Reversible and Irreversible Processes in Thermodynamics: Check Class 11 Physics Notes

Reversible thermodynamic processes are hypothetical. The idealisation is that the system and surroundings are at equilibrium without any energy loss while changing states and reversing themselves. An example of a reversible thermodynamic process is the melting of ice at 0°C, where solid and liquid are in balance at all times. 

Irreversible processes in thermodynamics occur in the real world. Neither the system nor its surroundings can go back to their original states after the transition. Consider how gas expands in a vacuum. As there is no external pressure to resist it, it cannot go back ‘naturally’ to its original state.

Here are the two explanations about reversible and irreversible processes in thermodynamics according to NCERT. 

1. “A thermodynamic process (state i → state f ) is reversible if the process can be turned back such that both the system and the surroundings return to their original states, with no other change anywhere else in the universe.”
2. “Imagine some process in which a thermodynamic system goes from an initial state i to a final state f. During the process the system absorbs heat Q from the surroundings and performs work W on it. Can we reverse this process and bring both the system and surroundings to their initial states with no other effect anywhere ? Experience suggests that for most processes in nature this is not possible. The spontaneous processes of nature are irreversible.”

Reversible and irreversible processes are not just interconnected with the fundamentals of thermodynamics in Physics. If you are preparing for IIT JAM within a short span, you have numerical questions to prepare. You have to consider the factors, such as friction and heat transfer, responsible for making a process irreversible or reversible. Such topic-based questions are pretty standard.  

For JEE Mains, too, you will have to solve questions based on how work is done in a reversible isothermal process, or how gases are compressed, and so on. These are quite simple. Further, you may want to go check JEE Main Paper Analysis 2025 to see whether the questions are tricky around this topic and chapter. 

In Class 11 Physics, you will need to know about these characteristics of reversible and irreversible processes.  

Reversible Processes Characteristics: 

Let’s look into some of the key concepts here. 

• Quasi-static process: This is the thermodynamic process that happens too slowly, the system stays in near-perfect equilibrium at every stage.
• No dissipative effects: There is no energy loss due to things like friction, turbulence, or unbalanced forces.
• Isothermal expansion: The gas expands while keeping its temperature constant, typically by absorbing heat from its surroundings.

Irreversible Processes Characteristics

1. Non-equilibrium states: These are the conditions inside the system where pressure, temperature, or other variables are uneven or unbalanced.
2. Free expansion: Such processes can expand without work done or exchanging heat. 
3. Viscosity: This is how a fluid resists to flow and energy dissipates. 

Assuming Reversibility: Real processes are irreversible due to friction.

Work in Free Expansion: No work is done despite volume change.

State Variables: $Q$ and $W$ are path-dependent.