In physics, thermodynamics deals with temperature, heat and their relation to energy, radiation, work, and properties of matter. The energy can be of any forms such as electrical, mechanical, or chemical energy. William Thomson coined the term thermodynamics in 1749. It is derived from two Greek words “thermes” meaning heat, and “dynamikos” meaning powerful.

When we say the word dynamic we think of motion or movement and energy. Thus, the term thermodynamics means heat movement or heat flow.


What is Thermodynamics?

Thermodynamics is the branch of physics which is concerned with the relationship between other forms of energy and heat. To be specific, it explains how thermal energy is converted to or from other forms of energy and how matter is affected by this process. Thermal energy is the energy that comes from heat. This heat is generated by the movement of tiny particles within an object. The faster these particles move, the more heat is generated.

Thermodynamics Timeline

Thermodynamics has many sections under it and is considered as a broad subject because it deals with topics that exist all around us and thus classification becomes necessary.

Thermodynamics Timeline

  • Classical Thermodynamics – In this section, the behaviour of matter is analyzed with a macroscopic approach. Units such as temperature and pressure are taken into consideration which helps the individuals to calculate other properties and to predict the characteristics of the matter that is undergoing the process.
  • Statistical Thermodynamics – In this section, every molecule is under the spotlight i.e. the properties of each and every molecule and ways in which they interact are taken into consideration to characterize the behaviour of a group of molecules.
  • Pure Component Thermodynamics – As the name itself states, this section tries to describe the behaviour of a system that has an unadulterated or pure constituent.
  • Solution Thermodynamics – This section attempts to describe the behaviour of a system that contains more than one chemical in the mixture.

Here, we will focus on the Classical part of the thermodynamics in our curriculum.

Laws of Thermodynamics

The laws of thermodynamics define the fundamental physical quantities like energy, temperature and entropy that characterise thermodynamic systems at thermal equilibrium. The laws represent how these quantities behave under various circumstances. The four laws of thermodynamics are given below:

To learn in details about all the laws of thermodynamics visit the links given below.

Zeroth Law of Thermodynamics

The Zeroth Law is the basis for the measurement of temperature. It states that “two bodies which are in thermal equilibrium with a third body are in thermal equilibrium with each other.”

Zeroth Law Of Thermodynamics Examples

As an example of the zeroth law of thermodynamics,

  1. consider two cups A and B with boiling water.
  2. When a thermometer is placed in cup A, it gets warmed up by the water until it reads 100°C.
  3. When it read 100°C, we say that the thermometer is in equilibrium with cup A.
  4. Now when we move the thermometer to cup B to read the temperature, it continues to read 100°C.
  5. The thermometer is also in equilibrium with cup B.
  6. From keeping in mind the zeroth law of thermodynamics, we can conclude that cup A and cup B are in equilibrium with each other.

The zeroth law of thermodynamics enables us to use thermometers to compare the temperature of any two objects that we like.

First Law of Thermodynamics

The first law of thermodynamics which is also known as the conservation of energy principle states that “energy can neither be created nor destroyed, but it can be changed from one form to another.” This law may seem abstract but if we look at a few examples of the first law of thermodynamics, we will get a clearer idea.

First Law Of Thermodynamics Examples

Following are a few examples:

  • Fans convert electrical energy to mechanical energy.
  • Plants convert the radiant energy of sunlight to chemical energy through photosynthesis. We eat plants and convert the chemical energy into kinetic energy while we swim, walk, breathe and when we scroll through this page.

Second Law of Thermodynamics

The second law of thermodynamics states that “Energy in the form of heat only flows from regions of higher temperature to that of lower temperature”. Many individuals take this statement lightly and for granted, but it has an extensive impact and consequence. This is why it costs money to run an air conditioner. The human body obeys the second law of thermodynamics too.

Second Law Of Thermodynamics Examples

One of the examples of the second law of thermodynamics can be sweating in a crowded room. Assume yourself to be in a small room full of people. You are very likely to feel warm and start sweating. Sweating is a mechanism the human body uses to cool itself. Here, the heat from your body is transferred to sweat. As the sweat absorbs more and more heat from the body it evaporates and transfers heat to the surrounding air, thereby, heating up the temperature of the room.

Third Law of Thermodynamics

The Third Law states, “The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K).” Entropy is sometimes called “waste energy,” i.e., the energy that is unable to do work, and since there is no heat energy whatsoever at absolute zero, there can be no waste energy.

Third Law Of Thermodynamics Examples

Let us consider steam as an example to illustrate the third law of thermodynamics step by step:

We know that steam is a gaseous state of water at higher temperatures. In this state,

  1. the molecules within it move freely and have high entropy.
  2. If one decreases the temperature below 100°C, the steam gets converted to water, where the movement of molecules is restricted, decreasing the entropy of water.
  3. When water is further cooled below 0°C, it gets converted to solid ice. In this state, the movement of molecules is further restricted and the entropy of the system reduces more.
  4. As the temperature of the ice further reduces, the movement of the molecules in them are restricted more and the entropy of the substance goes on decreasing.
  5. When the ice is cooled to absolute zero, ideally the entropy should be zero. But in reality, it is impossible to cool any substance to zero.

What is Enthalpy?

Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals the total content of heat of a system, equivalent to the system’s internal energy plus the product of volume and pressure.

Mathematically, the enthalpy, H, equals the sum of the internal energy, E, and the product of the pressure, P, and volume, V, of the system.

H = E + PV

What is Entropy?

The entropy is a thermodynamic quantity whose value depends on the physical state or condition of a system. In other words, it is a thermodynamic function used to measure the randomness or disorder of a system. For example, the entropy of a solid, where the particles are not free to move, is less than the entropy of a gas, where the particles will fill the container.

Different Types Of Energy

Different forms of energy along with formulas is tabulated below-

Internal Energy

\(U=\int TdS\;-\;PdV+\sum _{i}\mu _{i}dN_{i}\)

Helmholtz free energy

F = U – TS


H = U + PV

Gibbs Free Energy

G = U + PV – TS

Thermodynamics Problems


ΔG at 290 K for the following reaction \(2NO_{(g)}+O_{2(g)}\rightarrow 2NO_{2(g)}\)


\(\Delta H=-120kJ\; \; and \;\;\Delta S= −150JK^{-1}\)


To make the unit of ΔS same as ΔH we have to convert the unit of ΔS as follows,

\(\Delta S\;=\;-150:J/K(\frac{1: kJ}{1000: J}) \Rightarrow \Delta S\;=\;-0.15kJ/K\)

We know that,

\(G\;=\;U\;+\;PV\;-\;TS \Rightarrow \Delta G\;=\;\Delta H\;-\;T\Delta S\)


\(\Delta G\;=\;-120 kJ\;-\;(290K)\;(-0.150 kJ/K)\) \(\Rightarrow \Delta G\;=\;-120 kJ\;+\;43kJ \)\(\Rightarrow \Delta G=-77 kJ\)

Therefore, ΔG is -77kJ.

Practice Questions On Thermodynamics

Q1: State the laws of thermodynamics?

Ans:  The laws of thermodynamics are-

  1. First law of thermodynamics:
    Energy can neither be created nor be destroyed, it can only be transferred from one form to another.
  2. Second law of thermodynamics:
    The entropy of any isolated system always increases.
  3. Third law of thermodynamics:
    The entropy of a system approaches a constant value as the temperature approaches absolute zero.
  4. Zeroth law of thermodynamics:
    If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

Q2: What is the basic thermodynamics?

Ans: In simple terms, thermodynamics deals with the transfer of energy from one form to another.

Q3: Who is the father of thermodynamics?

Ans: Nicolas Léonard Sadi Carnot is often described as the “Father of Thermodynamics.”

Q4: What is entropy?

Ans: Entropy is the measure of the number of possible arrangements the atoms in a system can have.

Q5: Can energy be destroyed or lost?

Ans: Energy can neither be created nor be destroyed, it can only be transferred from one form to another.

Q6: What is an example of negative work?

Ans: When you’re pushing an object along the floor, the work done by Kinetic Friction is negative.

Q7. What is the thermodynamic process?

Ans: A thermodynamic process is a passage of a thermodynamic system from an initial to a final state of thermodynamic equilibrium.

Q8: What are the thermodynamic properties?

Ans: Thermodynamic properties may be extensive or intensive. Intensive properties are properties that do not depend on the quantity of matter. For example, pressure and temperature are intensive properties. volume, energy, and enthalpy are examples of extensive properties. Their value depends on the mass of the system.

Q9: Fans convert electrical energy into mechanical energy. This is explained by which law?

Ans: Fans convert electrical energy into mechanical energy. This is explained by First law of thermodynamics.

Q10: What is enthalpy?

Ans: Enthalpy is the measurement of energy in a thermodynamic system.

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Hope you have got a brief knowledge of Thermodynamics, Laws and terms and applications. For a better understanding of thermodynamics do read the related articles and answer the practice questions.

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