Physics · Thermodynamics · NEET
| Definition | System AND surroundings both return to start, no change in universe | Cannot return both; surroundings keep a permanent change |
| Speed | Infinitely slow (quasi-static) | Fast, finite speed |
| Dissipation | No friction, no viscosity, no loss | Friction, viscosity, sudden heat flow present |
| Equilibrium | System in equilibrium at every step | Passes through non-equilibrium states |
| Occurrence | Idealised, never fully real | All real / spontaneous natural processes |
| Example | Quasi-static isothermal expansion, frictionless piston | Free expansion, sudden compression, heat across finite gap |
Not exactly. Quasi-static means infinitely slow, so the system stays in equilibrium with the surroundings at every step. Reversible needs BOTH conditions: it must be quasi-static AND have no dissipative effects like friction or viscosity. So every reversible process is quasi-static, but a quasi-static process with friction is NOT reversible. Reversible = quasi-static + non-dissipative.
Real processes always have some dissipation: friction in the piston, viscosity in the gas, or a finite temperature difference that lets heat flow suddenly. NCERT says the spontaneous processes of nature are irreversible. To reverse them you would need to undo this lost energy exactly, which is not possible without changing the surroundings. So a truly reversible process is an idealised limit only.
No. This is the biggest confusion. In a cyclic process the SYSTEM returns to its start, but the SURROUNDINGS may keep a permanent change (heat dumped, work done against friction). Reversible needs the SYSTEM and the SURROUNDINGS to both return with no change anywhere in the universe. System-only return is not enough.
When heat flows across a large temperature gap (hot body to cold body directly), the process is fast and not quasi-static. NCERT gives the example of a hot vessel base cooling to room temperature; you never see the reverse happen on its own. For heat exchange to be reversible, the temperature difference between system and reservoir must be infinitesimal at every step (isothermal, quasi-static).
Reversible (ideal): a quasi-static isothermal expansion of an ideal gas in a cylinder with a frictionless piston. Irreversible (real): free expansion of a gas into vacuum, or sudden compression, or heat flowing across a finite temperature difference. Free expansion is neither quasi-static nor non-dissipative, so it is strongly irreversible.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It must be (1) quasi-static, meaning infinitely slow so the system stays in equilibrium with the surroundings at every stage, and (2) non-dissipative, meaning no friction, no viscosity, and no other energy loss. If either condition fails, the process is irreversible.
Only if it is carried out quasi-statically with a frictionless piston. A quasi-static isothermal expansion of an ideal gas is the standard example of a reversible process. But a fast isothermal change with friction would be irreversible. The label isothermal describes constant temperature; it does not by itself guarantee reversibility.
The Carnot engine uses only reversible steps (two isothermal and two adiabatic), which is why it has the maximum possible efficiency between two temperatures. Irreversibility always lowers efficiency. Questions on maximum efficiency, Carnot cycle and second law all rely on understanding reversible versus irreversible.
Free expansion (gas expanding into vacuum) is irreversible. It is sudden, not quasi-static, and the gas is never in equilibrium during the expansion. No work is done and no heat is exchanged, but you can never make the gas rush back into the corner on its own, so it cannot be reversed.
A reversible process is quasi-static, which means infinitely slow. In principle it takes infinite time because every step must keep the system in equilibrium with the surroundings. This is why a perfectly reversible process is an idealised construct and never exactly achieved in real life.
