IIT JEE Thermal Physics Study Material

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.  

Before we discuss the laws, we shall first discuss some of the words which will be used in these laws:

Heat: The amount or quantity of energy transmitted except than by work or by transfer of matter. This transfer or transmission may occur in any of the ways conduction, radiation or convective circulation.

Entropy: The word entropy is often confused with energy. Though the two terms are related yet they are different. Entropy refers to the measure of the level of disorder in a closed yet changing system. The system in questions is the system in which energy can only be transferred in one direction and that too from an ordered situation to a disordered state. Higher the entropy, higher is the disorder.   

Enthalpy: Enthalpy is the quantity that measures the total energy of a thermodynamic system. It includes the internal energy of the system and its thermodynamic potential as well as its volume and pressure.

Mathematically, it can be said to be equivalent to the sum of the internal energy of the system and the product of its volume multiplied by the intensity of pressure exerted on it by its environment.

First law of thermodynamics: The first law of thermodynamics is also termed as the law of conservation of energy. It states that energy can neither be created nor destroyed. Energy can only be distributed or it can change from one form to another. This can also be stated as the total energy and matter in the Universe remains stagnant, it just keeps on changing from one form to another.  

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:

Though the first law is immensely useful but it has certain limitations. Some of the drawbacks include:

  1. This law fails to indicate the direction in which the charge can occur. This would be clear with this example. When brakes are applied to stop a moving car work done against friction is converted into heat. Even when the car cools down, it does not start moving by converting heat energy into mechanical work.
  2. The law does not give any information with regard to the extent of change.
  3.  There is no information about the source of heat i.e. it does not give any clue regarding whether the body is cold or hot.
  4. Practically, it is not generally possible to convert the heat energy into an equivalent amount of mechanical energy.

View this video on the laws of Thermodynamics 

Second Law of Thermodynamics: This law is based on the concept of entropy. The entropy of an isolated system which is not in thermal equilibrium always tends to increase. The law can also be restated as in all energy systems, if there is no entry or exit of energy then potential energy of the state will always be lower than the original state.

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: This law is also related to the concept of entropy. It states that as the temperature tends to zero, the entropy of a system also approaches a constant value.      

Application of Hess Law: Hess law is a fundamental law of Physics. It states that the change in enthalpy that is associated with a chemical change is unaffected and is not concerned with the channel by which the chemical change occurs.   

Here we have not gone into the intricacies of the law as it has been discussed in detail in the later sections.

askIITians offers comprehensive study material covering thermodynamics for IIT JEE. It covers all the important topics like application of enthalpy, questions on thermodynamics, different types of enthalpy, isochoric process etc. IIT JEE solutions to various questions of previous papers of Thermal Physics and Thermodynamics are also provided enabling them to come up with flying colors in all the entrance exams like IIT JEE. 

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