Course Content
Class 11 Physics Chapter 1 Physical World
Section Name Topic Name 1 Physical World 1.1 What is physics? 1.2 Scope and excitement of physics 1.3 Physics, technology and society 1.4 Fundamental forces in nature 1.5 Nature of physical laws
Class 11 Physics Chapter 4 Motion In A Plane
4 Motion in a plane 4.1 Introduction 4.2 Scalars and vectors 4.3 Multiplication of vectors by real numbers 4.4 Addition and subtraction of vectors – graphical method 4.5 Resolution of vectors 4.6 Vector addition – analytical method 4.7 Motion in a plane 4.8 Motion in a plane with constant acceleration 4.9 Relative velocity in two dimensions 4.10 Projectile motion 4.11 Uniform circular motion
Class 11 Physics Chapter 5 Laws of motion
Section Name Topic Name 5 Laws of motion 5.1 Introduction 5.2 Aristotle’s fallacy 5.3 The law of inertia 5.4 Newton’s first law of motion 5.5 Newton’s second law of motion 5.6 Newton’s third law of motion 5.7 Conservation of momentum 5.8 Equilibrium of a particle 5.9 Common forces in mechanics 5.10 Circular motion 5.11 Solving problems in mechanics
Class 11 Physics Chapter 6 Work Energy and Power
Section Name Topic Name 6 Work Energy and power 6.1 Introduction 6.2 Notions of work and kinetic energy : The work-energy theorem 6.3 Work 6.4 Kinetic energy 6.5 Work done by a variable force 6.6 The work-energy theorem for a variable force 6.7 The concept of potential energy 6.8 The conservation of mechanical energy 6.9 The potential energy of a spring 6.10 Various forms of energy : the law of conservation of energy 6.11 Power 6.12 Collisions
Class 11 Physics Chapter 7 Rotation motion
Topics Introduction Centre of mass Motion of COM Linear Momentum of System of Particles Vector Product Angular velocity Torque & Angular Momentum Conservation of Angular Momentum Equilibrium of Rigid Body Centre of Gravity Moment of Inertia Theorem of perpendicular axis Theorem of parallel axis Moment of Inertia of Objects Kinematics of Rotational Motion about a Fixed Axis Dynamics of Rotational Motion about a Fixed Axis Angular Momentum In Case of Rotation about a Fixed Axis Rolling motion
Class 11 Physics Chapter 9 mechanics properties of solid
Section Name Topic Name 9 Mechanical Properties Of Solids 9.1 Introduction 9.2 Elastic behaviour of solids 9.3 Stress and strain 9.4 Hooke’s law 9.5 Stress-strain curve 9.6 Elastic moduli 9.7 Applications of elastic behaviour of materials
Class 11 Physics Chapter 11 Thermal Properties of matter
Section Name Topic Name 11 Thermal Properties of matter 11.1 Introduction 11.2 Temperature and heat 11.3 Measurement of temperature 11.4 Ideal-gas equation and absolute temperature 11.5 Thermal expansion 11.6 Specific heat capacity 11.7 Calorimetry 11.8 Change of state 11.9 Heat transfer 11.10 Newton’s law of cooling
Class 11 Physics Chapter 14 Oscillations
Section Name Topic Name 14 Oscillations 14.1 Introduction 14.2 Periodic and oscilatory motions 14.3 Simple harmonic motion 14.4 Simple harmonic motion and uniform circular motion 14.5 Velocity and acceleration in simple harmonic motion 14.6 Force law for simple harmonic motion 14.7 Energy in simple harmonic motion 14.8 Some systems executing Simple Harmonic Motion 14.9 Damped simple harmonic motion 14.10 Forced oscillations and resonance
Class 11th Physics Online Class For 100% Result
About Lesson

Mean free path

  • Mean free path is the average distance between the two successive collisions.
  • Inside the gas there are several molecules which are randomly moving and colliding with each other.
  • The distance which a particular gas molecule travels without colliding is known as mean free path.

Expression for mean free path

  • Consider each molecule of gas is a sphere of diameter (d).The average speed of each molecule is<v>.
  • Suppose the molecule suffers collision with any other molecule within the distance (d). Any molecule which comes within the distance range of its diameter this molecule will have collision with that molecule.
  • The volume within which a molecule suffer collision =<v>Δtπd2.
  • Let number of molecules per unit volume =n
  • Therefore the total number of collisions in time Δt =<v>Δtπd2xn
  • Rate of collision =<v>Δtπd2xn/Δt=<v>πd2n
  • Suppose time between collision τ =1/<v>πd2n
  • Average distance between collision = τ<v> = 1/πd2
  • 1/πd2n this value was modified and a factor was introduced.
  • Mean free path(l) = 1/√2 π d2n

Conclusion: – Mean free path depends inversely on:

  1. Number density (number of molecules per unit volume)
  2. Size of the molecule.
Class 11 Physics Chapter 13 Kinetic Theory Notes & NCERT Solution

Temperature inside the cylinder, T = 17°C =290 K

Radius of a nitrogen molecule, r = 1.0 Å = 1 × 1010 m

Diameter, d = 2 × 1 × 1010 = 2 × 1010 m

Molecular mass of nitrogen, M = 28.0 g = 28 × 10–3 kg

The root mean square speed of nitrogen is given by the relation:



R is the universal gas constant = 8.314 J mole–1 K–1

Therefore vrms=√ (3×8.314×290)/ (28×10-3)


The mean free path (l) is given by the relation:

l= KT/√2xd2xP

Where, k is the Boltzmann constant = 1.38 × 10–23 kgm2 s–2K–1

Therefore l= 1.38×10-23x290/√2x 3.14x (2×10-10)2x2.026×105

= 1.11 × 10–7 m

Collision frequency=vrms/l


= 4.58 × 109 s–1

Collision time is given as:


= 2×10-10/508.26 = 3.93 × 10–13 s

Time taken between successive collisions:

T’=l/vrms = 1.11×10-7m/508.26m/s= 2.18 × 10–10 s

T’/T= 2.18 × 10–10 s/3.93×10-13 = 500

Hence, the time taken between successive collisions is 500 times the time taken for a collision.

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