A thermodynamic process is a process by which a thermodynamic system goes from one state of equilibrium to another.
Although the child in the upper figure can hardly be considered a system in equilibrium, the example is useful to illustrate what happens in a thermodynamic system. Initially it is in a state 1 and, after exchanging (in this case matter and energy) with its surroundings, it goes to a final state 2. During the process, the chemical energy stored in the ice cream has been transformed into kinetic energy.
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Without going into formal definitions, thermodynamic processes can be:
- Quasi-static: those that take place infinitely slowly. Although there are exceptions, in general all the states a system goes through when it undergoes a quasi-static process are in equilibrium .
- Reversible: A process is reversible when its direction can be reversed without producing any change in the system or its surroundings. In the systems we will study, any reversible process will be quasi-static.
- Irreversible: those that are not reversible. For these processes, the intermediate states cannot be specified by any set of macroscopic variables and are not all in equilibrium. When an irreversible process takes place, the entropy of the universe increases.
A simple way to visualize the state of a system (generally a gas) and the processes it undergoes is to plot them in a Clapeyron diagram or pV diagram, like the one illustrated in the following figure.
The volume (with its units) occupied by the substance in each state is shown on the horizontal axis of the diagram. The pressure (with its units) of the substance is plot on the vertical axis. For each the state, the temperature is written in parentheses. For some substances, it can be determined by the equation of state; for example, when working with an ideal gas.
The states 1 and 2 are connected by a path that represents the process that the substance has undergone when it went from one state to another. It is usually represented with a solid line (in green in the figure) when it is reversible and with a dashed line (in red in the figure) when it is irreversible.
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