Difference Between Isothermal and Adiabatic Process

The main difference between an isothermal and an adiabatic process lies in the way they allow the exchange of heat. In an isothermal process, heat is exchanged but the temperature remains constant while in an adiabatic process, heat is not exchanged and the temperature has to change to maintain the lack of heat.

• Thermodynamics is the study of systems and things in terms of temperature, motion, and other physical properties.
• This applies to everything from single-celled organisms to mechanical systems in the universe.
• Therefore, thermodynamics can be defined as the branch of science that studies the relationships between heat energy and other types of energy.
• It also explains how thermal energy is converted into other types of energy and how they affect matter.

Key Terms: Thermodynamics, Isothermal Process, Adiabatic Process, Examples of Adiabatic Process, Boyle’s Law, Refrigerator, Heat pump, Temperature.

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What is Isothermal Process?

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A process in which the temperature of the system is kept fixed throughout is called an Isothermal Process.

• It is also known as the constant-temperature process.
• In this process, other factors like pressure, volume, amount of heat, etc can change but the temperature has to remain constant.
• The word “iso” means equal and thermal means “heat”.
• For a process to be isothermal, any heat flow into or out of the system must occur very slowly so that it maintains thermal equilibrium.
• In general, for an isothermal process none of the quantities ΔU, W, or Q is zero.
• But in some special cases, the internal energy of a system depends only on its temperature, not on its pressure or volume.
• The most common system following this special case is an ideal gas.
• For an ideal gas, if the temperature is constant, the internal energy is also constant i.e. ΔU = 0.
• Hence the first law of thermodynamics then implies that heat supplied to the gas equals the work done by the gas i.e. Q = W.

For an Isothermal process, the ideal gas equation i.e. PV = μRT gives

PV = constant

The above equation represents Boyle’s Law.

The work done in the isothermal process is given by

\(W=2.303 \mu RT \: \log \frac {V_2}{V_1}\)

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Examples of Isothermal Process

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The following are the examples of Isothermal process

• The melting and the evaporation which are the changes of state for any liquid follow the isothermal process.
• Refrigerator is another example present in households where many internal mechanisms occur but the temperature remains constant.
• The energy is transferred into the outside environment.
• Carnot engine cycle which has industrial applications.
• In the first step of the Carnot cycle, the work is done on the environment due to the heat being absorbed by the system to keep the temperature constant.
• A heat pump that is used to warm or cool the house is an example of an isothermal process.
• In a heat pump, heat is either removed from the house or added to the house while keeping the temperature constant.

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What is Adiabatic Process?

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A process in which no heat transfer takes place between a system and its surroundings is called an Adiabatic process. 

• Despite no transfer of heat or mass, the process still is a part of thermodynamics because the energy is derived from the internal system.
• The energy transfer takes place in the form of work done. 
• In the adiabatic process, the temperature changes to maintain the constant heat in the system.
• The adiabatic process can either be reversible, in which no change of entropy occurs, or irreversible in which entropy increases.
• For an adiabatic process, the entropy never decreases.
• If any process is occurring inside a good thermal insulator, then that process can be termed as adiabatic.
• The process is always carried out very quickly ensuring sufficient time for heat transfer.
• In an adiabatic process, Q = 0, so from the first law of thermodynamics, ΔU = -W.
• Hence the work done by the ideal gas results in a decrease in its internal energy.

The ideal gas equation for an adiabatic process is given by

\(PV^{\gamma}= constant\)

The work done in the adiabatic process is given by

\(W=\frac {\mu R(T_1-T_2)}{\gamma - 1}\)

The essential conditions for an adiabatic process are:

• No heat energy transfer should take place.
• Changes must occur at a spontaneous rate. 
• Specific heat is zero

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Examples of Adiabatic Process

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The following are the examples of Adiabatic process

• When ice is placed inside an icebox, no heat comes or goes out of the box making it an example of an adiabatic process.
• Devices like turbines, nozzles, and compressors work on the principle of the adiabatic process.
• In the pneumatic tire, the air gets released. This is also an example of the adiabatic process as in it, the gas is compressed which generates heat.
• The oscillation of the pendulum in a vertical plane is another example of the adiabatic process.
• Another example can be a quantum harmonic oscillator.

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Difference Between Isothermal and Adiabatic Process

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The main differences between isothermal and adiabatic processes are as follows

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Things to Remember

• Thermodynamics is the branch of physics that deals with the relationship between heat, work, temperature, and energy.
• The main difference between an isothermal and an adiabatic process lies in the way they allow the exchange of heat.
• In an isothermal process, heat is exchanged but the temperature remains constant.
• In an adiabatic process, heat is not exchanged and the temperature has to change to maintain the lack of heat.
• For an Isothermal process, the ideal gas equation is given by PV = constant.
• For an Adiabatic process, the ideal gas equation is given by PV^γ = constant. 
• The isothermal process follows Boyle’s Law.
• Examples of isothermal processes include changes of states (melting and evaporation), refrigerators, Carnot cycle, and heat pumps.
• Examples of adiabatic processes include ice in the icebox, and devices like turbines, nozzles, and compressors.

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