Isothermal and Adiabatic Process

Dhristi JEE 2022-24

Difference Between Isothermal and Adiabatic Process

The word ‘isothermal’ means constant temperature. An isothermal process is a thermodynamic process occurring at a constant temperature.

The word ‘adiabatic’ means isolated from surroundings. Adiabatic process means a process that neither allows the heat to transfer inside nor lets the heat out of the system.

For example, a reaction that takes place in a Dewar Flask is adiabatic. 

In this article, we will discuss adiabatic and isothermal, distinguish between isothermal and adiabatic processes, isobaric isochoric isothermal and adiabatic processes, and understand the process of isothermal adiabatic in detail.

Adiabatic and Isothermal Processes 

Now, we will understand the difference between adiabatic and isothermal:

Thermodynamics uses the concepts of the isothermal process, isochoric process, isobaric process, and adiabatic process to explain the behaviour of a thermodynamic system and its relationship to temperature changes. 

An isothermal process is a process that happens when there are no variations in the temperature of the system, but other parameters (volume, pressure) regarding the system can be changed accordingly. 

Adiabatic process describes a process that remains aloof from its surroundings. It is a process in which no heat transfer occurs between a system and its surroundings. Here, the temperature of the system can vary in order to avoid any heat transfer. This indicates that the main difference between isothermal and adiabatic processes is that the isothermal process takes place under constant temperature whereas the adiabatic process occurs under changing temperature.

Now, let’s Compare Isothermal And Adiabatic processes in a tabular form to understand isothermal and adiabatic processes:

Difference Between Isothermal Process and Adiabatic Process

Isothermal Process

Adiabatic Process

Transfer of heat occurs in this process.

No heat goes inside or leaves the system.

At a given volume of the substance, the pressure remains high.

At a given volume of the substance, the pressure remains low.

In an isothermal process, the temperature remains invariant.

The temperature varies because of the internal system changes.

Heat can be added or released to the system to keep the temperature constant.

There is no addition of heat nor the release of the heat because maintaining constant temperature doesn’t matter here.

The isothermal process has a slower transformation flow.

The adiabatic process has a faster transformation flow.

In an isothermal system, work done is because of the change in the net heat content of the system.

In an adiabatic process, the work done is because of the change in internal energy.

Now, we will understand a bit more about isobaric isochoric isothermal and adiabatic processes:

Adiabatic, Isothermal, Isobaric, and Isochoric, Processes

Adiabatic Process

An adiabatic process is a thermodynamic process that can take place without any heat transfer between a system and its surrounding. Here, neither heat nor energy is not transferred into or out of the system. Therefore, in an adiabatic process, the only way the energy transfer takes place between a system and its surroundings is the work. It is either an irreversible or reversible process and follows the below-mentioned conditions:

  • The time required to carry out the process should be minimal so that it should be carried out quickly so as to reduce the chances of heat getting transferred.

  • The instrument used to carry out the process should be insulated perfectly from the surrounding environment.

An adiabatic process can be quickly maintained by doing the process. For example, if we quickly press the piston in a cylinder filled with gas, there is not enough time for the system to transfer heat energy to the surroundings. In adiabatic processes, the work done by the system alters the internal energy of the system. Always remember that the total heat of the system is constant in an adiabatic process.

The below diagram shows the adiabatic process:

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Adiabatic equation

The adiabatic equation is:   PVγ = constant

Here P stands for the pressure of the system

V stands for volume of the system 

γ stands for adiabatic index

Adiabatic index

It is the ratio of heat capacity at constant pressure Cp to heat capacity at constant volume Cv.

\[\gamma = \frac{C_{P}}{C_{V}}\]

Adiabatic Expansion

The ideal behaviour of a system where the temperature keeps on increasing and pressure remains constant is termed adiabatic expansion. It refers generally to a closed system.

Adiabatic Compression: The external work done is equal to the increased internal energy of the air in the system. Here the heat is neither subtracted or added from the surrounding air to the system air. As there is an increase in the temperature of the system the pressure of the system tends to be more than the volume.


  1. Oscillation of a pendulum in a vertical plane.

  2. No heat is released or absorbed when an ice cube is put inside an icebox.

  3. Nozzles, compressors and turbines work on the principle of adiabatic efficiency.

Reversible Adiabatic Process

It is also known as the Isentropic process. In this process, there is neither transfer of matter nor heat. Hence it is known to be a reversible process. It could also be defined as a thermodynamic process in which the work transfers are frictionless and the system is adiabatic. It is vitally used as a model to show real process models in engineering and also for major comparisons between the systems.

Isothermal Process

An isothermal process is a thermodynamic process that takes place at a constant temperature. It means that an isothermal process occurs in a system where the temperature remains constant. However, to keep the temperature of the system constant, heat must be transferred into the system or shifted out of the system.

Generally, there are two conditions under which the isothermal process can work:

  1. The system slowly adjusts the temperature of the system with the temperature of its surrounding by releasing heat. This happens when the surrounding temperature (T) is less than the temperature of the system (TS) i.e., T < TS and there is no thermal equilibrium maintained.

  2. In the other case when the system slowly adjusts the temperature of the system with the temperature of its surrounding by absorbing heat. This happens when the surrounding temperature is greater than the temperature of the system and there is no thermal equilibrium maintained.

In simple terms, in the isothermal process: 

T = constant

This implies, the change in temperature will be zero i.e., \[\Delta T=0\] or \[dT=0\]

For ideal gasses we know that the internal energy (U) will be constant, then \[\Delta U=0\].

Apart from that, many factors of the system also vary during the continuation of an isothermal process such as internal energy. To maintain the temperature of the system, it can be kept in a tight cylinder. Then, by regulating the temperature of the cylinder, we can control the temperature of the system to an optimal level.

Below are the examples of the isothermal process:

  1. A phase change of matter

  2. Melting of matter, and

  3. Evaporation, etc. 

  4. Working in the refrigerator is an isothermal process. The temperature of the surroundings changes irrespective of changes in the internal temperature of the refrigerator. Excess heat is removed and transmitted to the surrounding.

  5. Working of a heat pump is again an isothermal process in this the heat from the surrounding is either brought inside the house or released outside the house depending upon the needs.

A common industrial use of the isothermal process is the Carnot heat engine

To maintain the temperature of the system, work should either be done on the system or be done by the system on the surrounding; doing work on the gas increases the internal energy, which, in turn, increases the temperature. 

However, if the temperature rises more than the required range, then work is done by the system on the surroundings. However, when the temperature of the system decreases, the energy is released to the surroundings in the form of heat.

Isobaric Process

An isobaric process has the word ‘bar’, where the bar is the unit of pressure. Another three letters added ‘iso’ make a process called the isobaric process. An isobaric process is a process that takes place under constant pressure.


  1. Freezing of water to ice or boiling of water to steam. In either of the scenarios gas, either contacts or expands and a net amount of work is done so as to maintain constant pressure.

Isochoric Process

The word ‘choric’ in isochoric stands for volume and the word ‘iso’ stands for equal. An isochoric process is one that takes place at a constant volume. It is also known as isometric process or constant-volume process.

If the work done, i.e., W = PΔV

At constant volume, ΔV =  0, i.e., no work is done by the system.

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FAQs (Frequently Asked Questions)

1.  Define the Thermodynamic Process.

A process occurs when the system changes from one set of values of its physical properties to another. The system reverts to its original state when all of its macroscopic physical properties regain their original values. Heat transfer and work are two core processes that alter the state of thermodynamic equilibrium. A quasi-static process is one in which the system changes so slowly that each succeeding state through which it passes remains at equilibrium. All the reversible processes occur very slowly or are quasi-static in nature. An equilibrium state is a resting state. During a reversible process, the system can deviate from equilibrium by an infinitesimal amount. There are other thermodynamic processes in equilibrium thermodynamics, viz: adiabatic, isochoric, and isobaric; where these processes are considered the thermodynamic variable that is kept constant.

2. What is an Isothermal Constant? State the Formula For the Isothermal Constant?

Isothermal Compression is the change in the volume of a system when the temperature is constant. It helps us determine the relationship between the pressure and density when the gas undergoes the compression process. For ideal gasses, when the temperature is kept constant, the internal energy of the system also remains constant, so Δ U = 0. The First Law of Thermodynamics states that Δ U = Q + W, it follows that for the compression or expansion process to occur Q = − W for ideal gasses. Though the isothermal compression process is a useful concept that determines the compressible attributes of a reservoir; however, is just a theoretical concept, and has no place in practical work.

3. State an example of an isochoric process.

The prominent example of the isochoric process is Otto Cycle. It is defined as a process that helps us understand how the engine turns the fuel into motion. In this, the chemical energy is transferred to mechanical energy. There is an increase in temperature in a car’s engine when the gasoline mixture is burnt. Along with the temperature, the pressure also increases while the volume tends to remain constant. 

4. Explain work done in an isochoric process.

Consider a closed container, in this when an ideal gas is poured and heated it will raise the pressure without changing the volume. Whereas, the volume and gas quantity remains constant. This can further be concluded as rising in energy is directly proportional to the increasing pressure and temperature. The work done in the isochoric process is always zero irrespective of the value of pressure be that positive or negative. Work done by the gas is an isochoric process as shown:  PV^n = Constant.

5. Explain the Brayton cycle.

It is an ideal cycle consisting of four thermodynamic processes- two reversible adiabatic processes and two isobaric processes.

  1. Reversible adiabatic process compression: The surrounding temperature is drawn inside the compressor and is pressurized.

  2. Isobaric heat addition: Now this compressed air is passed to the combustion chamber, the chamber is open for the absorption and release of pressure. The fuel is burned and thus the air is heated.

  3. Reversible adiabatic process expansion: This heated energy is further compressed under pressure so that it gives up energy.

  4. Isobaric heat rejection: The final process in which the final end product heat is rejected to close the cycle.

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