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The secret of quasi-static processes: How to maintain internal equilibrium in thermodynamics?
Quasistatic process, also known as quasi-equilibrium process, comes from the Latin word "quasi", which means "seem". It is a thermodynamic process that occurs slowly enough for the system to maintain an internal physical equilibrium. In such a process, although the effect does not necessarily affect the chemical properties, it can achieve an energy balance on the physical level. In the following article, we will explore the basic properties of quasi-static processes and how they maintain internal thermodynamic equilibrium.
Quasistatic processes are a series of physical equilibrium states characterized by infinitely slow changes.
A classic example is the quasi-static expansion process of a mixture of hydrogen and oxygen. During this process, the volume of the system changes very slowly, so that at every moment, its pressure remains uniform inside the system. Only in the quasi-static thermodynamic process can we accurately define the intensive quantities of the system (such as pressure, temperature, specific volume and specific entropy) at each moment in the entire process. If the process is too fast to achieve internal equilibrium, different parts of the system will exhibit different values of these quantities. A specific statement is: when the change equation of a state function includes pressure or temperature, it means that it is a quasi-static process.
The relationship between quasi-static processes and reversible processes is also worth noting. All reversible processes are quasi-static, but not all quasi-static processes require maintaining an equilibrium between the system and the environment and avoiding energy dissipation, which are the defining characteristics of reversible processes.
Many examples of quasi-static processes cannot be idealized as reversible, such as the slow heat transfer process between two objects with different temperatures.
In this case, even though the process proceeds very slowly, the states of the two objects in the composite system are still far from equilibrium because their thermal equilibrium requires that the temperatures of both objects must be the same. However, despite this, the entropy change of each object can still be calculated by Clausius' equation. Such an analysis can provide insight into the complexity of quasi-static processes.
PV-work in quasi-static process
In quasi-static processes, the calculation of work can be divided into different types according to the nature of these processes:
Isobaric processes: Under constant pressure, the expression for work is W = P (V₂ - V₁), where V refers to volume.Isochoric processes(Isochoric processes): At constant volume, the calculated result of work is zero.Isothermal processes: At constant temperature, work can be expressed as W = P₁V₁ ln(V₂/V₁), where the pressure changes with volume.Polytropic processes: The calculation of work usually varies with different variables, and the formula is W = (P₁V₁ - P₂V₂) / (n-1).
The influence of these quasi-static processes of different properties on physical phenomena not only shows the maintenance of stable states, but also reveals the complexity in practical applications. For example, engineers take the effects of friction into account when calculating dissipative entropy generation.
"In thermodynamics, the maintenance of internal equilibrium is an art, inseparable from scientific principles."
In summary, quasi-static processes play an important role in thermodynamics, helping us understand how to maintain internal equilibrium in complex systems. Across a variety of applications, the concept of quasi-static processes not only helps us predict system behavior but is also a cornerstone for designing efficient thermal cycling and heat exchange processes. Faced with so much thinking and discussion, we can’t help but ask: How can we further explore the mysteries of quasi-static processes in future thermodynamics research?
