# Examples of Zeroth Law of Thermodynamics

In a shivering winter season, if your mom prepares a nice hot coffee for you and you do not drink it within few minutes, then what happens?

It will cool down. Right !!

In this example, a zeroth law of thermodynamics plays an important role.

Don’t worry, I’ll explain to you everything about it. I’ll also explain to you many more real life examples of zeroth law of thermodynamics. (You will definitely love these amazing examples)

So, let’s get started with the examples of 0th law of thermodynamics.

Contents

## Applications/examples of Zeroth law of thermodynamics

### 1) All three blocks in thermal equilibrium with each other.

Just see this animation.

Block A is in contact with block C.

Similarly, block B is in contact with block C.

But block A and B are not in contact with each other.

Even though, after some time all the three blocks will reach the same temperature (In other words, these three blocks achieves thermal equilibrium with each other)

### 2) Hot coffee becomes cold after some time.

Why?

Why does hot coffee become cold if you do not drink it within a few minutes?

I’ll explain to you how the zeroth law of thermodynamics is responsible for this.

See, the hot coffee is surrounded by the cold air of the room. This hot coffee loses its heat to the surrounding air of the room.

Hot coffee will keep on losing the heat until its temperature becomes equal to the temperature of the surrounding air of the room.

Let say the surrounding temperature is 25 °C and the temperature of hot coffee is 70 °C. Then coffee will lose its heat until its temperature reaches 25 °C. (In other words, coffee will lose heat until it achieve thermal equilibrium with the surrounding)

Thus, the zeroth law of thermodynamics appears here in this example of a hot coffee cup.

### 3) A thermostat in your room.

Let say, a thermostat in your room shows 68 °F, which means that the air of your room and thermostat are in thermal equilibrium with each other

All the objects of your room (clock, cupboard, chair, table, fan, etc are also at 68 °F according to Zeroth law of thermodynamics)

### 4) Cold water and hot water

Similar to the example of a hot coffee cup, in one glass take an ice cube and in another glass, take hot water.

Now put them on a table, and you will see that after some time the ice will melt and in the other glass, the hot water will become cold.

After some time, the temperature of water of both the glasses as well as the air of the room becomes equal. This means that all these three are in thermal equilibrium with each other.

You can also do this experiment as an example of Zeroth law of thermodynamics.

### 5) Vegetables in your refrigerator

What will happen if you keep vegetables in the refrigerator for one night?

It will cool down and it will achieve the lower temperature as that of the air inside the refrigerator.

We can say that the vegetables are in thermal equilibrium with the air of a refrigerator.

Thus anything kept in the refrigerator is an example of Zeroth law of thermodynamics.

### 6) Thermometer for measuring body temperature.

Now, this is what you have already experienced when you had a fever.

Yes, a thermometer in contact with your body is an example of 0th law of thermodynamics.

But how?

Let me explain to you.

See, there is mercury inside the glass tube of a thermometer.

Now when the thermometer comes in contact with a hot body (a person having fever), it’s temperature rises.

But how much will be the temperature rise in mercury?

Now this is a big question.

Look, the mercury will rise up to the temperature of the human body which is in contact with it.

After few seconds, this mercury and human body will be at the same temperature. Thus we can say that they are in thermal equilibrium with each other.

Due you know what has happened to this mercury for reaching this thermal equilibrium?

Yes I mentioned that there is a rise in temperature of mercury.

When the temperature of mercury increases, it expands. And due to this expansion, the level of mercury inside the glass tube will rise. And we can come to know about the temperature of the human body through the readings marked on the thermometer.

This is how the principle of thermal equilibrium (or zeroth law of thermodynamics) appears here in thermometers.

### 7) Temperature measurement in refrigerator

Do you know, the temperature inside the refrigerator always remains in some range?

Also you might have seen that the refrigerator starts and stops after few minutes.

How does this happen? And why?

There is a temperature sensor inside the refrigerator which continuously monitors the inside temperature of the refrigerator.

Now the question is how this temperature sensor works?

It works on the zeroth law of thermodynamics. The temperature sensor monitors the system temperature by achieving thermal equilibrium with the air of the refrigerator.

After sensing the inside temperature of the refrigerator, it sends the signal to the refrigerator components to regulate the temperature.

Thus the temperature sensor in a refrigerator is an example of zeroth law of thermodynamics.

### 8) Temperature measurement in air conditioners.

The application of zeroth law of thermodynamics is the same as that of a refrigerator.

Here also the air conditioner has a temperature sensor which monitors the temperature of the room.

After this, it will send the signal to the electrical system to produce a cooling effect in the room. This cooling continues until the temperature of the room and temperature set by the user becomes equal. (In other words, the cooing of room is done by the air conditioning system until the room air achieves thermal equilibrium with the temperature set by the user)

Well, air conditioning is more complex and vast topic, but I have given you the simple application of zeroth law behind this temperature regulation inside the air conditioning system.

### 9) In food processing industries

You might have seen or at least heard about food preservation.

Food needs to be preserved at some specific lower temperatures.

For this a temperature sensing device is used. Here in the image, you can see the infrared thermometer (or thermal gun) which measures the temperature of the substance through the infrared rays coming from the device. This device is also known as Pyrometer

The working of this device is based on the zeroth law of thermodynamics. Here the temperature is sensed using the infrared light.

### 10) To measure atmospheric temperature

All of us have noticed that the temperature of atmosphere decreases in the winter season and it increases during the summer season.

But how much increase or decrease?

This exact temperature of the atmosphere is measured using a thermometer.

The working of this temperature is the same as discussed above in the first example.

### 11) Temperature measurement for industrial applications.

In many industries, the hot or cold fluids are supplied for specific applications.

As shown in the above image, the industrial thermometer measures the temperature of the fluid passing through the pipe.

This sensor attached to the thermometer achieves thermal equilibrium with the hot or cold fluids and it shows the readings on the dial. Thus this industrial thermometer is an example in which the zeroth law of thermodynamics is applied.

I hope you have understood all the examples of zeroth law of thermodynamics which are applicable in real life. For more information about zeroth law of thermodynamics, you can refer this detailed article A complete guide on Zeroth law of thermodynamics.”