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CBSE

Class 11

Physics NCERT Book

Chapter 13:

Ncert Books Class 11 Physics Chapter 13 Free Download

Q: 7. What is the importance of 'mean free path' in the kinetic theory of gases?

The concept of mean free path is crucial as it represents the average distance a gas molecule travels between successive collisions. It helps explain transport phenomena in gases, such as diffusion, viscosity, and thermal conductivity. While the kinetic theory assumes molecules are point masses that travel in straight lines, the mean free path provides a more realistic understanding of the actual, interrupted journey of a molecule within the gas, which is essential for more advanced applications of the theory.

An Overview of Ncert Books Class 11 Physics Chapter 13 Free Download

Ever wondered how gases behave in our daily life, like why a balloon pops or how air fills up a room? In Ncert Books Class 11 Physics Chapter 13 Free Download, you’ll uncover the core ideas behind the kinetic theory of gases. This chapter shows how scientists explain the motion of tiny particles and how those movements lead to things we can actually see and feel, such as pressure and temperature.

The best part? Vedantu offers a simple PDF download for this chapter, making it super easy for you to revisit difficult concepts or clear up any confusion. As you go through these lessons, you’ll also understand formulas and equations that are common in the CBSE Class 11 Physics syllabus. Need a syllabus snapshot? You can always refer to the Class 11 Physics Syllabus for a quick overview.

Use these resources to strengthen your basics and feel more confident when solving exam problems. And if you’re preparing for the best results, check out the Class 11 Physics Important Questions for extra practice.

Overview of NCERT Books for Class 11 Physics Chapter 13

Subject specialists have created NCERT Lessons for Class 11 Physics based on the most recent term – II CBSE Syllabus (2023-24) so that students may understand the topics more successfully. The NCERT Solutions for Class 11 Physics Kinetic Theory are provided to help students fully comprehend the ideas covered in this Chapter. In the CBSE Class 11 term – II examination, Kinetic Theory is one of the high scoring sections. Gas properties are simpler to comprehend than those of solids and liquids. All objects are made up of atoms, which are tiny particles that move around in a constant state of motion, attracting one another when they are separated by a little distance but repelling when pushed together.

This Chapter covers some of the most important ideas, such as different scientists' gas laws, atomic theory, the relevance and applications of constant values like the Boltzmann constant and Avogadro's Number, Kinetic Theory Postulates, and Specific Heat Capacities. A gas that fulfills perfectly at all pressures and temperatures is defined as an Ideal Gas, according to the NCERT Class 11 Physics Chapter 13. A simple theoretical model of gas is an Ideal Gas. There is no such thing as a perfect gas. The molecules are widely apart under low pressures or high temperatures, and molecular interactions are minimal. The gas acts as an ideal one when there are no interactions.

The molecular image of the matter is the foundation of the kinetic theory of gasses. A particular volume of gas is made up of a vast number of molecules that are constantly moving at random. The average distance between molecules at ordinary pressure and temperature is a factor of ten or more than the normal size of a molecule. The molecules' interaction is insignificant, therefore we may suppose they travel freely in straight lines according to Newton's first law.

Kinetic theory of gasses

The Kinetic theory of gasses is a theory based on the idea that gas molecules are extremely small spherical particles, from which many fundamental gas characteristics about their thermodynamic behavior may be inferred.

Overview of the sub-topics

Chapter 13 of Physics for Class 11 Kinetic Theory is divided into seven sections, the first of which is the Chapter's introduction. These lessons cover the Kinetic Theory, Specific Heat, Mean Free Path, and other gas-related concepts. The following are the subjects compiled:

Introduction - Before diving into the details, the first section lays out the fundamentals of the Kinetic Theory of Gasses and how it was developed. Students will study the structure and contents of gas molecules, as well as their characteristics and thermodynamic behavior when contrasted to liquids and solids. Questions about terminology like conduction, viscosity, diffusion, and specific heat capacities have been well addressed in NCERT Solutions Class 11 Physics Kinetic Theory.
Molecular Nature of Matter - This section goes into the molecular behavior of gasses in greater depth. It includes the Atomic Theory as well as its principles. Interatomic spaces, mean free paths, and dynamic equilibrium of gasses have all been emphasized. Students will be expected to comprehend Gay Loussac's Law and Avogadro's Law, as well as their relationship.
Kinetic Theory of an Ideal Gas - The basic idea of the Kinetic Theory of Gasses, as well as its postulates, are introduced here. This portion of the NCERT Solutions for Class 11 Physics Chapter 13 covers essential words and ideas such as the Boltzmann constant and Avogadro's number, as well as its applications in determining temperature, pressure, and volume of gasses. You'll also have to work out difficulties with mole number and molar mass.
Pressure of an Ideal Gas - Based on the fixed velocities of the gas particles, their momentum and consequently their pressure is calculated. This is explained in detail in Pascal's Law.
Kinetic Interpretation of Temperature - With the help of the Boltzmann constant, the link between kinetic energy, pressure, and temperature is calculated using the ideal gas equation and preceding derivations. Examples and activities about the above-mentioned ideas are included in Chapter 13 Physics Class 11 NCERT Solutions to reinforce the equations and derivations for effective learning.
Law of Equipartition Energy - Before establishing the Law of Equipartition of Energy, this portion covers some of the key concepts of the vibrational energy of molecules, rotational vibration, a moment of inertia, and degrees of movement of gas particles.
Specific Heat Capacity - In the computations of molar specific heat capacity of gasses, solids, and water at constant volume (cv) and constant pressure, the Law of Equipartition Energy is utilized (cp). There are also suggestions for eliminating gaps between projected and experimental specific heat capacity values. The relevance of quantum Physics and its use in numerical problems is conveyed in Class 11 NCERT's Kinetic Theory of Gases.
Mean Free Path - The last topic in Kinetic Theory of Gasses Class 11 Physics introduces the concept of mean free path and illustrates its usage through examples from everyday life examples. It is also calculated using a formula based on the size, density, and quantity of gas molecules. In Kinetic Theory of Gasses Class 11 Physics NCERT Solutions, students will discover a well-explained guide to numerical based on the formula that will help them improve their problem-solving abilities.

Key Points of Kinetic Theory

The following are some significant points from the Kinetic theory of gasses: -

In a gas, we should not have an inflated notion of intermolecular distance. In solids and liquids at typical pressures and temperatures, this is only about 10 times the interatomic distance. In gas, the mean free path is 1000 times larger than the molecule's size and 100 times the interatomic distance.
A fluid's pressure isn't just exerted on the wall. In a fluid, pressure exists everywhere. Because the pressure on both sides of any layer of gas inside the volume of a container is equal, the layer is in equilibrium.
Because of their rapid speeds and constant collisions, air molecules in a room do not descend and settle on the ground (due to gravity). At lower heights, there is a very modest rise in density in equilibrium (like in the atmosphere).

FAQs on Ncert Books Class 11 Physics Chapter 13 Free Download

1. What are the key postulates of the kinetic theory of gases that are frequently asked in the Class 11 Physics exam?

For the CBSE Class 11 exam, the most important postulates of the kinetic theory of gases to remember are:

All gas molecules are in continuous, random motion.
The volume of the gas molecules is negligible compared to the volume of the container.
There are no intermolecular forces of attraction or repulsion between gas molecules.
Collisions between molecules and with the container walls are perfectly elastic.
The average kinetic energy of gas molecules is directly proportional to the absolute temperature of the gas.

2. Which derivations from Kinetic Theory are considered important 5-mark questions for the 2025-26 board pattern?

The most crucial derivation from this chapter, often appearing as a long-answer or 5-mark question, is the expression for the pressure exerted by an ideal gas. A thorough understanding of the steps involved in deriving P = (1/3)ρv²_rms is essential for scoring full marks. You should also be prepared for questions that ask you to relate this pressure formula to the kinetic energy of the gas.

3. How does the kinetic interpretation of temperature connect the microscopic world of molecules to the macroscopic property we measure?

The kinetic interpretation of temperature provides a fundamental link between molecular motion and heat. It states that the absolute temperature (a macroscopic property measured with a thermometer) of a gas is a direct measure of the average translational kinetic energy of its molecules (a microscopic property). This means when you heat a gas, you are increasing the average speed and kinetic energy of its constituent molecules.

4. What types of numerical problems based on degrees of freedom and the law of equipartition of energy are important for the exam?

Important numericals often involve calculating the total internal energy and specific heats (C_v and C_p) for different types of gases. Expect questions where you must first identify the degrees of freedom for a given gas (monatomic, diatomic, or triatomic) and then apply the law of equipartition of energy, which assigns (1/2)kT of energy per degree of freedom, to find the molar specific heats and their ratio (γ).

5. Why is the root mean square (rms) speed a more significant measure for gas molecules than the simple average speed?

The rms speed is more significant because gas molecules move randomly in all directions. A simple average of their velocities would be close to zero, as velocities are vector quantities and would cancel each other out. The rms speed, however, involves squaring the velocities first (making them all positive), then taking the mean, and finally the square root. This provides a meaningful, non-zero measure of the typical speed of molecules, which is directly related to the gas's kinetic energy and temperature.

6. Under what conditions do the assumptions of the kinetic theory for an ideal gas fail to apply to real gases?

The ideal gas assumptions fail for real gases under conditions of high pressure and low temperature. This is because:

At high pressure, molecules are forced closer together, and the volume of the molecules themselves is no longer negligible compared to the container volume.
At low temperature, molecules move slower, and the weak intermolecular forces of attraction (van der Waals forces), ignored in the ideal model, become significant enough to affect their motion.

7. What is the importance of 'mean free path' in the kinetic theory of gases?

The concept of mean free path is crucial as it represents the average distance a gas molecule travels between successive collisions. It helps explain transport phenomena in gases, such as diffusion, viscosity, and thermal conductivity. While the kinetic theory assumes molecules are point masses that travel in straight lines, the mean free path provides a more realistic understanding of the actual, interrupted journey of a molecule within the gas, which is essential for more advanced applications of the theory.