Third law of thermodynamics

Third law of thermodynamics,

Definition of Third law of thermodynamics:

  1. See thermodynamics.

Meaning of Third law of thermodynamics & Third law of thermodynamics Definition

Third Law of Thermodynamics

“The entropy becomes constant or the change in entropy is zero when the temperature of the system approaches absolute zero.”

Before we start to define the Third Law of Thermodynamics first of all we have to know about that

What is Thermodynamics?

“Thermodynamics comprises of two words, ‘Therm’ means heat and ‘dynamics’ means motion. So basically Thermodynamics deals with the transfer of energy from one form to another form of energy respectively.

For a better explanation to define Thermodynamics some daily life examples are given as below:

  1. Engines that power our vehicles.

  2. Heating and cooling systems in Houses Apartments, etc.

  3. Design of buildings and vehicles.

So we see that thermodynamics is involved in our daily life

  1. There are three laws of thermodynamics

  2. The first law of thermodynamics.

  3. The second law of thermodynamics.

Third law of thermodynamics

The first two laws should be described in a summarized way because it relates to the third law of thermodynamics.

First Law of thermodynamics :

It states that:

“Energy can neither be created nor destroyed but it can change from one type of energy to a different type of Energy.”

So basically Heat is a form of energy and thermodynamics processes are therefore concern with the concept of conservation of energy.

Second Law of Thermodynamics:

It states that:

“Entropy of an isolated system always increases.”

In this statement, two words are the priority to describe it and that is Isolated System and Entropy.

Isolated System:

It is a thermodynamics system that can’t interchange either Matter or Energy outside the boundaries of the system.

Meaning of Second Law of Thermodynamics (Entropy):

It is simply a measure of disorder or randomness of the system and when the external energy is applied the randomness is increased.

The system is a particular area in which we have to observe it.

An explanation for the Second law of thermodynamics (Entropy):

For a better understanding of entropy consider a classroom in which students sit randomly, a snake is coming in the classroom and all the students left their place and sit at different seats until the snake leaves the classroom.

So basically Classroom is the system, a snake is external energy that has been applied and entropy phenomena occur in the form of disorder in the classroom.

Third Law of Thermodynamics :

The third law of thermodynamics is also called Nernst law.

It states that:

“The entropy becomes constant or the change in entropy is zero when the temperature of the system approaches absolute zero.”

Absolute Temperature : Absolute Zero is ( 0 K or -273 °C or -459.67 °F )

Meaning of Third Law of Thermodynamics :

It interprets in this way that Consider a student sits in the class and waiting for the teacher to start the class but the teacher is not coming due to some reason so the student leaves that class and the class is vacant.

By contrast this example with the statement with the above law we see that when the teacher is not coming or the absolute temperature is zero, all the students leave the class or the entropy becomes zero.

Another explanation to define the third law of thermodynamics.

Consider a block of ice that is chilled at a colder temperature. By continuing this chilling ice temperature decrease more and more and in a micro-level colder temperature slow down the internal sub-atomic movement decreasingly more until they arrived at that at least disordered state that is genuinely conceivable, which can be described using a constant value of entropy.


At absolute zero temperature entropy of perfectly crystalline substance becomes zero.

Mathematically, T= 0 K ------------> S= 0

Consequences of the Third Law of Thermodynamics:

In a real-life, no scientist can ever reach the absolute zero temperature. The lowest temperature that scientists can achieve is 184 Kelvin or -89.2 oC. I can contrast this consequence with our universe. Sun has the highest entropy in our universe. If the sun is diminished the whole universe is destroyed because the entropy is zero the randomness in this universe is also zero so there will be doomsday.

Relation of the Third law of Thermodynamics with First and Second law:

The second and third laws of thermodynamics prompt the end that inevitably, as all energy known to man changes into heat, it will arrive at a consistent temperature. Called thermal equilibrium, this condition of the universe is constant, however at a temperature higher than absolute zero.

The third law likewise bolsters ramifications of the first law of thermodynamics. This law states that the change in internal energy for a system is equal to the difference between the heat added to the system and the work done by the system:

ΔU = Q − W

Where U is energy_, Q_ is heat and W is work, all typically measured in joules, Btus, or calories).

This formula shows that more heat in a system means it will have more energy. That in turn necessarily means more entropy. Think of an ideal crystal at absolute zero – adding heat introduces some molecular motion, and therefore the structure is not any longer perfectly ordered; it’s some entropy. This is all to be concluded.