steps or path between the initial and final thermodynamic states is call the process.

Quasi-Equilibrium Processes:
* A process is call a quasi-equilibrium process if the intermediate steps in the process are all close to equilibrium. I this way we can characterize the intermediate states of the process using state variables (such as temperature, pressure, volume, entropy, etc. - See important thermal processes.)

* When a process is quasi-equilibrium we can plot the path of the process on say a pressure vs. volume work diagram since all the variable used to characterize the substance's intermediate states have well define values.

* Most of the process you will encounter will be quasi-equilibrium process and we will drop the "quasi-equilibrium" when talking about a particular process.

State Variable:
Examples of State Variables:
Internal Energy

* State Variables are Path Independent: meaning that the change in the value of the state variable will be the same no matter what path you take between the two states. This is not true of either the work W or the heat Q.

* If a system is carried through a cycle that returns it to its original state, then a variable will only be a state variable if variable returns to its original value.

* If X is a State Variable then:

* State Variables are only measurable when the system is in Equilibrium.

Reversible Processes:
* A process is reversible when the successive states of the process are Infinitesimally close to Equilibrium States. i.e. the process is quasi-equilibrium.

* With a reversible process it is possible to restore the system to its original state without needing an external agent or changing its surroundings.

* Reversible processes are an abstraction that aids the analysis of real processes.

* A reversible process is a standard of comparison for an actual system.

* Truly reversible thermal processes would require an infinite amount of time for completion.

Irreversible Processes:
* All Natural processes are Irreversible.

* The path of an irreversible process is indeterminate and cannot be drawn on a thermodynamic diagram. (We use a hashed line to indicate the path because the intermediate states are in non-equilibrium.)

* The Entropy of the universe always increases during an irreversible process.

* It is always possible to restore an irreversible process to its original state by a reversible process, but the Entropy of the universe can never be restored.

* An irreversible process always requires an external agent to restore it to its original state.

Examples of Irreversible Processes:
Heat Flow
Unrestrained Expansion
Inelastic Deformation
Chemical Reactions
Current Flow
Your House Getting Dirty

* A System is in Equilibrium if its Properties/Variables do not change with time.

Thermal Equilibrium
No Temperature or Pressure Gradients in the System.
Mechanical Equilibrium
No Unbalanced Forces or Torques in the System.
Chemical Equilibrium
No tendency of the System to undergo Chemical Reaction or Diffusion.
Electrical Equilibrium
No Electrical Potential Gradients in the System.