Reversible and Irreversible Processes
Imagine pushing a box across a rough floor. You push, it moves, and when you stop, it stays put. To bring it back, you have to push again from the other side. The energy you put in turned into heat — you can't get that energy back to use again. That's an irreversible process.
Now imagine a frictionless pendulum. It swings down, then up to exactly the same height on the other side, then back. No energy is lost. If you filmed it and played the film backwards, you couldn't tell which was forward and which was reverse. That's the closest real-world picture of a reversible process.
The Intuition
A reversible process is one that can be reversed by an infinitesimally small change in a condition — and when reversed, it leaves no trace on the surroundings. The system and the surroundings both return exactly to their starting states.
An irreversible process is everything else. Once it happens, you cannot undo it without spending extra energy or leaving some change behind. Every real process in nature is irreversible.
Reversible processes are idealisations. No real process is perfectly reversible — but some come very close (slow compression of a gas in a frictionless piston, for example).
The Precise Statement
A process is reversible if:
- It can be reversed by an infinitesimal change in an external variable.
- The system passes through a continuous sequence of equilibrium states.
- When reversed, both the system and the surroundings are restored to their initial states — no net change anywhere.
A process is irreversible if any of these fail. In practice, irreversibility arises from:
- Friction, viscosity, or other dissipative effects
- Finite temperature or pressure differences
- Unrestrained expansion (like a gas rushing into a vacuum)
- Mixing of different substances
- Chemical reactions that don't proceed through equilibrium
The Entropy Connection
Here is the sharpest distinction. For any process:
ΔSuniverse=ΔSsystem+ΔSsurroundings
- Reversible process: ΔSuniverse=0
- Irreversible process: ΔSuniverse>0
The entropy of the universe always increases for a real (irreversible) process. A reversible process is the theoretical limit where entropy stays constant — and that limit is never reached in practice.
Why This Matters for Exams
You will be asked to classify processes and to calculate entropy changes. The key steps:
- Identify the cause of irreversibility — is there friction? A finite temperature difference? Unrestrained expansion?
- For reversible processes, you can use dS=TdQrev directly, and the work done is the maximum possible.
- For irreversible processes, you cannot use dQ/T directly for the system — you must find a reversible path between the same initial and final states to compute ΔS.
| Feature | Reversible | Irreversible |
|---|---|---|
| Path | Through equilibrium states | Through non-equilibrium states |
| Reversal | Possible by infinitesimal change | Impossible without extra work |
| ΔSuniverse | Zero | Positive |
| Work output | Maximum | Less than maximum |
| Real existence | Idealisation only | All real processes |
A Concrete Example
Consider a gas expanding against a piston. …