Irreversible Process – What Must Be True? In any natural (irreversible) thermodynamic process, the entropy generation is positive; equivalently, the entropy of the universe increases during the process.

Introduction / Context:
Reversibility classifies processes by whether they can be undone without leaving changes in the surroundings. The second law states that real, natural processes are irreversible and produce entropy. Recognizing the correct hallmark of irreversibility is fundamental to thermal system analysis and efficiency estimation.

Given Data / Assumptions:

• Closed or open systems may be considered; surroundings are large reservoirs.
• Irreversibilities include friction, unrestrained expansion, heat transfer across a finite temperature difference, mixing, and inelastic deformation.
• Standard sign conventions for heat and work.

Concept / Approach:

The combined system (system + surroundings) obeys ΔS_universe = S_gen ≥ 0. For an internally reversible process, S_gen = 0; for an irreversible one, S_gen > 0. The presence or absence of heat loss is not a defining feature, because a process can be adiabatic yet still irreversible (e.g., throttling, rapid expansion).

Step-by-Step Solution:

Verification / Alternative check:

Examples: Throttling valve (Joule–Thomson) has Q ≈ 0 and W ≈ 0 but S_gen > 0 due to internal friction and mixing. Heat transfer across a finite ΔT yields S_gen = Q * (1/T_cold − 1/T_hot) > 0.

Why Other Options Are Wrong:

Net heat loss is not necessary; adiabatic yet irreversible paths exist. Zero work is not required; turbines produce work irreversibly. Spontaneous reversal violates the second law. Constant system entropy implies reversibility only when the environment change is appropriately accounted for.

Common Pitfalls:

Equating “irreversible” with “adiabatic” or “no heat”. Focusing on system-only entropy without considering surroundings. Assuming irreversibility always reduces temperature or pressure (it need not).

Final Answer:

Entropy generation is positive (S_gen > 0).