Thermodynamics: Definitions, Laws, Types, Importance and Applications

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Let’s start with the definition first!

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Definition of thermodynamics-

Thermodynamics is an important branch of science that deals with energy transfer and its effect on the state of a system.

It can also be defined as:

“Thermodynamics is the study of the relationship between heat and work. It also deals with energy interaction with the physical system”.

The field of science, that deals with the Energies possessed by gases and vapours, is known as Thermodynamics”.

Laws of thermodynamics-

Thermodynamics basically deals with four laws ie-

Zeroth low, first low, second law and third law of thermodynamics.

  1. Zeroth law of thermodynamics
  2. First low of thermodynamics
  3. Second Law of thermodynamics
  4. Third law of thermodynamics

1. Zeroth Law of Thermodynamics-

The Zeroth law deals with thermal equilibrium and establishes a concept of temperature. The zeroth law of thermodynamics provides the basis for the measurement of the temperature of a system.

It says that If two thermodynamic systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other”.

If we look at it like this- If A=C and B=C then A must be equal to B.

Let me consider two systems B and C, which are in thermal equilibrium with each other. Accordingly, no change occurs in their physical properties when they are brought into good contact.

Likewise, let system C be separately equal in temperature to another system A so that no change occurs when they communicate with each other.

It is then observed fact that systems A and B will not undergo any physical changes when brought into contact. Thus systems A and B are taken to be in thermal equilibrium.

zeroth law of thermodynamics diagram

Examples of the Zeroth Law of Thermodynamics-

Here are some examples of this zeroth law-

  • Measuring the temperature of the system or body with temperature-measuring equipment
  • Cold water and hot water after some time at the same place
  • Hot coffee being cold in time

2. First Law of Thermodynamics-

The first law deals with the concept of internal energy. It says that energy neither be created nor be destroyed, Only Energy changes its form from one to another and it is also known as the law of conservation of energy.

Examples of the First Law of Thermodynamics-

Here are some examples of this first law-

  • Fuel Energy(Chemical Energy) changes into Kinetic Energy to run vehicles.
  • Solar energy changes into Electricity Energy
  • Bulbs change Electricity Energy into thermal energy to light up

3. Second Law of Thermodynamics-

The second law deals with the limit of converting heat into work and introduces the principle of increase in Entropy. The entropy of an isolated system always increases without putting any external effort to lower the entropy.

The equilibrium state of matter also comes under the second law where the lower-temperature matter always tries to gain temperature from matter with higher temperature.

Examples of the Second Law of Thermodynamics-

Here are some examples of this second law-

  • Melting of an ice cube
  • Being cooler of your coffee mug with time
  • The messy room looks messier with time

4. Third Law of Thermodynamics-

The third law deals with absolute zero Entropy. It says a Decrease in temperature happens due to decreasing kinetic energy and when kinetic energy gets zero, the temperature of the system also gets zero and hence Entropy is zero.

So, Absolute zero entropy is not possible in the real world due to the equilibrium state of matter states in the second law of thermodynamics.

Examples of the Third Law of Thermodynamics-

Here are some examples of this third law-

  • Calculating absolute entropy
  • Adiabatic cooling
  • predicting the temperature changes

Types of thermodynamic systems-

There are three types of Thermodynamic systems:

  1. Closed system
  2. Open system
  3. Isolated System

1. Closed System-

This is a system of fixed mass and identity whose boundaries are determined by the space of the matter (working substance) occupied in it.

In this system, only energy can transfer from the system boundary. The mass of the system remains constant.

closed system of thermodynamics diagram

In a closed system, in which a cylinder is fitted with a movable piston, If heat is supplied to the system from some external source, the temperature of the gas or vapour will be increased and further piston will rise and the boundary of the system moves.

In other words, the heat and work energy cross the boundary of the system during the process but the mass will not.

Fig. cylinder piston arrangement in a closed system-

cylinder piston arrangement in closed system

Examples of the Closed System of Thermodynamics-

Here are some examples of a closed system-

  • pressure cooker]
  • Radiator
  • Refrigerator
  • Motor-car
  • Battery

2. Open System-

A thermodynamic system in which the mass of the working system and energies (heat or work) both flow through the system boundary, is known as an open system.

The mass within the system does not necessarily remain constant, it may change depending on mass inflow and mass outflow. Most Engineering systems are open systems. 

open system in thermodynamics

In an air compressor system, the mass (low-pressure air) enters the system (air compressor) and crosses the system boundary in the form of mass (high-pressure air).

In this system work (energy) from driving, the shaft enters the system and crosses the boundary in the form of heat (Energy).

air compressor in open thermodynamics system

Examples of the Open System of Thermodynamics-

Here are some examples of an open system-

  • Gas turbine
  • Steam turbine
  • boilers
  • Steam engines
  • IC Engine

3. Isolated System-

A system which is completely uninfluenced by the surroundings is called an isolated system.

In other words. “In an isolated system, neither energy can transfer from system boundary nor mass can transfer”.

Examples of the Isolated System of Thermodynamics-

The perfect example of an Isolated system is a Thermos flask in which the walls of the flask are made up of heat-resistant material.

Concept of thermodynamic system and surroundings-

“The thermodynamic system may be defined as a definite area or a space, where some thermodynamic process is taking place.”

Another definition of a thermodynamic system is:

“A thermodynamic system represents a prescribed and identifiable (fixed) quantity of matter under consideration to analyse a problem, to study the changes in its properties due to exchange of energy in the form of heat and work.”

A little observation will show that a thermodynamic system has its boundaries and anything outside the boundaries is called its surroundings


In other words, “A line which differentiates system and surroundings is known as Boundary” and “The region outside the system boundary is known as the surrounding“. System and surroundings collectively are known as the “universe“.

thermodynamics system

Importance of thermodynamics-

The Laws, Principles and Concepts of thermodynamics are important and indispensable tools in the innovation, design, improvement and development of engineering processes, equipment and devices which deal with the effective utilization of Energy.

Applications of Thermodynamics-

Some important applications of thermodynamics in the field of Energy technology are:

  • Power-producing devices eg., I.C. engines and gas turbines, steam power plants and nuclear
  • power plants
  • Power-consuming devices eg, fans, blowers and compressors, refrigeration and air-conditioning
  • Direct energy conversion devices
  • Chemical process plants.
  • A large number of processes in various fields such as agriculture, textiles, dairy, drugs and the pharmaceutical industry are also governed by thermodynamic principles.


Wrapping Up-

This is it for Thermodynamics. I hope you gained the information, you are looking for and further satisfied your curiosity.

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Till then, take care of yourself and your loved ones. Peace !!

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