Adiabatic Vs Isothermal: In the domain of Physics, especially in the subject of thermodynamics, there are two often considered concepts that are regularly used in industrial practical application. These approaches are the adiabatic and isothermal processes.

These two processes are the opposite fronts of the coin. They are the poles situated at opposite ends so to speak. Firstly, differently known as an isocaloric process, the adiabatic process is when there’s no transfer of heat from or towards the fluid being worked on. Besides, adiabatic would indicate impassable if defined actually. Thus, heat is not able to pass through.

When there’s a definite gain or heat loss in the surroundings then the process is known as adiabatic. Because the temperature can fluctuate in an adiabatic process due to internal system diversities, the gas in the system may favor to cool down when expanding. In this relation, it would also mean that its pressure is significantly lesser correlated to the other process (isothermal) at a given volume.

As acknowledged, the process at the other extreme end that endorses the transfer of heat to the surroundings, and thus, forming the overall temperature constant (do not change) is named an isothermal process. If you come to think of it, the word isothermal when clarified literally would mean ‘iso’ (the same), ‘thermal’ (temperature). Hence, there is the same temperature.

Adiabatic Vs Isothermal
Adiabatic Vs Isothermal

In a thermodynamics system, the two main processes associated are adiabatic or isothermal. It is regarded as the erstwhile when the transformation (variations in temperature) is fast acceptably that no heat was significantly transferred between the outside environment and the system. When the change is very slow in that same system then the process is isothermal because the temperature of the system remains the same through the exchange of heat with the outer environment.

What is Adiabatic?

An adiabatic system or process is one in which there is no final change in heat energy. Adiabatic processes are correlated to the First Law of Thermodynamics. This law describes that when heat energy is placed into a system, it will either fluctuate the internal energy of the system or it will do work.

This is associated to the law of the conservation of energy which states that matter and energy cannot be created or destroyed. In reference to thermodynamics, heat energy in a system must do something. It will either fluctuate the internal energy of the system, do work, or some merger of both. It cannot just disappear.

In an adiabatic system, pressure, volume, and temperature will reform in such a way that the heat energy remains constant. Adiabatic processes are most clearly seen in gases. Adiabatic heating in gas will cause the temperature to increase as pressure on the gas increases.

Adiabatic Process
Adiabatic Process

If the pressure on the gas decreases, this will induce the temperature to drop, resulting in adiabatic cooling. With adiabatic heating, gas will be compressed, and thus work will be done on the gas by the environment. If adiabatic cooling appears, this will result in the gas expanding and the gas will do work on the environment.

An example where adiabatic processes are important is in the situation of a piston, such as a piston in a diesel engine. As pressure from the piston increases, the gas will contract. With decompression, the gas will expand again, moving the piston. This is controlled by adiabatic processes.

Adiabatic processes are essential in meteorology. If a package of air rises, the pressure on the package of air will decrease and this will cause the air temperature to decrease due to adiabatic cooling. On the other hand, if an air mass is pushed against the ground, it will cause the pressure on the air mass to increase, warming up the air mass.

Because air pressure decreases with height, the temperature will decrease with height in the atmosphere. The rate at which the temperature decreases with increasing height is known as the adiabatic lapse rate.

Adiabatic Process Equation

Below is the adiabatic process equation:

PVγ = constant

  • P is the pressure of the system
  • V is the volume of the system
  • γ is the adiabatic index and is characterized as the ratio of heat capacity at constant pressure Cp to heat capacity at constant volume Cv

Reversible Adiabatic Process

The reversible adiabatic process is also named an Isentropic Process. It is a glorified thermodynamic process that is adiabatic and in which the work transfers of the system are frictionless; there is no transfer of heat or of matter and the process is reversible. Such an idealized process is essential in engineering as a model of and basis of comparison for real processes.

Adiabatic Expansion

Adiabatic expansion is described as an ideal behavior for a closed system, in which the pressure is constant and the temperature is decreasing.

Adiabatic Compression

Adiabatic compression of the air is described as the compression in which no heat is added or subtracted from the air and the internal energy of the air is increased which is equal to the external work done on the air. The pressure of the air is more than the volume as the temperature increases during compression.

Adiabatic Process Example

There are several instances, some are stated below:

  • It is a process where there is gas compression and heat is developed. One of the easiest examples would be the release of air from a pneumatic tire.
  • Adiabatic Efficiency is used to devices such as nozzles, compressors, and turbines. One of the excellent applications of the adiabatic process.
  • The pendulum oscillating in a vertical plane is an example of it.
  • A quantum harmonic oscillator is also an example of an adiabatic system.
  • When we led the ice into the icebox, no heat goes out and no heat comes in.

What is Isothermal?

An isothermal process is one where the temperature goes on constant even if the pressure and volume vary. In thermodynamics, pressure, temperature, and volume are presented by Boyle’s gas law. If one is held constant, the others will change infractions to each other. If the temperature of a gas is held constant, the pressure and volume of the gas will be inversely proportional.

An illustration of an isothermal process is a change of phase. When a substance, such as water, reaches its melting point or boiling point, the pressure and temperature will remain unchanged as the phase, volume, and heat energy change.

Isothermal Process
Isothermal Process

Isothermal processes form the base of heat engines which are used in electrical power plants, cars, airplanes, rockets, and other machines that are important for modern times. Isothermal processes are also important in biology, geology, space science, planetary sciences, and many other fields.

Examples of Isothermal Process

An isothermal process occurs in systems that have some defines of regulating the temperature. This process occurs in systems ranging from highly structured machines to living cells. A few examples of an isothermal process are given below.

  • Changes of state or phase changes of different liquids through the process of melting and evaporation are examples of the isothermal process.
  • One of the examples of the industrial application of the isothermal process is the Carnot engine. In this engine, some sections of the cycles go through isothermally.
  • A refrigerator works isothermally. A set of changes take place in the procedure of a refrigerator but the temperature inside remains constant. Here, the heat energy is extracted and transmitted to the surrounding environment.
  • Another example of an isothermal process is the heat pump. The heat is either extracted from the house and dumped outside or the heat is brought inside the house from outside to warm the house. In either case, the goal is to keep the house at the desired temperature setting.

Difference Between Isothermal and Adiabatic Process

In order to separate between isothermal and adiabatic processes, the major difference between isothermal and adiabatic processes, as clear from the intro, is in the manner of heat transfer in-between the surrounding and the system.

The heat change by heat transfer is used to differentiate between isothermal and adiabatic processes as in the isothermal process, heat transfer results to control a constant temperature while there is no such heat transfer in-between the system and the surrounding. Some of the more important difference between isothermal and adiabatic process in tabular form is given below as a implies to compare between isothermal and adiabatic process:

  1. There is heat transfer in the isothermal process while there is no heat transfer in this thermodynamic adiabatic process.
  2. At any given volume, the pressure is more following the ideal gas equation. The pressure is less at a given volume.
  3. As the name suggests the temperature of an isothermal process remains constant. Since, there is no transfer of heat, as the internal energy changes the temperature of an adiabatic process changes.
  4. From a thermal reservoir near the system, heat can be added or removed from the system, in order to keep the temperature constant. By definition, there is no change in the heat in an adiabatic process and so there is no addition or subtraction of the heat.
  5. Any transformation in such a process is slow. Any transformation in an adiabatic process is fast.

What is Isobaric?

In an isobaric process, the pressure in a system goes on constant. Under isobaric conditions, volume and temperature are directly affected. If temperature increases, so must the volume. This can be understood by placing a balloon in a freezer. The pressure both inside the balloon and outside will remain constant, but the balloon will occur to shrink in volume as it cools down.

Another example is a weighted piston that is shifted by heated gas in a cylinder. As the gas is heated, the gas temperature grows, and the gas expands, driving on the piston. If the piston were fixed and could not move, the pressure in the gas would grow instead of the gas expanding and the system would not be isobaric.

Isobaric processes are critical in the construction of heat engines since certain heat engines rely on isobaric processes to transform heat energy into mechanical energy.

What is Isochoric Process?

An isochoric process is a thermodynamic process in which the volume remains unchanged. Since the volume is unchanged, the system does not work, and W = 0. (“W” is the sign for work.) This is maybe the easiest of the thermodynamic variables to control since it can be achieved by placing the system in a sealed container that neither expands nor contracts.

First Law of Thermodynamics

To understand the isochoric process, you need to understand the first law of thermodynamics, which states:

“The change in a system’s internal energy is equal to the difference between heat added to the system from its surroundings and work done by the system on its surroundings.”

Applying the first law of thermodynamics to this situation, you find that:

delta-Since delta-U is the change in internal energy and Q is the heat transfer into or out of the system, you see that all of the heat either comes from internal energy or goes into increasing the internal energy.

Constant Volume

It is possible to do work on a system without changing the volume, as in the case of stirring a liquid. Some sources use “isochoric” in these cases to mean “zero-work” regardless of whether there is a change in volume or not. In most straightforward applications, however, this nuance will not need to be considered—if the volume remains constant throughout the process, it is an isochoric process.

Example Calculation: Here we have an example of a calculation involving the isochoric process.

Assume an isochoric heat addition in an ideal gas. In an ideal gas, molecules have no volume and do not interact. According to the ideal gas law, pressure varies linearly with temperature and quantity, and inversely with volume. The basic formula would be:

PV = nRT


  • p is the absolute pressure of the gas
  • n is the amount of substance
  • T is the absolute temperature
  • V is the volume
  • R is the ideal, or universal, gas constant equal to the product of the Boltzmann constant and the Avogadro constant
  • K is the scientific abbreviation for Kelvin

In this equation the symbol R is a constant called the universal gas constant that has the same value for all gases—namely, R = 8.31 Joule/mole K.

The isochoric process can be expressed with the ideal gas law as:

p/T = constant

Since the process is isochoric, dV = 0, the pressure-volume work is equal to zero. According to the ideal gas model, the internal energy can be calculated by:

∆U = m cv ∆T

where the property cv (J/mole K) is referred to as specific heat (or heat capacity) at a constant volume because under certain special conditions (constant volume) it relates the temperature change of a system to the amount of energy added by heat transfer.

Since there is no work done by or on the system, the first law of thermodynamics dictates ∆U = ∆Q. Therefore:

Q = m cv ∆T


In an adiabatic system, there is no final change in heat. When a gas expands, the temperature will drop, leading to adiabatic cooling. If a gas is compressed, the temperature will increase, leading to adiabatic heating. Adiabatic processes are important in atmospheric science. In an isothermal process, the temperature is stable, and pressure and volume are inversely related to each other. An example of an isothermal process is a change of phase.

During the change of phase, the temperature of a object will not change even though its heat and volume change. In an isobaric system, the pressure remains constant and volume will increase or decrease with temperature. If a volume of gas is placed in a freezer, for example, the volume of gas will decrease in amount since the pressure is constant while its temperature is descending.