What is Thermodynamics – Definition, Laws, Applications

What is thermodynamics, the most commonly asked question in chemistry. We will explore thermodynamics, it’s basics, characteristics, etc. along with a lot of examples.

What is thermodynamics?

Let us try to understand, what is thermodynamics? Have you visited any thermal power plants? Or do you have any idea, how does electricity produce?

If we visit any, thermal power plant, you can understand,

  • how does power plant consume fuel?
  • how does fuel convert into electricity?

This conversion of electrical power from the thermal energy is associated with heat, work or energy and the concept of thermodynamics came.

what is thermodynamics heat work energy basics
Fig. 1 What is thermodynamics heat work energy basics

Definition of Thermodynamics

Thermodynamics is defined as the science, which is related to work, heat, and its related properties.

Hence, it is defined as: ‘’thermodynamics is a science deal with heat and work and the properties related to heat and work.’’ – P. K. Nag.  Thermodynamics means,

  • Study of heat
  • Study of work
  • Energy conversions
  • Relation to matters.
  • It states the relationship between heat, work & energy.

Meaning of Thermodynamics

Thermodynamics is a GREEK word and it consists of two different words, THERME and DYNAMICS.

  • THERME means HEAT.
thermodynamics meaning
Fig. 2 Thermodynamics meaning

Hence, from the GREEK words or meaning, it is understood that thermodynamics is related to the heat as well as the power. In a simple way, it is basically,

  • Heat
  • Work
  • Power or energy
  • The relation between heat, work & energy.

As thermodynamics is related to heat & work, we have to study the behavior of matters. This is called the thermodynamics approach.

Thermodynamic Approach

There are two thermodynamics approaches to study the behavior of matters:

  • Macroscopic approach,
  • Microscopic approach,

Macroscopic Approach

In this approach, a certain quantity of matter is considered instead of a molecular level activity. This approach is also known as the classical approach.

  • The pressure is determined by this approach. Like pressure, temperature, volume, etc. are also analyzed or determined.
  • A certain quantity of matters is the main concern.
  • No consideration of the molecular level.
  • No consideration of the structure of matter.
  • No knowledge of individual molecules.
  • A bulk or collection of molecules has been considered as a single matter.
  • Variables are able to measure directly or indirectly.
macroscopic microscopic approach
Fig. 3 Macroscopic Approach and Microscopic Approach

Microscopic Approach

In this approach, the behavior of a system is determined by considering the molecular level. It is also is known as the statistical approach.

  • The matter consists of myriads of molecules.
  • Individual molecules are studied.
  • Individual molecules should be known.
  • Molecular effects can not be sensed by human beings.
  • Each molecule has a specific position, velocity, and energy.
  • Actions are dependent on the nature of matter.
  • Each molecule is considered in the microscopic approach as a single component.
  • No instrument can measure the variable directly or indirectly.

Branches of Thermodynamics

Thermodynamics is a vast subject and it has four branches. These are,

  • Classical Thermodynamics
  • Statistical Thermodynamics
  • Chemical Thermodynamics
  • Equilibrium Thermodynamics

Classical Thermodynamics

  • In the case of classical thermodynamics, the macroscopic approach is adopted.
  • Pressure and temperature, etc. are the main variables which helps to calculate the other properties.
  • Study of heat, work, and energy exchange.
  • Study of the basic level of the behavior of matter.

Statistical Thermodynamics

  • Atomic or molecule levels opt.
  • The macroscopic approach is considered.
  • The behavior of molecules is studied.

Chemical Thermodynamics

  • Study of energy-related to chemical reactions.
  • Energy interaction associated with chemical reactions.
  • Related to heat and works along with the change of states.

Equilibrium Thermodynamics

  • Related to energy transfer with surroundings.
  • Related to mass transfer with surroundings.
  • The study is related to the equilibrium state.
  • No flow between the system and the surrounding in equilibrium.
  • No unbalanced forces will be in the system.

Thermodynamic Terms

There are few terms in thermodynamics, which is required to know to understand the basics of thermodynamics. Thermodynamic terms are,

  • The system, Boundary, Surrounding & Universe
  • Thermodynamic Systems
  • Thermodynamic Process
  • Thermodynamic Properties
  • Thermodynamic state
  • Thermodynamic equilibrium
  • Internal energy
  • Enthalpy
  • Entropy

Let us learn these all, in brief

System, Boundary, Surrounding & Universe


  • The system means the space or the region where the process or the properties are analyzed or measured.


  • The system is bounded by a real or imaginary surface which is known as a boundary.
  • It separates the system and surroundings.
  • The boundary may be fixed or movable, based on the type of system.


  • Anything external to the system is simply known as the surrounding.
  • It is outside of any system.


  • We have learned system and surroundings, now, if we combine these two, both together will become the universe.
  • We can write, System + Surrounding = Universe.
system boundary surrounding universe
Fig. 5 System boundary surrounding & universe

Thermodynamic Systems

If we observe any thermodynamic system, we understand two types of exchange can happen between system and surrounding.

  • Mass exchange
  • Energy exchange

So, a thermodynamic system is divided into three types based on two types of exchanges,

  • Open system
  • Closed system
  • Isolated system.

Open System Open system, as the name suggests, means open and mass or energy can easily transfer between system and surrounding across its boundary.

open system
Fig. 6 Open system

It means,

  • Open to surroundings
  • Mass transfer happens between systems & surroundings.
  • Energy transfer may happen.
  • The mass and energy can be in or out of the system.
  • Mass and energy together can be transferred.
  • It is known as control volume.

Boiling milk is an open container is an open system. Closed System A closed system means, as the name suggests, it is separated from the surrounding.

closed system
Fig. 7 Closed system

It means,

  • It has a physical boundary.
  • No mass can transfer within the boundary.
  • There will not be any insulation associated with the system boundary.
  • Since the boundary is not insulated, energy transfer happens.

Cooking vegetables in a pressure cooker is an example of a closed system. Isolated System When a system is totally isolated from the surrounding, it will be called as isolated system.

isolated system
Fig. 8 Isolated system

In this system,

  • There will not be any interaction between system & surrounding,
  • There will not be any mass transfers.
  • There will not be any energy transfers.

Thermos Flask is an isolated system, which we use in our daily life.

Thermodynamic Process

It is the process when a system undergoes a change from one equilibrium state to another equilibrium state. It means in a system, pressure or volume or energy, etc. can be changed.

There are many types of a thermodynamic process, as follows:

  • Isothermal Process
  • Isobaric Process
  • Reversible Process
  • Irreversible Process
  • Isochoric Process
  • Adiabatic Process
  • Steady-State Process

Isothermal Process

  • ISO came from GREEK word Isos and it means equal.
  • Isothermal means equal thermal or no change in temperature.

Check a very informative VIDEO from Catalysis by Vedantu

Isobaric Process

  • ISO means equal in GREEK.
  • Baric means related to pressure.
  • Isobaric means equal pressure or no change in pressure.

Reversible process

  • A reversible process is defined as the process which can be reversed back to its original process.
  • A reversible process is applicable if the changes are infinitely small.
  • It is an ideal process, as it does not happen in reality.

Irreversible process

  • The irreversible process means the thermodynamic process will not be able to reverse back to its original state or original properties.
  • The natural process, which we see around us, all are irreversible.

Isochoric Process

  • If a thermodynamic system doesn’t undergo any change in volume, it will be called the isochoric process.
  • The system will not be able to work.

Adiabatic Process

  • The process where no heat transfer happens between the system and surrounding is known as the adiabatic process.
  • No heat transfer will be happened into the system or out of the system.

Steady-state process

  • If there is no change in internal energy, a steady-state process can occur.

Thermodynamic Properties

We will learn thermodynamic properties here. There two types of thermodynamic properties,

  • Intensive properties
  • Extensive properties

Intensive properties

  • When the property is not changed with the change of the amount of matter or substance.
  • Intensive property means independent of the amount of matter or size.
  • Remember ‘IN’ means INdependent.
  • Example – temperature, density, conductivity, etc.

Extensive properties

  • Extensive properties depend on the quantity of matter or it’s size.
  • Example – Mass, Volume, Weight, etc.
intensive extensive properties
Fig. 9 Intensive Properties & Extensive Properties

Thermodynamic State

  • The thermodynamic state is defined as a state at which we can study or analyze its thermodynamic properties, its conditions, or other variables at a specific time.
  • At any particular state, all the thermodynamic properties have fixed values.

Thermodynamic Equilibrium

  • We have already learned macroscopic properties, now, if there is no change in any of macroscopic properties, we say, the system is in thermodynamic equilibrium.
  • It is possible if the system is totally isolated from the surroundings.
  • No temperature potential present within the system.
  • No pressure differentials present within the system.
  • No temperature or pressure difference between the system and the surrounding.
  • There will not be any kinds of chemical reactions within the system.

Type of Equilibrium There are different kinds of equilibrium,

  • Thermal equilibrium: No temperature difference within the system.
  • Mechanical equilibrium:  No pressure difference within the system
  • Phase equilibrium:  No change in the mass of each phase.
  • Chemical equilibrium:  No difference in the chemical composition over the period of time.

What is internal energy?

The internal kinetic energy and internal potential energy together form the internal energy of a system.

  • Internal energy means is created due to the motion of molecules in the system.
  • It refers to the microscopic level.
  • The internal kinetic energy in the system comes from the motion of the molecules.
  • Internal kinetic energy is related to the temperature directly. Kinetic energy is increased with the in temperature.
  • Internal potential energy comes from the type of chemical bonds or substance.


We have learned that thermodynamics is related to energy. Now, do you have an idea of how this energy can be measured?

Yes, it is another thermodynamic term, enthalpy which is used to measure the energy of a thermodynamic system.

  • The total heat content of a system is nothing but enthalpy.
  • It is the sum of internal energy and the product of pressure and volume.

It is written as,

  • h = u + pv


  • h : Enthalpy
  • U : Internal energy
  • P : Pressure
  • V : Volume


Entropy is a thermodynamic function to measure the disorderliness of a system.

  • Entropy is used to measure the thermal energy which is unavailable to work due to disorder.
  • Work is produced by the ordered molecular action in any system.
  • The entropy of a gas is more than the solid, as molecules in the gas moves freely with respect to the solid.

Laws of Thermodynamics

There are several rules and processes in thermodynamics, hence, to simplify all the rules or regulations, laws of thermodynamics are introduced. There is a total of four nos. of thermodynamics laws.

  • Zeroth law of thermodynamics
  • First law of thermodynamics
  • Second law of thermodynamics
  • Third law of thermodynamics

Zeroth Law of Thermodynamics

The zeroth law of thermodynamics states that if two thermodynamic systems are in thermal equilibrium with another thermodynamic system, then they all are in thermal equilibrium with each other. Let’s consider, any three systems,

  • System A
  • System B
  • System C
thermal equilibrium zeroth law of thermodynamics
Fig. 11 Thermal equilibrium  – The zeroth law of thermodynamics

Now, if System A and System B are in thermal equilibrium and also system A and system C also in thermal equilibrium, then system A, system B, and system C will be in thermal equilibrium.

First Law of Thermodynamics

This first law is related to the conservation of energy. It states that energy can never be created or destroyed, but it can be changed from one form of energy to another form of energy.

  • To understand, let us take some water in a container. There will not be any change when the water in a normal condition. We will heat the water and we will see after heating, water changed into vapor. Here, water is changed into vapor due to heat input.
  • If we put coal in the furnace of the power plant, then only we get electricity through the process.  Coal has stored thermal energy which is used to get heat energy. So, energy is the same, but it is changed into different forms.

Second Law of Thermodynamics

This law is related to entropy. As per this law, entropy is always increased in an isolated system.

  • It means, there should be an external energy source to transfer heat energy from a lower temperature region to a high-temperature region.
  • For example, if you switched ON the air conditioner, then only it will cool the room. Hence, external power is required to run the air conditioner or to transfer heat from room to outside.
thermodynamics second law
Thermodynamics second law

Third Law of Thermodynamics

Based on this third law of thermodynamics, when the temperature of a state approaches to absolute zero, entropy approaches to a constant value.

  • The entropy of a pure crystal, as per the third law of thermodynamics,  is zero, at absolute zero temperature.
  • The entropy of any system is the measurement of the disorder.

Example of the third law of thermodynamics,

  • Let’s take a piece of ice, and cooled it to absolute zero temperature. So, entropy at that absolute temperature will be zero. It is theoretical and practically it is not possible.
  • Now, increase the temperature of that ice, what will happen? It will start to melt and entropy will increase.
  • Let’s increase the temperature again to a little, you will see the entire ice is converted into water and entropy will be further increased.
  • Further increase in temperature of the water, will be changed into steam. Now, in this case, molecules move faster and entropy will further increase.

Why Thermodynamics is Required?

We have already discussed the basics of thermodynamics, and understand the requirements. Few reasons are captured to explain why thermodynamics is essential. These are,

  • We are always associated with fuels to get energy and thermodynamic helps to select a better efficient system.
  • Minimization of power consumption
  • Better fuel economy
  • Monitoring environment impacts
  • Optimization of environmental impacts
  • Power conservations
  • Finding new resources of energy
  • Analysis of diseases which occurred from the biological systems
  • Research & development of renewable energy
  • Reducing the emission to the environment
  • Diagnosis of extreme human activities

Application of Thermodynamics

The application of thermodynamics is huge. We see dewdrops are on the glass early morning. Do you know is it the application of thermodynamics? Yes! It is. Let’s see a few of the thermodynamics applications to have a better understanding.

  • We can not live without electricity and it is produced in different kinds of power plants using thermodynamic laws.
  • The engine is the heart of various cars, bikes, trucks, airplanes, ships, spacecraft, etc., and is it working based on thermodynamics laws.
  • Fans, or blowers, or compressors, etc. also working on the basis of the law of thermodynamics.
  • In the refrigerators, industrial refrigeration, air conditioners, cold storages, etc. are working on thermodynamics.
  • In extremely cold weather, we use heaters which is another application of thermodynamics.
  • All plants like thermal power plants, renewable energy-based power plants, nuclear power plants, hydroelectric power plants, etc., are working on thermodynamics principles.
  • Solar, tides, wind, water waves, geothermal energies are based on thermodynamics.
  • Heat transfer is a part of thermodynamics, which is the main working principle of evaporators, condensers, coolers, radiators,  etc.
  • In various medical applications, like a thermometer, etc.

Example of Thermodynamics

To understand thermodynamics, we will take the example of a car and see how is it related to Thermodynamics? What do we do in the car?

  • We fill the oil tank,
  • Start the car,
  • Car moves

If the fuel tank does not have fuel or oil, the car will not move.  But the tank has fuel, the car will move. So, what do you think? Why is the car moving only when it has fuel?

thermodynamic examples
Fig. 15 Thermodynamic examples

Let’s try to understand, step by step

  • Energy is stored in the oil or fuel.
  • We all know that engine has its own combustion chamber.
  • Fuel is converted into heat energy in the presence of air in the chamber.
  • This heat energy is then converted into mechanical energy which is transferred to the driving shaft which rotates the wheels.

As the wheel rotates, the car will move. Exhaust gas is released through the silencer from the car. Hence, we get a grip on how thermodynamics is related to energy and its transformations.

Thermodynamics Examples in Daily Life

Whether we are traveling in an airplane or resting in an air-conditioned space or sleeping in a heated room, the application of thermodynamics is everywhere. We have listed a few thermodynamics examples in daily life,

  • Our transportation system.
  • Heating, air conditioning, etc.
  • Medical etc.

Limitations of Thermodynamics

Thermodynamics has some limitations as well:

  • Thermodynamics doesn’t specify the requirement of time i.e., how long it takes.
  • The mechanism of the process is not understood.
  • Thermodynamics is not specified the internal structure of molecules or atoms.

Further Study


We have learned the basics of thermodynamics, its branches, along with a lot of examples. If you have any questions, please feel free to ask.


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