Why is there a Zeroth law of Thermodynamics? (2 Reasons)

The exact reason why there is a Zeroth law of thermodynamics is mentioned below:

#Reason 1: Zeroth law of thermodynamics is very important because it shows that the physical quantity named “temperature” also exists in nature.

#Reason 2: It was discovered after the first, second and third law. And it is important to keep this law before the three laws without changing the order, hence it is named “Zeroth”.


Just image two cylinders having equal pressures.

Two cylinders
Image source (pikrepo)

Let’s say both these cylinders have an equal pressure of 5 bar. 

Now imagine that we connect both these cylinders using a pipe.

What will happen to their pressures?

The pressure of both the cylinders will remain the same only. (i.e 5 bar only). Because initially they were at 5 bar pressure.

And finally we can say that both the cylinders are in mechanical equilibrium with each other.

In short we are using the property named “PRESSURE” to say that the cylinders are in mechanical equilibrium.

(See how many times I have bold the term “pressure” in above sentences)

Now let’s come to the main point.

Let’s say the two blocks A and B are brought in contact with each other and no heat transfer takes place between them.

Why is there a Zeroth law of Thermodynamics

This condition is known as thermal equilibrium.

Now in the previous example of cylinders, we used the physical property named “pressure” to define mechanical equilibrium.

Now here, in order to define this thermal equilibrium, there must be some physical property just like the pressure.

This property is “TEMPERATURE“.

The zeroth law of thermodynamics defines the physical property called “temperature”.

This is the reason why there is a zeroth law of thermodynamics.

Why “zeroth”, and why not first, second, third or fourth?

In 1935, Ralph H Fowler discovered the title “Zeroth law of thermodynamics.”

There was no discovery of any term like “temperature” before 1935. 

All the 3 laws of thermodynamics were already discovered before 1935 without defining temperature.

Fowler realized that thermal equilibrium is a very important phenomenon and it should be defined before first law. 

The zeroth law is very important which gives the definition of temperature or it shows that some physical quantity named “temperature” also exists. 

According to Fowler, this law should be kept first for proper understanding of thermodynamics laws.

But already so many books were published with the first law, second law and third law. 

So changing the numbers of these laws can create a lot of confusion. 

Thus he was forced to adopt the number “zero” for his law.

And finally this law was named “zeroth law of thermodynamics”.

I hope you have got the answer to your query.

Don’t forget to check the ultimate guide on Zeroth law of thermodynamics.

(This guide has all the important knowledge about Zeroth law of thermodynamics along with real life examples and lots more. You will love this guide for sure.)

Also see: 

Suggested important topics:

Leave a Comment