Thermal Physics


Table of Content

Thermal Physics is a vital branch of Physics and covers various topics like thermodynamics, statistical mechanics, kinetic theory etc. Thermal Physics deals with diverse branches including key areas like the study of heat, temperature and heat transfer. The various concepts of thermodynamics are closely related to thermal energy which is one of the basic forms of energy. These concepts have far reaching applications. In most of the energy transformations there is a production of thermal energy in the form of heat. Thermal physics also forms the basis of various ancient cycles of heat. This is also the reason behind the invention of steam engine, rotors, shipments and various other revolutionary machines.

We shall discuss some of the topics included under thermal physics in brief as they have been described in detail in the coming sections. We proceed with the laws of thermodynamics which describe the physical entities like temperature and energy and form the basis of thermodynamic systems.  


When a cold body ‘A’ is placed in contact with a hot body ‘B’ something is transferred from hot body ‘B’ to the cold body ‘A’ which results in a rise in temperature of the cold body. This transference stops when the two have acquired same temperature. This indicates that only a part of energy of ‘B’ is transferred to ‘A’. This part is called heat. This transfer or transmission may occur in any of the ways conduction, radiation or convective circulation.Before we discuss the laws, we shall first discuss some of the words which will be used in these laws:

The part of thermal energy which flows from one body to another due to temperature difference is called heat. By convention, heat given to a body is taken as positive while that taken out of the body is taken as negative. Heat is a scalar quantity.


Temperature is defined as the degree of hotness of a body. it is also a scalar quantity. The state of equality in the temperature of two bodies is known as thermal equilibrium. 


The physical quantity entropy is a measureable property and describes the ability of the system to do work. Entropy of a substance is, therefore, said to be a measure of the degree of disorder prevailing among its molecules, just as the temperature is a measure of the degree of hotness of a substance. At the absolute zero of temperature the thermal motion completely disappears so that the disorder and hence the entropy tends to zero and the molecules of a substance are in perfect order, i.e., well arranged. Higher the entropy, higher is the disorder. The entropy of any isolated system increases and approaches, more or less rapidly, to the inert state of maximum entropy.


dS = dQ/T


Enthalpy is an extensive thermodynamic property and is given the symbol H. Enthalpy is given by the equation,

H = U+PV

Therefore, enthalpy is defined as the sum of the internal energy and the product of pressure and volume.

First Law of Thermodynamics

The first law of thermodynamics states that, “If the quantity of heat supplied to a system is capable of doing work, then the quantity of heat absorbed by the system is equal to the sum of the increase in the internal energy of the system, and the external work done by it”.

Therefore, first law of thermodynamics signifies that, “energy can neither be created nor destroyed, but it can only be transformed from one form to another”. 

Thus, dQ= dU+dW

If work is done by the surroundings on the system (as during the compression of a gas), W is taken as positive so that dQ = dU+W. if however work is done by the system on the surroundings (as during the expansion of a gas), W is taken as negative so that dQ = dU – dW.

Mathematically, the first law of thermodynamics can be stated as

First law of thermodynamics

First law of thermodynamics

Let us discuss some of the limitations of the first law of thermodynamics:

Limitations to the First Law of Thermodynamics

First law of thermodynamics which tells us that heat can be converted into work and viceversa is merely a quantitative statement of the equivalence of heat and work. It has the following limitations.

(a) It does not tell us whether any particular process can actually occur. According to the first law of thermodynamics heat may flow from higher temperature to lower temperature nad vice versa. Pratically we know that heat cannot flow from a lower temperature to a higher one. This is not predicted by the first law.

(b) According to the first law, we could convert the whole (100%) of heat, available to us, into work and vice versa, while pratically we know that it is not possible to do so. Thus, first law does not restrict our ability to convert heat into work or work into heat.
Above quoted handicaps lead to the formulation of another law of thermodynamics called second law of thermodynamics.

View this video on the laws of Thermodynamics:- 

Second Law of Thermodynamics

In terms of entropy the second alw of thermodynamics may be stated as follows:

The entropy of an isolated system is fully conserved in every reversible process, i.e. for every reversible process the sum of all changes in entropy taking place in an isolated system is zero. If the process is not reversible one, then the sum of all changes in entropy taking place in an isolated system is greater than zero. In general we can say that in every process taking place in an isolated system the entropy of the system either increases or remains constant.

Furthermore, if an isolated system is in such a state that its entropy is a maximum, any change from that state would necessarily involve a decrease in entropy and hecnce will not happen. Therefore, the necessary condition for an equilibrium of an isolated system is that its entropy shall be maximum. 

It is to be noted that the above statements are applicable to isolated system only. The more usual forms of the second law of thermodynamics are as follows.

(a) It is impossible to construct a device which, operating in a cycle has the sole effect of extracting heat from a single reservior and performing an equivalent amount of work. (Kelvin-Planck Statement)

(b) It is impossible for heat to flow from a cooler body to another hotter body without the aid of any external agency.(Clausius Statement)

 Most of these limitations of the 1st law of thermodynamics have been taken care off in the second law. The second law of thermodynamics helps us in estimating whether the reaction is actually viable or not and it also tells the direction of heat. The second law also clarifies that it is not possible to convert energy completely into work.
Second law of thermodynamics

Third Law of Thermodynamics

In accordance to third law of thermodynamics, “ the heat capacities of all solids tends to zero as the absolute zero of temperature is approached and that the internal energies and entropies of all substances become equal there, approaching their common value asymptotically.” This simple statement follows neither from the first law (law of conservation of energy) nor from the second law (law of transmutability of energy) and is thus of the nature of a new law, usually called the third law of thermodynamics.

This theorem is useful in explaining the nature of bodies in neighbourhood of absolute zero temperature. Its importance lies in the fact that it permits the calculations of absoluted values of entropy and the physical interpretation of thermodynamic propperties such as Helmholtz nad Gibb’s free energies etc. It can be conceived that as the temperature of a system tends to absolute zero, its entropy tends to a constant value which is indepent of pressure and state of agregation etc. We may put it equal to zero so that the entropy of every substance becomes normalized in an absolute way.

Related Resources

You might like to refer some of the related resources listed below:

Click here for the Detailed Syllabus of IIT JEE Physics.

Look into the Sample Papers of Previous Years to get a hint of the kinds of questions asked in the exam.

You can get the knowledge of Useful Books of Physics.


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