Transformation of energy

Reversible and Irreversible Processes: Concept, Definition and Examples

Reversible and irreversible processes: concept, definition and examples

In the study of thermodynamics, reversible and irreversible processes play a fundamental role in understanding the behavior of thermodynamic systems and energy transformations.

These two types of processes represent opposite extremes in terms of efficiency and energy loss.

Let's see in detail what these two thermodynamic processes mean and how they differ from each other.

Reversible Processes: Definition and Examples

A reversible process is defined as one that can be completely reversed at each of its stages, leaving no residual change in the system or the surroundings. In a reversible process, the system remains in thermodynamic equilibrium at all times, which implies that the properties of the system do not undergo abrupt changes or discontinuities during the process.

A key feature of reversible processes is that they follow a smooth path on the thermodynamic state diagram, either on a pressure-volume diagram (PV diagram) or on a temperature-entropy diagram (TS diagram).

In addition, reversible processes have the property of being quasistatic, that is, they take place in a series of infinitesimally close equilibrium states.

Examples of Reversible Processes

  1. Isothermal expansion of an ideal gas: In an ideal gas, an isothermal expansion at a constant temperature is a reversible process. It is done slowly to maintain thermal equilibrium with the surroundings and to ensure that the pressure and volume change smoothly.

  2. Reversible Adiabatic Compression: In a reversible adiabatic process, rapid compression takes place without heat transfer to or from the system. For it to be reversible, it must be done very quickly to minimize power dissipation and keep the system in equilibrium at all times.

  3. Mixing of ideal gases: If two ideal gases initially separated and in equilibrium are mixed, the mixing process is considered reversible, as long as it is carried out slowly and equilibrium is maintained at each stage. There is no heat transfer or abrupt changes in the properties of the system during the process.

  4. Carnot Cycle: The Carnot cycle is a theoretical thermodynamic cycle consisting of two reversible adiabatic processes and two reversible isothermal processes. It is considered the most efficient cycle theoretically possible and is used as a reference to compare other real cycles.

  5. Phase equilibrium in a closed system: When a closed system containing two phases of a substance (for example, liquid and vapor) reaches an equilibrium between the phases, the phase change process is reversible. The system is in equilibrium at all times, and changes in pressure and temperature occur smoothly and continuously.

Irreversible Processes: Definition and Examples

In contrast to reversible processes, irreversible processes are those that cannot be completely reversed, resulting in irreversible changes to the system and its environment. These processes involve a net loss of useful energy in the form of heat dissipation or entropy generation.

Irreversible processes are characterized by not being in thermodynamic equilibrium at all times, which means that the properties of the system can undergo abrupt changes and discontinuities as the process occurs. These processes are usually fast, non-quasistatic, and are carried out under non-ideal conditions.

Examples of Irreversible Processes

Common examples of irreversible processes include:

  • The combustion of a match is an irreversible process since the molecular composition of the fuel cannot be reconstructed.

  • Free expansion of a gas: If a separator or barrier is quickly removed from a gas contained in a container, the gas will expand rapidly into the available space. This process is irreversible, as there is no time for the system to adjust to the changes and restore itself to a steady state. Furthermore, the work done during the expansion cannot be fully recovered.

  • Thermal Conduction: When there is a finite temperature difference between two systems, heat is transferred from the region of higher temperature to the region of lower temperature. This process is irreversible due to unavoidable heat losses and entropy generation, resulting in decreased efficiency and a tendency towards greater disorder in the system.

  • Friction in a mechanical system: Friction, present in practically all mechanical systems, is an irreversible process. When two surfaces slide or rub against each other, mechanical energy is converted to heat due to resistance and friction, generating energy losses and increasing the entropy of the system.

  • Conversion of solar energy to electricity in solar panels: The conversion of solar energy into electricity in photovoltaic solar panels involves irreversible processes. As solar radiation hits the solar cells, some of the energy is converted to electricity, but some of the energy is also dissipated as heat due to the internal resistances of the panel and the generation of entropy in the process.

Importance and Consequences

The distinction between reversible and irreversible processes has significant implications for the efficiency and performance of thermodynamic systems. Reversible processes are considered ideal due to their high efficiency and minimal loss of useful energy. However, in practice, it is difficult to achieve fully reversible processes due to the inevitable losses due to friction, resistance and other forms of irreversibility.

Irreversible processes, on the other hand, are common in everyday life and in most technological applications. These processes are associated with the production of entropy, which is a measure of the dispersal of energy and the tendency towards greater disorder in the system. 

The generation of entropy in irreversible processes is linked to the dissipation of energy in the form of heat, which limits the efficiency and performance of the systems.


Published: May 17, 2023
Last review: May 17, 2023