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Thermodynamics is the study of the concepts and ideas of Heat and Temperature and their interrelation. Thermodynamics is a branch of Physics that explains how thermal energy is changed to other forms of energy and the significance of thermal energy in matter. The behavior of heat, work, and temperature, along with their relations to energy and entropy are governed by the Four Laws of Thermodynamics. The four laws of Thermodynamics are Zeroth’s Law of Thermodynamics, the First Law of Thermodynamics, the Second Law of Thermodynamics, and the Third Law of Thermodynamics. This article covers the Introduction of Thermodynamics, Branches of Thermodynamics, Basic Concepts of Thermodynamics, and Examples of Thermodynamics in Daily Life.

What is Thermodynamics?

In Physics, thermodynamics is a branch that deals with the concept of energy, heat and temperature and their interrelation with radiation, energy and physical characteristics of matter.

Let’s split the term “thermodynamics” into its two components, “thermo” and “dynamics.” The term “thermo” refers to heat, whereas the term “dynamics” refers to a mechanical motion that requires “work.” The field of physics that studies the relationship between heat and other types of energy is called thermodynamics.

Creating a clear boundary makes thermodynamics much simpler. The “system” refers to everything that is contained within the boundary, and the “surroundings” refers to all that is outside of it. Once the boundary diagram has been created, the flow across system boundaries can be used to describe the movement and transfer of energy. The word “universe” is inclusive. In other words, it refers to both the surroundings and the system.

Branches of Thermodynamics

The study of Thermodynamics is classified into several branches listed below:

Classical Thermodynamics

The behavior of matter is examined using a macroscopic perspective in classical thermodynamics. In order to determine the characteristics and predict the characteristics of the matter conducting the process, individuals take into account units like temperature and pressure.

Statistical Thermodynamics

The development of atomic and molecular theories in the late 19th and early 20th centuries gave rise to statistical mechanics, also known as statistical thermodynamics, which added an interpretation of the microscopic interactions between individual particles or quantum-mechanical states to classical thermodynamics. This field explains classical thermodynamics as a natural consequence of statistics, classical mechanics, and quantum theory at the microscopic level. It does this by connecting the microscopic, bulk properties of materials that can be observed on the human scale to the macroscopic, individual atom, and molecule properties.

Chemical Thermodynamics

Chemical thermodynamics is the study of how energy interacts with chemical processes or state changes in accordance with the laws of thermodynamics. Determining the spontaneity of a certain transition is the main goal of chemical thermodynamics.

Equilibrium Thermodynamics

Equilibrium thermodynamics is the study of matter and energy transfers in systems or substances that can be moved from one state of thermodynamic equilibrium to another by agents in their environment. The phrase “thermodynamic equilibrium” refers to a condition of equilibrium in which all macroscopic flows are zero. In the case of the most basic systems or bodies, this means that their intensive properties are uniform and that their pressures are perpendicular to their boundaries. Unbalanced potentials or driving forces between the system’s macroscopically diverse components do not exist in an equilibrium state.

Non-equilibrium Thermodynamics

Systems that are not in thermodynamic equilibrium are the focus of the field of thermodynamics known as non-equilibrium thermodynamics. The majority of systems in nature are not in thermodynamic equilibrium because they are not in stationary states and are subject to fluxes of matter and energy to and from other systems on a continual and irregular basis. More general notions than those covered by equilibrium thermodynamics are needed for the thermodynamic study of non-equilibrium systems.

Basic Concepts of Thermodynamics

Thermodynamic Systems

A collection of an extremely large number of atoms or molecules confined within certain boundaries such that it has certain values of pressure (P), volume (V) and temperature (T) is called a thermodynamic system. 

Anything outside the thermodynamic system to which energy or matter is exchanged is called its surroundings. Taking into consideration the interaction between a system and its surroundings, a system said to be an open system if it can exchange both energy and matter with its surroundings may be divided into three classes-

  1. Open system: A system is said to be an open system if it can exchange energy and matter with its surroundings. See figure 1(A).
  2. Closed system: A system is said to be a closed system if it can exchange only energy (not matter) with its surroundings. See figure 1(B).
  3. Isolated system:  A system is said to be isolated if it can neither exchange energy nor matter with its surroundings. See figure 1(C).
Types of Thermodynamic Systems


System and Surroundings

The term “surroundings” refers to everything outside the system that affects how it behaves. There is a boundary separating the system from its surroundings. It could be stationary, mobile, or fictitious. It won’t take up any space in terms of mass or volume. For example, consider a closed beaker with liquid inside as shown below. The liquid inside the beaker is the system, while the outline of the beaker represents the boundary of the system. And matter outside the system and boundary is called its surroundings.

Thermodynamics Systems



Heat is energy that is transmitted between objects or systems as a result of a temperature difference. Heat is conserved energy, which means it cannot be created or destroyed. However, it can be moved from one location to another. Additionally, heat can be transformed into and out of various types of energy.


The work done by a system or on a system during a process depends not only on the system’s starting and final states but also on the path chosen for the process. When a force acting on a system moves the body in its own direction, work has been done. Force and displacement combine to create the work (W) that is done to or by a system.

Internal Energy

The kinetic and potential energies of the molecules are added up to form internal energy. The system’s internal energy is represented by the letter U. Kinetic energy is the energy that molecules or atoms possess due to their motion. Two molecules have some potential energy because they are attracted to one another. The total kinetic and potential energy of the atoms or molecules that make up a system is what is known as the system’s internal energy.

Thermodynamic Properties or Variables 

A thermodynamic system can be described by specifying its pressure, volume, temperature, internal energy, enthalpy, and the number of moles. These parameters or variables are called thermodynamic variables. 

Thus, the variables which are required to specify the state of the thermodynamic system are called thermodynamic variables. Entropy is a measurement of energy present in a system or process but is not available to do work. It is also defined as the measure of disorder in the system. Enthalpy is the measurement of the total energy of a thermodynamic system.

Types of Thermodynamic Variables

  1. Intensive variables: The variables which are independent of the size of the system are called intensive variables. e.g. Temperature, pressure, and specific beat capacity. 
  2. Extensive variables: The variables which depend on the size or mass of the system are called extensive variables. e.g. Volume, energy, entropy, heat capacity, and enthalpy.

Thermal Equilibrium 

Thermal Equilibrium


When two bodies having different temperatures are placed in contact, then the energy flows from a body at a higher temperature to a body at a lower temperature. The flow of energy continues from one body to another to attain the same temperature. When both the bodies in contact have the same temperature and there is no energy for the body till both bodies are between them, then these bodies are in thermal equilibrium. Thus, two bodies or systems in contact are said to be in thermal equilibrium if both are at the same temperature. 

  • A thermodynamic system is said to be in thermodynamic equilibrium if its variables like pressure, volume, temperature, number of particles, etc. do not change with time. 
  • Any isolated system is in thermodynamic equilibrium.

Thermodynamic Processes

Any process in which the thermodynamic variables of a thermodynamic system change is known as the thermodynamic process.

  1. Quasi-Static Process (Quasi-Static means nearly static): A process in which the system departs only infinitesimally from the equilibrium state is known as a quasi-static process In this process, the change in pressure or change in volume or change in temperature of the system is very very small.
  2. Isothermal Process: A process in which the pressure and volume of the system change at constant temperature are called the isothermal process. In this case, P and V change but T is constant. i.e. dT (change in temperature) = 0. 
  3. Adiabatic Process: A process in which pressure, volume, and temperature of the system change, but there is no exchange of heat between the and its surroundings is called the adiabatic process. In this case, P. V and T change but Q = 0. The system should be compressed or allowed to expand suddenly so that there is no time for the exchange of heat between the system and its surroundings. Since these two conditions are not fully realized in practice, so no process is perfectly adiabatic.
  4. Isochoric Process:  A thermodynamic process that takes place at constant volume is called the isochoric process. It is also known as the isovolumic process. In this case, dV = 0.
  5. Isobaric process: A thermodynamic process that takes place at constant pressure is called the isobaric process. In this case, dP = 0.
  6. Cyclic process: A cyclic process consists of a series of changes that return the system to its initial state.

Thermodynamic Potentials

The stored energy in a system is measured by its thermodynamic potentials. Potentials measure how a system’s energy transforms from its initial state to its final one. Depending on the constraints of the system, such as temperature and pressure, different potentials are used.

Different forms of thermodynamic potentials are mentioned below:

  • Internal energy (U): It is equal to the sum of the ability to do work and the ability to release heat.
  • Gibbs energy (G): It is the ability to do non-mechanical work.
  • Enthalpy (H): It is the ability to do non-mechanical work and the ability to emit heat.
  • Helmholtz energy (F): It is the ability to do both mechanical and non-mechanical work.


In a thermodynamic system, energy is measured by enthalpy. Enthalpy is a measure of a system’s total heat content and is equal to the system’s internal energy plus the sum of its volume and pressure. Enthalpy is a property or state function that resembles energy; it has the same dimensions as energy and is therefore measured in joules or ergs. The value of enthalpy is entirely dependent on the temperature, pressure, and composition of the system, not on its history.


Entropy is the measurement of the amount of thermal energy per unit of temperature in a system that cannot be used for useful work. Entropy is a measure of a system’s molecular disorder or randomness since work is produced by ordered molecular motion. Entropy theory offers a deep understanding of the direction of spontaneous change for many common events.

Laws of Thermodynamics

Thermodynamic systems at thermal equilibrium are characterized by fundamental physical constants like energy, temperature, and entropy, which are defined by thermodynamic laws. These thermodynamic principles describe how these quantities act in different situations.

Zeroth Law of Thermodynamics

According to the Zeroth Law of Thermodynamics, if two bodies are separately in thermal equilibrium with a third body, then the first two bodies are likewise in thermal equilibrium with each other. This indicates that if system A is in thermal equilibrium with system B, and system C is likewise in thermal equilibrium with system B, then both systems A and C are in thermal equilibrium.

First Law of Thermodynamics

Energy cannot be generated or destroyed, according to the first law of thermodynamics, but it can be converted from one form to another. Heat, internal energy, and work are all addressed by the first law of thermodynamics. Energy cannot be generated or destroyed, according to the first law of thermodynamics, but it can be converted from one form to another. According to this law, some of the heat provided to the system is utilized to change the internal energy, while the remaining is used to perform work.

First Law of Thermodynamics


Mathematically, it may be expressed as



  • The heat given or lost is denoted by ΔQ.
  • The change in internal energy is denoted by ΔU.
  • W stands for work done.

The above equation can alternatively be written as follows:


As a result of the above equation, we may deduce that the quantity (ΔQ – W) is unaffected by the path taken to change the state. Furthermore, when heat is applied to a system, internal energy tends to rise, and vice versa.

Second Law of Thermodynamics 

In an isolated system, the second law of thermodynamics asserts that entropy always increases. Any isolated system progresses spontaneously toward thermal equilibrium or the state of maximum entropy. The universe’s entropy is always increasing and never decreasing.

Second Law of Thermodynamics


Third Law of Thermodynamics

The third law of thermodynamics states that when the temperature approaches absolute zero, the entropy of a system approaches a constant value. At absolute zero temperature, the entropy of a pure crystalline solid is zero. If the perfect crystal has only one state with minimum energy, this assertion holds true.

Third Law of Thermodynamics


Thermodynamics Examples in Daily Life

  • Melting of Ice Cubes: Drinks with ice cubes become cooler as the heat from the drink is absorbed. If we neglect to drink it, it will eventually warm back up to room temperature by absorbing heat from the environment. The first and second laws of thermodynamics govern how all of this works.
  • Sweating in a Crowded Room: In a crowded room, everyone begins to sweat. By transmitting body heat to the sweating, the body begins to cool down. Sweat evaporates, heating the space. Again, this occurs as a result of the application of the first and second laws of thermodynamics. Keep in mind that heat is not lost but rather moved until equilibrium is reached with the least amount of entropy.
  • Flipping a Light Switch: Different kinds of power plants, including thermal and nuclear ones, are studied using thermodynamics.

FAQs on Thermodynamics

Question 1: Define Isolated system.


A system is said to be isolated if it can neither exchange energy nor matter with its surroundings.

Question 2: What is the significance of Thermodynamics Laws?


Temperature, energy, and entropy, which describe the physical quantities that define thermodynamic systems at thermal equilibrium, are defined by the laws of thermodynamics.

Question 3: Can energy be lost or destroyed?


No, the energy can not be destroyed and created. But it can only be transferred from one form of energy to other.

Question 4: What does negative work look like?


An example of negative workis when you’re pushing an object along the floor, the work done by Kinetic Friction is negative.

Question 5: Do the laws of thermodynamics apply to the human body?


The human body really does follow the law of thermodynamics. You start to feel warm and begin to sweat when you are in a crowded room with other people. This is how the body cools itself.

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Last Updated : 24 Aug, 2022
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