Reversible and Irreversible Processes

Overview

A reversible process can be retraced through the same states while restoring both system and surroundings; an irreversible process cannot be exactly undone without leaving a net change in the surroundings.

Learning Outcomes

• A reversible process can retrace the same path in reverse without a net change in the surroundings.
• A truly reversible process is an idealization approached by extremely slow, controlled changes.
• Friction, viscosity, turbulence and finite temperature differences cause irreversibility.
• An irreversible process cannot restore both system and surroundings exactly.
• Heat conduction, convection and radiation across finite differences are irreversible.
• Natural processes have a preferred direction and are generally irreversible.

Intermediate Level

Reversible Process

A reversible process can be retraced in exactly the reverse order through the same equilibrium states, restoring the system and surroundings. It must proceed extremely slowly and without dissipative effects such as friction.

• The system remains arbitrarily close to equilibrium.
• Driving differences in pressure or temperature are infinitesimally small.
• No friction, turbulence or other dissipation occurs.
• Reversing a tiny change in conditions reverses the direction of the process.

Irreversible Process

An irreversible process cannot be retraced exactly without leaving some change in the surroundings. Real processes occur with finite gradients, friction or dissipation and therefore produce irreversible effects.

• Expansion against a finite pressure difference
• Heat flow across a finite temperature difference
• Motion with friction
• Mixing, diffusion, turbulence and decay
• Conduction, convection and radiation

Direct Comparison

Textbook Examples

Very slow melting/freezing or evaporation/condensation can be treated as approximately reversible when the temperature difference is extremely small. Friction, biological decay, conduction, convection and radiation are irreversible.

Common Mistakes

• Reversible does not merely mean that the system can be returned to its initial state.
• Both the system and surroundings must be restored.
• A slow process is only approximately reversible if dissipative effects are also negligible.
• Ordinary melting or evaporation with finite temperature differences is not perfectly reversible.

Short-Question Preparation

Sticky Notes

• A reversible process can retrace the same path in reverse without a net change in the surroundings.
• A truly reversible process is an idealization approached by extremely slow, controlled changes.
• Friction, viscosity, turbulence and finite temperature differences cause irreversibility.
• An irreversible process cannot restore both system and surroundings exactly.
• Heat conduction, convection and radiation across finite differences are irreversible.
• Natural processes have a preferred direction and are generally irreversible.

60-Second Revision

Reversible means exact path retracing with both system and surroundings restored; it is an ideal, quasi-static, non-dissipative limit. Irreversible processes involve finite gradients or dissipation, and all natural processes are irreversible to some degree.

Summary

A reversible process can be retraced through the same states while restoring both system and surroundings; an irreversible process cannot be exactly undone without leaving a net change in the surroundings.

Ask-the-Tutor Modes

• Explain it simply.
• Compare the processes.
• Explain the P–V graph.
• Solve an energy balance.
• Test me with an MCQ.