# Zeroth law of thermodynamics

This is really easy, trust me.

I’ll explain you the entire statement of zeroth law of thermodynamics. Don’t worry.

When I was student, I was wandering why this law is named “Zeroth”, why not first, second, third, etc???

It should be like first law, second law, third law, etc… But it is named zero. Why?

Are you having the same question?

Don’t worry, you will get the answer just in few seconds.

I will also explain you zeroth law of thermodynamics in easy way and you will love this.

I will also show some practical examples of zeroth law of thermodynamics which happens around you.

Contents

## Why is there a zeroth law of thermodynamics?

Well, why this law begins with the number zero? The number should begin with one, two, three, etc…

I’ll explain.

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

Fowler came upon one text – “every physical quantity must be measurable in numeric term.”

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

And main thing all the three laws of thermodynamics were already discovered before 1935 without defining temperature.

Fowler realized that thermal equilibrium is very important phenomenon and it should be defined before 1st law of thermodynamics.

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

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”.

## What is zeroth law of thermodynamics?

Now I want to explain you the definition of zeroth law of thermodynamics with an example. So let us consider three bodies A, B and C.

Zeroth law of thermodynamics states that

“If two bodies A and B are in thermal equilibrium with third body C, then body A and B are also in thermal equilibrium with each other.”

I know this may seems little bit difficult to you. So let me explain you in simple way.

As shown in the above animation, consider three bodies A, B and C having different temperatures.

Body A is represented by blue color, body B is represented by green color and body C is represented by red color.

Initially, all these three bodies are at different temperatures.

Let body A and B are separated from each other as shown in above animation.

But both of them are in thermal contact with the third body C.

After some time, body A will come in thermal equilibrium we the body C and similarly body B will also come in thermal equilibrium with body C.

After this condition, if body A and body B are kept together in contact with each other, then there will be no heat exchange between the body A and body B (because body A and body B are at same temperature).

This indicates that body A and body B are now in thermal equilibrium with each other. (You can see that both the bodies have the same color now)

Mathematically, these situation can be represented as;

If, Ta = Tc and Tb = Tc, then according to zeroth law of thermodynamics Ta = Tb. Where Ta, Tb and Tc are the temperatures of body A, B and C respectively.

Now I want to explain you the most important and practical thing. But if you do not know the basics, then kindly check the article on basics of thermodynamics.

## What did you find from the zeroth law of thermodynamics?

From zeroth law of thermodynamics we found that there is one physical property which indicates whether the bodies are in thermal equilibrium or not. And that property is Temperature.

If the temperature of the bodies are same, then the bodies are in thermal equilibrium with each other.

In simple terms, we can say that 0th law of thermodynamics helps us in finding the temperature of the body.

## Application of zeroth law of thermodynamics

1). To measure human body temperature.

You have definitely experienced this when you had fever.

0th law of thermodynamics helps us in finding the temperature of a human body.

For example, when a thermometer is kept in contact with a human body suffering from fever, the mercury gains heat and it expansion in the thermometer tube.

After few seconds, mercury achieves thermal equilibrium with the human body. At this condition, the temperature of thermometer will be same as the temperature of human body. And we can measure the readings on the thermometer

Thus in this way, zeroth law of thermodynamics is used to find the body temperature using thermometer.

2). To measure atmospheric temperature.

Climate may be hot or cold in various regions.

This atmospheric temperature is measured using thermometer, and thermometer works on the principle of zeroth law of thermodynamics.

3). Temperature measurement for industrial applications.

For measuring the temperature in industries, thermometer is used.

Temperature gun (or infrared thermometers) also used nowadays which gives the temperature of the surface on which the infrared light falls.

4). Temperature measurement in air conditioners.

To maintain the temperature of the room, the thermometer measures the temperature of air inside the room and accordingly the air conditioner works based on the load inside the room.

5). Temperature measurement in refrigerator

The refrigerator also uses thermometer to check the inside space temperature of the refrigerator and accordingly it maintains the temperature.

6). In food processing industries

To regulate the temperature in food processing industries, thermometers are used, which note down the temperate of the room or particular area. And then further cooling as let requirements.

## What is temperature?

What will happen if you keep on hitting the nail with the hammer?

It will become little bit warm. Why?

What happens when you rub your hands together?

Yes you are right. Your hands will become warm. But why such things happens?

This happens because when we hit the nail with hammer or we rub our hands, the velocity of molecules becomes fast, they start to move faster.

Because of this, the kinetic energy of these molecules increases and thus we feel our hand warmer when they are rubbed together.

In general, the warmer body possess more kinetic energy and colder body possess less kinetic energy.

## How will you define temperature?

Recently, we have seen in the above examples that, if kinetic energy is more, then the body is warm (or hot) and if kinetic energy is less, then the body will be cold.

Thus,

Temperature is a measurement of the average kinetic energy of the molecules in any object.

Temperature can also be defined as a degree of hotness or coldness of an object.

There are many devices used for measuring temperature.

Here I have listed few temperature measuring devices.

1). Liquid in glass thermometer

3). Constant volume gas thermometer and constant pressure gas thermometer

4). Thermocouple

5). Silicon diode

6). Thermistors

7). Bimetallic device

8). Bulb and capillary sensors

9). Resistance temperature detector (RTD)

10). Sealed bellows

The value of temperature is indicated using temperature scales like

## Heat (you will love this concept)

This boy touched the hot stove by mistake and feels the stove very hot.

But this boy is feeling cold on holding the ice in his hand.

Let me explain you the phenomenon which occurs over here and you will surely understand the concept of heat.

When you touch the stove, the thermal energy enters your hand because the stove is warmer than your hand.

But when you touch a piece of ice, then the case is reverse. Your hand is warmer than ice. So thermal energy will pass out of your hand.

What have you observed from these two examples?

• Heat always flows from higher temperature to lower temperature.
• Heat flows due to temperature differences between the two objects.

Now it’s time to frame the definition from these concepts.

Definition of heat:

Heat is defined as a flow of thermal energy due to differences in temperatures.

Heat is simply thermal energy in transit.

Very easy, isn’t it?

Now, let me ask you a simple question.

Why you wear sweater during winter and not during summer?

Let us discuss this with some thermodynamic approach. You feel hot in summer season because the heat enters your body from the surrounding.

While in winter season, what do you think? Something like “Cold” enters to your body?

No, it is not so.

Actually, your body looses heat to the surrounding (because in winter, your body is warmer than surrounding. Remember, heat always flows from higher temperature to lower temperature)

This is the reason, why you feel cold in winter season.

And in order to avoid this heat loss from the body, we have to wear sweater in winter.

In other words, sweater avoids the flow of heat from the body to the surrounding.

Heat is type of energy, so it’s SI unit is same as the unit of energy i.e Joule. It is denoted by “J”. In British Thermal Unit (BTU), the unit of heat is often used as “calorie”. It is denoted by “C”.

Remember that,

1 calorie = 4.18 Joules

So finally I want to summarize the difference between temperature and heat.

## How heat transfer occurs?

In our day to day activities, we see heat flow taking place surrounding us.

Let us understand with some examples.

When hot coffee is poured into a cup, the cup also gets hot after few minutes.

Why?

Why all of this water gets heated just by heating the bottom of the container?

When I use to stay away from my home in summer noons, I feel warm. Why?

All these examples shows the transfer of heat from one point to other.

In the first example, you can see that cup also gets heated when hot coffee is poured into it. This happens because the heat is transferred from the inner side of the cup to the outer side of the cup.

In general we can say that this occurs due to direct contact of cup with the hot coffee. In thermodynamics, this phenomenon is termed as conduction.

Remember the below sentence for conduction.

Conduction: Energy is transferred by direct contact.

Keep reading. Amazing examples are on the way!!

Now, in the second example we see that entire water gets heated up just by heating the bottom of the container.

How this happens?

This occurs because the heat energy is transferred from bottom layer of water to the top layer of water. And this heat transfer takes place due to motion of water molecules.

In thermodynamics, this phenomenon is termed as convection.

Remember the below sentence for convection.

Convection: The energy is transferred due to motion of molecules.

Now, in the third example I told you that we feel warm when we use to stay outside the house in summer noons.

Well, we are not in physical contact with the sun. Even though we feel warm.

This occurs due to electromagnetic waves coming from the sun to the earth. This is termed as radiation heat transfer in thermodynamics.

Remember the below sentence for radiation.

Let me summarize conduction, convection and radiation in single example. This will help you understand the modes of heat transfer in a better way.

## What is Thermal Equilibrium in zeroth law of thermodynamics?

I will keep this simple.

Let’s take an example for better understanding, and later on I will explain you the definition of thermal equilibrium.

Consider one body at higher temperature and other body at lower temperature.

Let say the temperature of hot body is 80 °C and the temperature of cold body is 20 °C.

Now what will happen when we bring both the body in contact with each other?

Did you remember the concept of heat transfer which we discussed earlier in this article?

Yes, the heat transfer will start now when both the bodies will be in contact with each other.

Heat will start to flow from higher temperature body to lower temperature body. (Remember heat always flows from higher temperature to lower temperature)

In other words we can say that, hot body becomes cold and cold body becomes hot. Or else we can say that hot body looses heat while cold body gains heat.

After some time, the temperature of both the bodies will be equal. Let say 50 °C.

After achieving this stage, there will be no heat transfer between the bodies. This stage is called thermal equilibrium.

Thermal equilibrium:

“The condition under which two objects in physical contact with each other exchanges no heat energy is termed as thermal equilibrium. During this condition, the temperature of both the bodies remains same”

Just like thermal equilibrium, let me tell you something about mechanical equilibrium and chemical equilibrium.

Mechanical equilibrium:

“A system is said to be in mechanical equilibrium if the net force acting on the system is zero”

For example, if the pressure force inside the system is same at all the points and does not changes with time, then the system is said to be in mechanical equilibrium.

Chemical equilibrium:

“A system is said to be in chemical equilibrium if it’s chemical composition does not changes with time and no chemical reaction takes place in the system”

Or

“A reaction is said to be in chemical equilibrium if the rate of forward reaction and rate of reverse reaction is equal”

Now what is thermodynamic equilibrium?

Its very simple, it is a combination of all these above mentioned three equilibrium.

Thermodynamic equilibrium:

“Two systems are said to be in thermodynamic equilibrium when they are in thermal, mechanical and chemical equilibrium with each other”

## Phase diagram and triple point

### Phase diagram

Phase?

Diagram?

The diagram showing phase is called phase diagram. Hehehe…..

Just kidding!!!

But the actual meaning is somewhat similar to this.Now, you might have heard about solid, liquid and gas. What are they?

They are the phases of a matter. Right !!!

Now phase diagram is nothing but a graph of pressure vs temperature which shows the phase of matter at given temperature (T) and pressure (P).

Here, I will explain you the phase diagram in brief.

As shown in the image, three phases solid, liquid and gas are shown and they are separated by three lines.

Fusion curve (or melting curve or freezing curve)

AB line indicates that solid phase and liquid phase are in equilibrium. And this line is known as Fusion curve.

I’ll explain you the practical example of solid liquid equilibrium.

The above image shows the ice and water in equilibrium state.

Neither ice is melting nor water is converted to ice. This curve is also known as melting curve or freezing curve.

This is amazing. Right?

Let’s move further.

Vaporization curve (or condensation curve)

AC line indicates that liquid phase and gaseous phase are in equilibrium. And it is known as vaporization curve. (It’s simple, because we know that when water is converted to vapor, we call it as vaporization. And here liquid and gaseous phase are in equilibrium with each other)

Let us see one example.

You can see in the image that water and vapor both are in equilibrium with each other.

Neither water is converted to vapor nor vapor is condensed to water.

Such a phase is represented as a vaporization or condensation curve on the phase diagram.

Sublimation curve (or deposition curve)

OA line indicates that solid and vapor state are in equilibrium with each other. This curve is known as sublimation curve.

But do you know what is sublimation?

Sublimation is nothing but a conversion of solid state directly to the vapor state.

You might have seen that when we ignite the camphor, it gets converted to vapor. It does not melt, but it directly evaporates.

In above image you can see that the dry ice (solid carbon dioxide) which is in solid state, gets directly converted to gaseous state. It does not comes in to liquid state before achieving gaseous state.

Such a phenomenon is called sublimation.

Thermodynamics is Amazing!!!

### Triple point

Have you ever seen any object in solid, liquid and gaseous state at a same time?

In above image, point A indicates that the object is in solid state, liquid state as well as gaseous state and that also at a same time.

OMG!!!

This situation occurs at particular temperature and pressure.

But let us frame the definition of triple point.

Triple point: The temperature and pressure at which all the three phases of matter i.e solid, liquid and gas exist in equilibrium with each other is known as triple point of that matter.

The triple point of water is obtained at the pressure of 4.58 mm Hg and 273.16 K temperature. Triple point of water is used to fix the scale of thermometer.

## Summary (Read this if you are in hurry)

### What is 0th law of thermodynamics?

0th law of thermodynamics definition/statement can be given as;

“If two bodies A and B are in thermal equilibrium with third body C, then body A and B are also in thermal equilibrium with each other.”

### What did you find from the zeroth law of thermodynamics?

From 0th law of thermodynamics we found that there is one physical property which indicates whether the bodies are in thermal equilibrium or not. And that property is Temperature.

### Application/examples of 0th law of thermodynamics

1). To measure human body temperature.

2). To measure atmospheric temperature.

3). Temperature measurement for industrial applications.

4). Temperature measurement in air conditioners.

5). Temperature measurement in refrigerator.

6). In food processing industries.

### What is temperature?

• Temperature is a measurement of the average kinetic energy of the molecules in any object.
• Temperature can also be defined as a degree of hotness or coldness of an object.

### Heat

Definition of heat: Heat is defined as a flow of thermal energy due to differences in temperatures.

Heat is simply thermal energy in transit.

### How heat transfer occurs?

The modes of heat transfer are mentioned below.

Conduction: Energy is transferred by direct contact.

Convection: The energy is transferred due to motion of molecules.

### What is Thermal Equilibrium?

“The condition under which two objects in physical contact with each other exchanges no heat energy is termed as thermal equilibrium. During this condition, the temperature of both the bodies remains same”

### Phase diagram and triple point

Phase diagram: Phase diagram is nothing but a graph of pressure vs temperature which shows the phase of matter at given temperature (T) and pressure (P).

Triple point: The temperature and pressure at which all the three phases of matter i.e solid, liquid and gas exist in equilibrium with each other is known as triple point of that matter.