Quasi-Static Process: 15 Important Explanations

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FAQ/Short Notes

  • What is a quasi static force ?
  • Is reversible process quasi static ?
  • Is adiabatic process quasi static ?
  • What are the examples of Quasistatic processes in our daily lives ?
  • Why a reversible process is necessarily a Quasi static process ?
  • Since the pressure is uniform in the quasi static process, how can there be any work done?

Quasi-static process definition

It can be defined in simple words process happening very slowly, and all state passed by this process is in equilibrium.

The meaning of the word “Quasi” is almost. The static means the thermal properties are constant concerning time. All the reversible processes are quasi. The slow rate of the process is the main characteristic of the this process.

Non quasi-static process

It is not realized for any finite difference of the system. Most of the processes happening around us (in nature) can be termed as non quasi-static process.

Both of the process can be well understood by diagram as shown below,

Quasi-static process
Diagram Quasi static and non Quasi static process

It helps analyze. It is primarily studied in books and references. We already know the introductory study of thermodynamic starts with quasi processes. We can readily notice work PdV in this diagram. The curve of non quasi looks half-circle type. The quasi-static method is represented by a straight line.

Difference between quasi-static and reversible process

We can define a reversible process as if the system restores its initial or starting stage and there is no effect of the process on the surrounding.

In a reversible process, the process follows the same path in the forward and reverse functions. There is no impact of the system on the surrounding. Ideally, this type of process can not be possible due to friction.

It can be defined in simple words process happening very slowly, and all state passed by this process is in equilibrium.

There is no friction present in this process. So, we can say that ideally, the processes are reversible.

There is no entropy generation in both processes. We can make any process reversible if we continue the process at a prolonged rate.

Example of quasi-static process

We can consider the static compression process as an example of the quasi-static process. In this process, the system’s volume will change very slowly, but the pressure of the system remains throughout the process.

Compression process with cylinder and piston is shown in figure below,

piston cylinder 1
Compression process of quasi-static process

Characteristics of quasi-static process

It is a thermodynamic process where the process occurs at a very slow rate. We can say the process occurs at near to rest condition.

Every point or stage in the this process is considered in equilibrium conditions.

We can say that control on the quasi process is effortless. In the non quasi-static process, the control can be challenging compared to ideal quasi. The reason behind it is the speed of the process.

It is a thermodynamic process in which the time taken for the complete process will be infinite.

It is highly efficient as there is no loss in this process. There is no friction or heat generation due to friction. In the case of a non quasi process, friction is present, which is ultimately loss so less efficient than quasi.

This process is reversible in nature.

Device working on quasi-static process produce maximum work

Common quasistatic processes

Ideally, the quasi reversible process can not possible practically. There is always some loss in any system. With some assumptions, we can consider some processes as quasi processes.

  • Ideal Gas processes at a slow rate.
  • Compression process at a prolonged rate
  • Reversible Processes.
  • Growth of tree

Huge Temperature Reservoir

Condition for an ideal gas in a quasi-static adiabatic process

If we consider the quasi adiabatic process, there is some condition to be satisfied. If ideal gas is compressed from state 1 to state 2, then

P1 and V1 Is the initial condition of the system,

P2 and V2 Is the final condition of the system,

The condition for the system is,

PV^{gamma }= Constant

We can write this condition for both condition as below,

P1V1^{gamma }= P2V2^{gamma }

Conditions for quasi-static process

It is a thermodynamic process where the process occurs at a very slow rate. We can say the process occurs at near to rest condition.

Every point or stage in this process is considered in equilibrium conditions.

We can say that control on this process is very easy. In the non quasi-static process, the control can be challenging compared to quasi. The reason behind it is the speed of the process.

It is a thermodynamic process in which the time taken for the complete process will be infinite.

This process is highly efficient as there is no loss. There is no friction or heat generation due to friction. In the case of a non quasi process, friction is present, which is ultimately loss so less efficient than quasi.

This process is reversible in nature.

Device working on this process produce maximum work

Difference between quasi-static and non quasi-static process

It can be defined in simple words that it is the process happening very slowly, and all state passed by this process is in equilibrium.

This process is always reversible in nature.

There is no friction or loss present.

It is not realized for any finite difference of the system. Most of the processes around us (in nature) can be termed a non quasi-static process.

The non quasi process is always irreversible.

There is always friction and loss present in the system.

We can write relation for entropy generation,

dS = frac{dQ}{T} + I

Where dS denotes entropy change in system

The entropy change in the system can be positive, negative, or zero.

Heat transferred in an infinitesimal quasi-static process

Heat transfer equation for this ideal process can be written in following foam for calculation,

dQ = left ( frac{Cv}{nR} right )cdot left ( Vcdot dP right )+left ( frac{Cp}{nR} right )cdot left ( Pcdot dv right )

Here

dQ = Heat transfer

Cv= Constant volume heat capacity

n= no. of moles of substance

R= ideal gas constant

Cp= Constant pressure heat capacity

V = volume,

dV = Differential volume

P= Pressure,

dP = Differential pressure

Importance of quasi-static process

It is proposed in 1909 as a ” quasi-static process.” It is an essential process in the field of thermodynamic for analysis. It is providing maximum output work in the system. Though this process is ideal, this process in the various study is vast.

In this process, the system remains in equilibrium for infinitesimal time. The reasons behind its importance in the field of engineering are

1. This process is easy for analysis

2. Any device working on this process produces maximum work. There is no loss of any energy.

Non quasi-static process example

Every process in nature is a non quasi-static process,

Those processes do not occur at a prolonged rate. You can consider any processes non quasi-static process.

  • Fast Heat transfer,
  • Fast compression,
  • Expansion,

Non-quasi-static cyclic process

It is not realized for any finite difference of the system. Most of the processes around us (in nature) can be termed a non quasi-static process.

We can readily notice the curve in the diagram given below. As we know, the non quasi-static process does not return with the same path. The backward process is always with a different direction to be considered a cyclic process.

Quasi-static process diagram

The diagram for both of the process shown below for the expansion process.

Quasi 1
Quasi-static and Non Quasi-static Diagram

Quasi-static process entropy

We can write relation for entropy generation,

dS = frac{dQ}{T} + I

Where dS denotes entropy change in system

The entropy change in the system can be positive, negative, or zero.

Quasi-static process equation

It can be derived for various processes in thermodynamics. The equation for different process with the constant property is given below,

Process with Constant pressure (Isobaric process)

W_{12}= int PdV

Process with Constant volume (Isochoric process)

W_{12}= int PdV = 0

Process with Constant temperature (Isothermal processe)

W_{12}= P1 V1cdot lnfrac{V1}{V2}

Polytropic process

W_{12}= frac{P1V1 - P2V2}{n-1}

FAQs

What is quasi static force ?

It can be stated as the force applied very slowly on the system. Due to this force, the system deforms very slowly with infinite time. This type of force can be defined as a quasi-static force.

Is reversible process quasi static?

This process is always reversible.

There is no friction or loss present.

It is not realized process for any finite difference of the system. Most of the processes around us (in nature) can be termed a non quasi-static process.

Is adiabatic process quasi static ?

An adiabatic process is a process with no heat transfer. It is also considered as an isentropic process means constant entropy of the system.

There are some conditions of the process to be quasi.

If the adiabatic process occurring at a very slow rate, then it can be considered as quasistatic adiabatic process

What are the examples of Quasi static processes in our daily lives?

It is an ideal process in nature; still, the process that occurs very slowly can be considered as quasi.

Growth of tree,

Why a reversible process is necessarily a Quasi static process?

This process is always reversible in nature.

There is no friction or loss present. There is no heat loss at all in this process

It is not realized for any finite difference of the system. Most of the processes around us (in nature) can be termed a non quasi-static process.

Since the pressure is uniform in the quasi static process, how can there be any work done ?

If the pressure is constant in any system with the this process, the work done can be given by the following equation,

Process with Constant pressure (Isobaric process)

W_{12}= int PdV

     

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