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Third law of Thermodynamics

Last updated date: 01st Dec 2023
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Introduction to The Third law of Thermodynamics

The third law of thermodynamics defines that the entropy of flawless crystals at a temperature of (O K) is Zero. It majorly impacts how modern industries work and the principles on which our machines are designed. These laws are applicable to everything in the universe. These laws are stated in reference to some standard and explain the physical conditions in terms of thermodynamics. Third law of thermodynamics in particular talks about the universe as a whole and the magnitude of randomness in it. This law basically considers everything as a single system and sets rules for the randomness of components of this system with respect to its dependency on temperature. In the field of science, it is very important to have fixed standards and to have an idea of what’s practically possible. So, the third law talks about absolute zero as a standard state.


Entropy is a physical quantity that indicates the molecular disorder of a system or else stated as the randomness of a system. Mathematically, it is the amount of thermal energy of a system that is unavailable for doing useful work. This is because work is derived from the ordered movement of molecules of a system. Entropy as a concept helps to understand the spontaneity of any process to come to a conclusion that which processes are thermally possible or impossible. This explains the phenomenon of the irreversibility of reactions.

The entropy of a system depends on temperature. It increases with the increase in temperature and decreases with a decrease in temperature. 



ΔS is the entropy change in the system

Q is the heat absorbed 

T is the temperature 

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The Statement of the Third Law

This law deals with the concept of “absolute zero”. Absolute zero is the lowest temperature on the Kelvin Scale which is zero. This is equal to -273.15 on a Celsius scale and -459.7 on a Fahrenheit scale.

The third law of thermodynamics states that when the temperature of a system approaches absolute zero, the entropy of that system approaches a constant value.

This suggests that there is a minimum measure of randomness of a system and that occurs when the temperature reaches absolute zero. If an isolated system is constituted by the combination of two thermodynamic systems, then any energy exchange in any form between those two systems is bounded.

This can also be explained by seeing that at absolute zero, the system does not contain any heat which tells the presence of all the molecules at their lowest energy points. This law should make sense as our basic understanding of thermodynamics tells us that as the temperature decreases, the heat of a system which is just a collection of kinetic energies of the system also decreases. Thus, at some point, the kinetic energy reduces to a complete stop which means no randomness.

Contradiction with The Other Laws of Thermodynamics

While the third law states about absolute zero as a state, on the other hand, the second law of thermodynamics suggests that the temperature can never actually become zero. This is because, through the second law, we know that heat cannot spontaneously be moved from a colder body to a hotter body. When a system tries to approach absolute zero, it actually has a tendency to draw heat energy from the external environment and if that happens, it will never actually reach absolute zero.

As the first law states that energy can neither be created nor destroyed, here the heat energy has to be drawn from somewhere outside of the system which finishes the chances of the system reaching absolute zero 

Did you know?

  • Heat Death of the universe also known as the Big Chill or Big Freeze is an ultimate fate of the universe which states that the universe has diminished to a state of no thermodynamic energy.

  • Heat Death of the universe suggests that at a point in time, the universe would be unable to sustain any processes that involve an increase in entropy.

  • There are theories that suggest that the origin of all life was a result of a bunch of atoms suffering an increase in their entropy when exposed to light.

  • All laws of thermodynamics are universal in nature and can be applied to any entity however large or small it is.

FAQs on Third law of Thermodynamics

1. How does the Third Law of Thermodynamics impact modern science and technology?

Ans. The third law of thermodynamics puts forth a very important point about the entropy of the universe. This comes in contradiction with the theory that our universe is expanding. In a universe which is expanding at an accelerated rate, entropy increases as temperature decreases. If at its end, the universe reaches 0K temperature, then its entropy would consequently be zero which is not practically possible. 

Some practical applications include the aim to order things to reduce entropy which in turn is to decrease in temperature. This is used for adiabatic cooling which uses external methods to order the materials resulting in very low temperatures. This method uses magnetic energy to do so and is thus called magnetic cooling.

2. What is the role of the Third Law of Thermodynamics in refrigeration?

Ans. The third law tells us that there cannot be any environmental parameter that can be manipulated to cause a change in the entropy of a system at absolute zero. Thus, refrigeration constitutes a cycle of alternating steps. First is keeping the temperature constant and changing other parameters to bring out heat from a system eventually resulting in entropy decrease. The next step is to insulate the system and reverse the change in the parameter. This results in a fall in the temperature. So basically the alternate isothermal and adiabatic processes work in the principle of third law and help in better refrigeration.