Quasistatic process | Definition, Types & Work

Whenever there is a discussion about the thermodynamic processes, there is a discussion about the quasistatic process. These are the process that requires infinite time to complete. But are these processes practically possible?

In this blog post, you will get an answer to that and learn about the quasistatic process.

Quasistatic process

The quasistatic process, also known as the quasi-equilibrium process, refers to an idealized process that happens extremely slowly so that the system remains in internal thermodynamic equilibrium. The term quasi-static means ‘almost static’. Therefore, these processes are considered almost static, i.e., the rate of change of system properties is practically negligible. The need for a system to be in thermodynamic equilibrium throughout the process is crucial to define the macroscopic state of the system at each instant of the process. The quasistatic process can be shown with a well-defined path in graphical representations.

Graphical representation of quasistatic and non-quasistatic process
Graphical representation of the quasistatic and non-quasistatic process

The quasistatic process can not be physically seen in nature because there is no finite change in the system properties. And as a result, no process in nature is quasi-static, and the completion time of such a process is infinite.

Even though no process in nature is quas-static; but, practical applications require well-defined intermediate system properties to quantify the change in properties. As a result, all the processes are analyzed, considering them quasistatic.

For example, the melting of ice at a constant temperature. A piece of ice kept in a container at 0° C when heated by the addition of small of heat with time will result in the melting of the ice. And by keeping the heating addition extremely small, the process becomes quasistatic.

Reversible process

Reversible processes are quasistatic processes with no dissipation effect. As a result, all reversible processes are quasistatic processes.

For example, reversible adiabatic process, reversible isothermal process, etc.

Work done in the Quasistatic process

For the quasistatic process, the intermediate stages of the process also remain in thermodynamic equilibrium with the surrounding. As a result, these processes have well-defined paths.

The work done for the such process are:

  • Isothermal process: For these processes, the change in temperature remains zero, i.e., throughout the process, the temperature of the system remains constant. For these processes, the work done becomes:

$$ W = mRT \ln (\frac{T_f}{T_i})$$

  • Isobaric process: These are the process for which the change is pressure remains zero, i.e., throughout the process, the pressure within the system remains constant. The work done for such a process is:

$$ W = P(V_f – V_i)$$

  • Isochoric process: For these processes, the change in volume remains zero, i.e., throughout the process, the system’s volume remains constant. Since there is no volume change; therefore the work done for such a process is zero:

$$ W = 0$$

Conclusions

The quasistatic process is a thermodynamic process, characterized by an infinitesimally small change in system properties. And as a result, the completion time of such a process is infinite. Even though no process in nature is quas-static; but, for practical applications, all the processes are analyzed, considering them quasistatic.

Some key learnings from the post:

  • Quasistatic process: An idealized process that happens extremely slow such that the system remains in internal thermodynamic equilibrium throughout the process.
  • Reversible process: The quasistatic process with no dissipation effect.

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