Adiabatic Vs Isothermal: Exhaustive Comparative Matrices And Detailed Facts


Both Adiabatic and Isothermal processes are integral part of thermodynamics but both of them are totally different from each other.

An Adiabatic process undergoes in such a way that no heat enters or leaves the system during the whole process i.e.

An Isothermal process is a process where temperature remains constant throughout the whole process i.e.

Adiabatic vs Isothermal Process

The major differences between Adiabatic and Isothermal process are listed below:

Adiabatic ProcessIsothermal Process
Heat transfer takes place during the process.No any transfer of heat and mass during the process.
Temperature remains constant.Temperature of an adiabatic process changes due to internal system variation.
Work done is due to the net heat transfer in the system.Work done is mainly the outcome of change in internal energy inside the system.
Transformation occur in the system is very slowTransformation occur in the system is very fast.
To maintain the temperature constant , addition and subtraction of heat take place.There is no any change in heat, so no any addition or subtraction of heat take place

Adiabatic curve vs Isothermal curve

Certain differences can be observed in between Adiabatic and Isothermal processes depending on the changes occur in pressure, volume, temperature etc. during the process.

Adiabatic curveIsothermal curve
This curve is a representation of the relation between pressure and volume of a given mass of gas when there is no change  in temperature during the whole process.This curve is a representation of the relation between pressure and volume of a given mass of gas when there is no transfer of heat throughout the whole process.
It is represented by the equation PV=constant It is represented by the equation, This image has an empty alt attribute; its file name is image-39.png
  
adiabatic vs isothermal
P-V diagram of Isothermal and Adiabatic path

Image Credit: lumenlearning

In the above figure both Isothermal and Adiabatic curves are plotted. Both the processes Isothermal

This image has an empty alt attribute; its file name is image-38.png

and Adiabatic (Q=0) start from the same point A. In case of Isothermal process to maintain the temperature constant, heat transfer takes place between the system and the surrounding due to which during the Isothermal process more work has to be done.

Pressure remains higher in Isothermal process than the adiabatic process generating more work.The final temperature and pressure for the Adiabatic path(point C) is below the Isothermal curve indicating a lower value though the final volume of both the processes are same.

Adiabatic Expansion vs Isothermal Expansion

P-V curves for Adiabatic Vs Isothermal Expansion

Image Credit: A_level_Physics

In the above figure represents the isothermal and adiabatic expansion of an ideal gas which is initially  at a pressure p1.

For both Adiabatic and Isothermal expansion volume starts at Vand ends at V2 (V2> V1). If we integrates the curves in the figure above , we will get positive work for both the cases which implies work done is done by the system only.

In case of Expansion process, Wisothermal>Wadiabatic .

That means Isothermal expansion does greater work than Adiabatic expansion.

Work done in an adiabatic process ,

Work done in an Isothermal expansion process

 In adiabatic expansion work is done by the gas which implies work done is positive, since Ti >Tf  the temperature of the gas goes down. The final pressure obtained in an Adiabatic expansion is lower than the final pressure of Isothermal expansion. The area under the isothermal curve is larger than that under the adiabatic curve which implies more work is done by the isothermal expansion than by the adiabatic expansion.

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Adiabatic vs Isothermal Humidification

In both adiabatic and isothermal humidification processes, approximate 1000 BTU’s per pound (2.326 KJ/kg) of water are necessary to transform water from a liquid to a vapour.

Humidification occurs when the water has absorbed enough heat to evaporate. Two common  methods of humidification used are: isothermal and adiabatic. In isothermal humidification, boiling water is the main source of energy. In adiabatic humidification, surrounding air is used as the source of energy.

 In adiabatic humidification, the air and water is in direct contact, which is not heated. Generally a wetted medium or a spray mechanism is required to spray water directly in to the air and heat from the surrounding atmosphere causes the evaporation of water.

In Isothermal humidification, steam vapour is produced from external energy and steam is injected directly into the air. An outside energy source like natural gas, electricity or a steam boiler is always necessary for steam humidification process. These energy sources transfer energy to water in its liquid form and then transformation of liquid to vapour takes place.

Isothermal as well as Adiabatic humidification are used in commercial and industrial applications to sustain a set-point humidity level in their working areas.

Adiabatic and Isothermal Humidification Processes

Image Credit: pdfs.semanticscholar.org

The above diagram represents the psychometric process for both adiabatic or evaporative and isothermal or steam humidification. To humidify the air up to the set point condition, for Adiabatic humidification the air follows the path from D to C and in case of Isothermal humidification the air follows the path B to C.

For both the process of humidification, external energy source is required to heat the air before humidification from A to B and From A to D.

Work done Adiabatic vs Isothermal

Isothermal process follow PV=constant whereas  Adiabatic process follow PV=constant where ꝩ>1.

In case of both Isothermal expansion and compression processes, the work done is greater than the magnitude of work done for an Adiabatic process. Though the work done during an Adiabatic compression is less negative than the Isothermal compression, the amount of work is compared only in terms of magnitude.

Work done in an Adiabatic process

Work done in an Adiabatic process

Work done in an Isothermal process

Adiabatic vs Isothermal Bulk Modulus

Using the Bulk Modulus of a gas we can measure its compressibility .

When a uniform pressure is applied on a gas, the ratio of the change in pressure of the gas to the volumetric strain within the elastic limits is called as Bulk modulus. K is used to denote Bulk Modulus .

Image Credit: concepts-of-physics.com

The compression of a gas due to the application of pressure

Image credit: concepts-of-physics.com

Bulk modulus, K=- VdP/dV

The negative sign indicates when the gas is compressed due to the application of pressure, volume of the gas decreases.

A change in pressure of a gas is observed both in Adiabatic and Isothermal process.

For Isothermal process, PV=constant

In case of Isothermal process, Bulk modulus is equal to its pressure.

For Adiabatic process,

Adiabatic vs Isothermal PV diagram

A PV diagram is most widely used in thermodynamics to describe corresponding changes in pressure and volume in a system. Each point on the diagram represents different state of a gas.

PV diagram of Isothermal Process and Adiabatic Process is similar but  Isothermal graph is more tilted.

                                                            

Isothermal Process, Image Credit: energyeducation
Adiabatic Process, Image Credit: energyeducation

Image credit: physics.stackexchange

From the PV diagram of Isothermal process, we can see an ideal gas maintain a constant temperature by exchanging heat with its surrounding. On the other hand PV diagram of Adiabatic process represents an ideal gas with changing temperature by maintaining no heat exchange between system and surrounding.

Sangeeta Das

I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at https://www.linkedin.com/in/sangeeta-das-57233a203/

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