Short Answer:
Reversible processes are those that can return to their original state without leaving any net change in the system or the surroundings. Meanwhile, an irreversible process cannot return to its original state without inducing changes to the system. This main difference will determine how these processes are represented on a PV diagram.
Detailed Answer:
Reasoning, why irreversible processes can't be described in a PV diagram.
An irreversible process cannot be represented by a path because these processes are dependent on the initial and final states of the system, not the path taken to arrive at the final state. Essentially, the process cannot be 'reversed' by simply traversing the opposite direction along a path on a PV diagram. Due to these inherent changes and loss of information about how it got there (like dissipated heat), an irreversible process is usually represented by a dotted line or simply by the initial and final states.
Reversible Vs. Irreversible Processes
Understanding the difference between reversible and irreversible processes is crucial when studying thermodynamics. In an idealized sense, reversible processes are those that can undergo a change and then return to their original state without any net exchange of energy with the surroundings. This concept is often represented as a smooth, continuous curve on a pressure-volume (PV) diagram since each infinitesimal step along the way is in perfect equilibrium.
In contrast, irreversible processes happen more commonly in reality. These processes include friction, rapid expansion or compression, and mixing of different substances. Because they involve changes that cannot be undone without affecting the surroundings or the system itself — like heat loss to the environment — they do not follow a clear path on a PV diagram. This is mainly due to the inability to trace back the exact intermediate states the system went through as it transitioned from its initial to its final state.
For additional details please refer to URL:
https://www.vaia.com/en-us/textbooks/physics/essential-university-physics-2-edition/chapter-18/problem-5-why-cant-an-irreversible-process-be-described-by-a/
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Shailesh (Sky) Kadakia, Former Professor
University of Pittsburgh, Johnstown PA
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