Thermodynamics - definition, branches, laws, equations, formulas and examples (2023)

Thermodynamicsis the study of the concepts and ideas of Heat and Temperature and their interrelationship. Thermodynamics is a branch of physics that explains how thermal energy is transformed into other forms of energy and the importance of thermal energy in matter. The behavior of heat, work, and temperature, along with their relationships to energy and entropy, are governed byfour 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 Introduction to 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 temperatureand its interrelationship with radiation, energy, and the physical characteristics of matter.

Let's break down the term "thermodynamics" into its two components, "thermo" and "dynamics." The term "thermo" refers to heat, while the term "dynamic" refers to a mechanical movement that requires "work." The field of physics that studies the relationship between heat and other types of energy is calledthermodynamics.

Creating a clear limit makes thermodynamics much simpler. The "system" refers to everything that is contained within the boundary, and the "environment" refers to everything that is outside of it. Once the contour diagram has been created, the flow through the system boundaries can be used to describe the motion and energy transfer. The word "universe" is inclusive. In other words, it refers to both the environment and the system.

Branches of Thermodynamics

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

classical thermodynamics

The behavior of matter is examined using a macroscopic perspective in classical thermodynamics. To determine the characteristics and predict the characteristics of the matter that drives the process, people take into account units such as 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 states from quantum mechanics to thermodynamics. classical. 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 bulk microscopic properties of materials that can be observed on a human scale with the individual macroscopic properties of the atom and molecule.

chemical thermodynamics

Chemical thermodynamics is the study of how energy interacts with chemical processes or changes of state according to the laws of thermodynamics. Determining the spontaneity of a certain transition is the main objective 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 an equilibrium condition in which all macroscopic fluxes 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 macroscopically diverse components of the system do not exist in a state of equilibrium.

non-equilibrium thermodynamics

Systems that are not in thermodynamic equilibrium are the focus of the field of thermodynamics known as nonequilibrium thermodynamics. Most systems in nature are not in thermodynamic equilibrium because they are not in steady states and are subject to continuous irregular flows of matter and energy to and from other systems. More general notions than those covered by equilibrium thermodynamics are needed for the thermodynamic study of systems that are not in equilibrium.

Basic Concepts of Thermodynamics

Thermodynamic Systems

A set of an extremely large number of atoms or molecules confined within certain limits so that they have certain values ​​of pressure (P), volume (V), and temperature (T) is called a thermodynamic system.

Anything outside the thermodynamic system in which energy or matter is exchanged is called a neighborhood. Taking into account the interaction between a system and its environment, a system is said to be an open system, if it can exchange energy and matter with its environment, it can 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. system closed:A system is said to be a closed system if it can only exchange energy (not matter) with its surroundings. See figure 1(B).
  3. Isolated system:A system is said to be isolated if it cannot exchange energy or matter with its surroundings. See figure 1(C).

Thermodynamics - definition, branches, laws, equations, formulas and examples (1)

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system and environment

The term "surroundings” refers to everything outside the system that affects its behavior. there is onefronteraseparate the system from its environment. It can be fixed, 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 the matter outside the system and the border is called neighborhood.

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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 place to another. In addition, heat can be transformed into various types of energy.

To work

The work done by or on a system during a process depends not only on the initial and final states of the system, 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 work (W) that is done to or by a system.

internal energy

The kinetic and potential energies of the molecules add to forminternal energy. The internal energy of the system is represented by the letter U. Kinetic energy is the energy possessed by molecules or atoms due to their movement. Two molecules have some potential energy because they are attracted to each other. The total kinetic and potential energy of the atoms or molecules that make up a system is known as the internal energy of the system.

Thermodynamic properties or variables

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

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

Types of thermodynamic variables

  1. Intensive variables:Variables that are independent of the size of the system are called intensive variables. for example. Temperature, pressure and specific mixing capacity.
  2. Extended variables:Variables that depend on the size or mass of the system are called extensive variables. for example. Volume, energy, entropy, heat capacity and enthalpy.

thermal balance

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When two bodies with different temperatures come into contact, energy flows from a body with a higher temperature to a body with a lower temperature. The flow of energy continues from one body to another to reach the same temperature. When both bodies in contact have the same temperature and there is no energy for the body until both bodies are between them, then these bodies are in thermal equilibrium. Thus, it is said that two bodies or systems in contact are in thermal equilibrium if both are at the same temperature.

  • It is said that a thermodynamic system is in thermodynamic equilibrium if its variables such as pressure, volume, temperature, number of particles, etc. don't change over time.
  • Any isolated system is in thermodynamic equilibrium.

Thermodynamic processes

Any process in which thethermodynamic variablesof a thermodynamic change of the system is known asthermodynamic process.

  1. Quasi-static process (quasi-static means quasi-static):A process in which the system deviates only infinitesimally from the equilibrium state is known as a quasistatic process. In this process, the change in pressure, volume, or temperature of the system is very small.
  2. Isothermal process:A process in which the pressure and volume of the system vary at constant temperature is called an isothermal process. In this case, P and V change, but T is constant. that is, dT (temperature change) = 0.
  3. Adiabatic Process:A process in which the pressure, volume, and temperature of the system change, but no heat is exchanged between the system and its surroundings, is called an adiabatic process. In this case, P. V and T change, but Q = 0. The system must be suddenly compressed or expanded so that there is no time for heat exchange between the system and its surroundings. Since these two conditions are not fully satisfied in practice, no process is perfectly adiabatic.
  4. Isochoric process:A thermodynamic process that occurs at constant volume is called an isochoric process. It is also known as the isovolumetric process. In this case, dV = 0.
  5. isobaric process:A thermodynamic process that occurs at constant pressure is called an 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 energy stored in a system is measured by its thermodynamic potentials. Potentials measure how the energy of a system changes from its initial state to its final state. Depending on system limitations, 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 capacity to do work and the capacity to release heat.
  • Gibbs energy (G):It is the ability to perform non-mechanical work.
  • Enthalpy (H):It is the ability to perform non-mechanical work and the ability to emit heat.
  • Helmholtz energy (F):It is the ability to perform mechanical and non-mechanical work.


In a thermodynamic system, energy is measured byenthalpy. Enthalpy is a measure of the total heat content of a system and is equal to the internal energy of the system 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 depends entirely on the temperature, pressure, and composition of the system, not on its history.


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

Laws of Thermodynamics

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

Zero law of thermodynamics

In accordance withZero law of thermodynamics, if two bodies are separately in thermal equilibrium with a third body, then the first two bodies are equally in thermal equilibrium with each other. This indicates that if system A is in thermal equilibrium with system B and system C is also in thermal equilibrium with system B, then both systems A and C are in thermal equilibrium.

First Law of Thermodynamics

Energy is neither generated nor 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 covered byFirst Law of Thermodynamics. Energy is neither generated nor destroyed, according to the first law of thermodynamics, but it can be converted from one form to another. According to this law, part of the heat supplied to the system is used to change the internal energy, while the rest is used to do work.

Thermodynamics - definition, branches, laws, equations, formulas and examples (4)

Mathematically, it can be expressed as


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  • The heat given up or lost is denoted by ΔQ.
  • The change in internal energy is denoted by ΔU.
  • W represents the work done.

The above equation can alternatively be written as follows:


As a result of the above equation, we can deduce that the quantity (ΔQ – W) is not affected by the path taken to change state. Also, when heat is applied to a system, the internal energy tends to increase and vice versa.

Second law of thermodynamics

In an isolated system, theSecond law of thermodynamicsstates that entropy always increases. Any isolated system progresses spontaneously towards thermal equilibrium or the state of maximum entropy. The entropy of the universe is always increasing and never decreasing.

Thermodynamics - definition, branches, laws, equations, formulas and examples (5)

Third law of thermodynamics

The third law of thermodynamics states that as 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 minimal energy, then this statement is true.

Thermodynamics - definition, branches, laws, equations, formulas and examples (6)

Examples of thermodynamics in daily life

  • Melt ice cubes:Drinks with ice cubes get colder as the heat of the drink is absorbed. If we stop drinking it, it will eventually return to room temperature, 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 room full of people, everyone starts to sweat. By transmitting body heat to perspiration, the body begins to cool down. The sweat evaporates, heating up the room. Again, this is the result of applying the first and second laws of thermodynamics. Note that heat is not lost but rather moved around until equilibrium with the least amount of entropy is reached.
  • Activate a light switch:Different types of power plants, including thermal and nuclear, are studied using thermodynamics.

Thermodynamics FAQ

Question 1: Define isolated system.


A system is said to be isolated if it cannot exchange energy or matter with its surroundings.

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Question 2: What is the meaning of the Laws of Thermodynamics?


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

Question 3: Can energy be lost or destroyed?


No, energy cannot be destroyed and created. But it can only be transferred from one form of energy to another.

Question 4: How is the negative work?


An example of negative work is when you push an object against the ground, the work done by kinetic friction is negative.

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


The human body actually follows the law of thermodynamics. You start to feel hot and sweaty when you are in a crowded room. This is how the body cools down.

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