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Basics Concepts of Thermodynamics

  • Last Updated : 29 Dec, 2021

Thermodynamics is concerned with the ideas of heat and temperature, as well as the exchange of heat and other forms of energy. The branch of science that is known as thermodynamics is related to the study of various kinds of energy and its interconversion. The behaviour of these quantities is governed by three main laws of thermodynamics, which provide a quantitative description. The word thermodynamics was coined by William Thomson in 1749. The following section dives into some of the most commonly used thermodynamic terminologies.

What is Thermodynamics?

Thermodynamics is a field of science that investigates the links between heat, work, and temperature, as well as their interactions with energy, radiation, and the physical properties of matter. Thermodynamics, in its broadest sense, is associated with the transfer of energy from one location to another and from one form to another. 

Heat is a type of energy that corresponds to a specific quantity of mechanical work, which is the key concept. It discusses how thermal energy is transformed to or from other forms of energy, as well as how this process affects matter. The energy derived from heat is known as thermal energy. 

The movement of microscopic particles within an object generates heat, and the faster these particles move, the more heat is produced. Thermodynamics is unconcerned about the rate at which these energy transformations occur. It is based on the change occurring in the initial and final states. It’s also worth noting that thermodynamics is a macroscopic field of study. This means it is concerned with the bulk system rather than the molecular structure of matter.

Basic Concepts of Thermodynamics

Thermodynamics has its own set of terms related to it. A thorough comprehension of the fundamental ideas ensures a deep understanding of the numerous topics covered in thermodynamics, avoiding any misunderstandings. Thermodynamic concepts are covered below.


The system refers to a specific part of the universe that is being observed. A thermodynamic system is a part of matter with a defined boundary on which we concentrate our attention. The system boundary can be real or imaginary, and it can be fixed or movable. 

Physical qualities and chemical compositions are said to be homogeneous if they are consistent throughout the system. A system, on the other hand, is considered to be heterogeneous if it is made up of parts with varied physical and chemical properties. Depending on how matter and energy travel in and out of a system, it can be divided into three categories. There are three different types of systems.

  • Open System – An open system is one that can exchange matter as well as energy with its surroundings. As a result, mass and energy can be transmitted between the system and its surroundings in an open system. Since it gains and loses matter as well as energy, hot coffee in an open flask is an example of an open system. An open system is a steam turbine, for example.
  • Closed System – A closed system exchanges energy but does not exchange matter with its surroundings. The transmission of energy takes place across the closed system’s boundary, but the transfer of mass does not. Closed systems include refrigerators and gas compression in piston-cylinder assemblies. Coffee in a stainless flask is also an example of a closed system because energy, but not matter, can flow through the stell walls.
  • Isolated System – An isolated system is one that is unable to exchange matter or energy with its surroundings. There is no such thing as a system that is completely isolated. An isolated system, on the other hand, is one that is entirely sealed to prevent matter intake or outflow and is thermally insulated to prevent heat flow. The universe is believed to be self-contained. Since it cannot gain or lose energy or matter, hot coffee in a corked thermos flask is an example of an isolated system.


The remaining portion of the universe that is not a part of the system is known as the surrounding. The term “surrounding” refers to everything outside the system that has a direct impact on the system’s behaviour. In other words, anything outside of the system is included in the surroundings. Together, the system and its surroundings make up the universe. 

The alterations in the system, on the other hand, have no effect on the entire universe. As a result, the surroundings are that portion of the remaining universe that can interact with the system for all practical purposes. In general, the surroundings are defined as an area of space in its close surroundings.

Different Branches of Thermodynamics

The following are the four branches of thermodynamics.

  1. Classical Thermodynamics– The behaviour of matter is studied using a macroscopic approach in classical thermodynamics. Individuals consider units such as temperature and pressure, which aids in the calculation of other properties and the prediction of the characteristics of the matter undergoing the process.
  2. Statistical Thermodynamics– Every molecule is in the spotlight in statistical thermodynamics, which means that the properties of each molecule and how they interact are taken into account to characterise the behaviour of a group of molecules.
  3. Chemical Thermodynamics– The study of how work and heat interact in chemical reactions and state transitions is known as chemical thermodynamics.
  4. Equilibrium Thermodynamics– Equilibrium thermodynamics is the study of energy and matter transitions as they approach equilibrium.

Thermodynamic Properties

Thermodynamics is concerned with massive chemical entities such as atoms or molecules. The macroscopic properties of a system are those that originate from the bulk behaviour of matter. Thermodynamic properties are defined as system characteristics that can be used to specify the system’s state. Thermodynamic properties are classified into two categories, as shown below.

  1. Extensive property– Extensive properties are system properties whose value is dependent on the amount or size of material present in the system. The value of extensive characteristics is dependent on the system’s mass. Internal energy, entropy, free energy, and enthalpy are only a few of the many properties.
  2. Intensive property- Intensive properties are system properties whose value is independent of the amount or size of substance present in the system. The properties of an intensive substance are those that are independent of the amount of substance present. Vapour pressure, pressure, viscosity, surface tension, and temperature are only a few of the many intensive properties.

Thermodynamic Equilibrium

All properties of a system have fixed values at any given state. As a result, even if the value of one property changes, the system’s state changes. While a system is in equilibrium, the value of its properties does not change when it is isolated from its surroundings.

  • Thermal equilibrium– A system is said to be in thermal equilibrium when the temperature is the same throughout the entire system.
  • Mechanical equilibrium– A system is said to be in mechanical equilibrium when there is no change in pressure at any point in the system.
  • Chemical equilibrium– A chemical equilibrium is defined as a system in which the chemical composition of a system does not change over time.
  • Phase equilibrium– When the mass of each phase of a two-phase system reaches an equilibrium level, it is called phase equilibrium.

If a thermodynamic system is in chemical equilibrium, mechanical equilibrium, and thermal equilibrium,  and the relevant parameters cease to vary with time, then it is said to be in thermodynamic equilibrium.

Thermodynamic Process

A process can alter the state of a thermodynamic system. A process, in other words, specifies the path or procedure by which a system transitions from one state to another. The process may be accompanied by a material and energy exchange between the system and the surrounding. 

When there is an energetic shift within a system that is related to changes in pressure, volume, and internal energy, it is called a thermodynamic process.

Some typical forms of thermodynamic processes have their own set of characteristics, which are listed below.

  1. Isothermal process- A process in which there is no change in temperature, i.e., the system’s temperature remains constant. Heat is either supplied to or removed from the system in such a system.
  2. Adiabatic process– When a system does not exchange heat with its surroundings, no heat leaves or enters the system. There is no heat transmission into or out of the system during this process. The temperature of the system is constantly changing in such a process. The systems in which such activities take place are thermally isolated from the rest of the surroundings.
  3. Isobaric process– A process in which the system’s pressure remains constant, i.e. no pressure change occurs.
  4. Isochoric process– A process in which the volume of the system remains constant, i.e. there is no change in volume and no work is done by the system.
  5. Reversible process- A reversible process is one in which the direction of flow can be changed at any point in the process by making a slight change in a variable such as a temperature or pressure. Throughout this process, the system maintains a virtual state of equilibrium with the surroundings at all times.
  6. Irreversible process- A process that cannot be reversed is known as irreversible. The opposing force is not the same as the driving force. Natural processes are irreversible.
  7. Cyclic process- A cyclic process is one in which a system goes through a sequence of modifications before returning to its initial condition.

Laws of Thermodynamics

Thermodynamic laws specify the essential physical qualities that characterise thermodynamic systems in thermal equilibrium, such as energy, temperature, and entropy. The laws of thermodynamics describe how these quantities react in various situations.

Thermodynamics is governed by four laws, which are described below.

  • 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 utilised to change the internal energy, while the remaining is used to perform work.

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.
  • 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.

Sample Questions

Question 1: Is a cup of tea on a table a system that is open, closed, or isolated?


Since it gains and loses matter as well as energy, a cup of tea on a table is an open system.

Question 2: What are the differences between spontaneous and non-spontaneous processes?


A spontaneous process is irreversible and can only be reversed by an outside force.

Non-spontaneous processes are those that do not occur on their own under specified conditions and only occur when an external force is continuously provided.

Question 3: What role does thermodynamics play in our daily life?


Thermodynamics is a particularly essential branch of physics and chemistry since it deals with the study of energy and its conversion, as well as the various forms and abilities of this energy to accomplish work. It assists us in understanding the heating and cooling systems in our homes and buildings in our daily lives. It also aids in the investigation of the vehicle’s engines. The rules of thermodynamics describe the process by which energy is turned into heat, which is subsequently transported and converted into productive work.

Question 4: Which one of the properties between pressure and force is an intensive property?


Intensive properties are system properties whose value is independent of the amount or size of substance present in the system. Since pressure is the force exerted per unit area, it is an intense property.

Question 5: Which one of the properties between heat capacity and specific heat is extensive and which is intensive?


Heat capacity is a broad property that is determined by the mass of the substance. However, since specific heat is a heat capacity per unit mass, it is independent of the amount of substance. As a result, specific heat is an intensive property.

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