Isothermal Process

Physics, studies systems and objects to measure their temperatures, motions and other physical characteristics. It can be applied to anything from single-celled organisms to mechanical systems to galaxies, stars and planets and the processes that govern them. In physics, thermodynamics is a branch that deals with the relationships between heat energy and other forms of energy. It describes how thermal energy is converted into other forms of energy and how it affects matter.

An isothermal process is a thermodynamic process in which the temperature of a system remains constant. The transfer of heat into or out of the system happens so slowly that thermal equilibrium is maintained. At a particular constant temperature, the change of a substance, object or system is known as the Isothermal Process. Usually, there are two phenomena under which this process can take place. If a system is in contact with a thermal reservoir from outside, then, to maintain thermal equilibrium, the system slowly adjusts itself with the temperature of the reservoir through heat exchange. In contrast, in another phenomenon, no heat transfer occurs between a system and its surrounding. In this process, the temperature of the system is changed in order to keep the heat constant. This process is known as the Adiabatic Process.

Isothermal and Adiabatic Process DifferenceExamples of Isothermal ProcessWhat is Boyles Law

Difference Between Isothermal and Adiabatic Process

An isothermal process is a process that occurs under constant temperature but other parameters of the system can be changed accordingly. On the other hand, in an adiabatic process, heat transfer occurs to keep the temperature constant. The main difference between isothermal and adiabatic process is that the isothermal process occurs under constant temperature, while the adiabatic process occurs under varying temperature. The work done in an isothermal process is due to the change in the net heat content of the system. Meanwhile, the work done in an adiabatic process is due to the change in its internal energy.

Examples of Isothermal Process

An isothermal process occurs in systems that have some means 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 in 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 parts of the cycles are carried out isothermally.
  • A refrigerator works isothermally. A set of changes take place in the mechanism of a refrigerator but the temperature inside remains constant. Here, the heat energy is removed and transmitted to the surrounding environment.
  • Another example of an isothermal process is the heat pump. The heat is either removed 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.

What is Boyle’s Law?

An isothermal process is of special interest for ideal gasses. An ideal gas is a hypothetical gas whose molecules don’t interact and face an elastic collision with each other. Joule’s second law states that the internal energy of a fixed amount of an ideal gas only depends on the temperature. Thus, the internal energy of an ideal gas in an isothermal process is constant.

In an isothermal condition, for an ideal gas, the product of Pressure and Volume (PV) is constant. This is known as Boyle’s law. Physicist and chemist Robert Boyle published this law in 1662. Boyle’s law is often termed as Boyle–Mariotte law, or Mariotte’s law because French physicist Edme Mariotte independently discovered the same law in 1679.

Boyle’s Law Equation

The absolute pressure exerted by an object of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.

There are a couple of ways in which the above-stated law can be expressed. The most basic way is given as follows:

PV = k

where P is the pressure, V is the volume and k is a constant.

The law can also be used to find the volume and pressure of a system when the temperature is held constant in the system as follows:

PiVi =  Pf Vf


  • Pi is the initial pressure
  • Pf is the final pressure
  • Vi  is the initial volume
  • Vf is the final volume

The way people breathe and exhale air out of their lungs can be explained by Boyle’s Law. When the diaphragm contracts and expands, lung volume decreases and increases respectively, changing the air pressure inside them. The pressure difference between the interior of the lungs and the external air produces either inhalation or exhalation.

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When scientists study isothermal processes in systems, they examine heat and energy and their relation and also the mechanical energy it takes to change or maintain the temperature of a system. Such understanding helps biologists study the regulation of temperature in living organisms. It also comes into play in planetary science, space science, engineering, geology, and many other branches of science.

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