# Kinetic Theory Notes, Class 11 Physics | SwaVid Kinetic Theory, NCERT Class 11 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers. Canonical: https://www.swavid.com/cbse/class-11/physics/chapter/kinetic-theory Source: https://www.swavid.com/cbse/class-11/physics/chapter/kinetic-theory # Kinetic Theory ## Formulas and definitions ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 11 Physics on SwaVid ### Molecular nature of matter ### Behaviour of gases ### Kinetic interpretation of pressure ### Kinetic interpretation of temperature ### Law of equipartition of energy ### Specific heat capacities of gases ### Mean free path ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 11 › Physics › Kinetic Theory Notes written by SwaVid from the NCERT textbook. Class 11 Physics, Chapter 12. Key ideas, formulas and common mistakes. CBSE prints 20 marks for Units VII-IX together, not for this chapter alone. Short answer. This chapter develops the microscopic model of gases to explain macroscopic quantities like pressure, temperature, and internal energy. It shows how molecular collisions produce pressure and how absolute temperature directly measures the average translational kinetic energy of molecules. Source: NCERT Physics Part II (Reprint 2026-27) , Chapter 12, Kinetic Theory Matter consists of atoms or molecules in continuous random motion. In gases, interatomic distances are much larger than molecular sizes, so intermolecular forces are negligible except during brief elastic collisions. An ideal gas obeys the equation of state P V = μ R T = k B N T at all pressures and temperatures. Real gases approach ideal behaviour at low pressures and high temperatures. Gas pressure arises from the rate of momentum transferred to container walls by colliding molecules. The pressure formula is P = (1/3) ρ v rms ², which connects microscopic molecular speeds to macroscopic pressure. The average translational kinetic energy of a molecule is proportional to absolute temperature and equals (3/2) k B T. Absolute zero corresponds to the temperature where molecular translational kinetic energy becomes zero. In thermal equilibrium, the total energy of a dynamical system is shared equally among its degrees of freedom. Each translational or rotational degree of freedom contributes (1/2) k B T of energy per molecule, while each vibrational mode contributes k B T. Molar heat capacities depend on the degrees of freedom of the gas molecules. For a gas with f degrees of freedom, C v = (f/2) R and C p = C v + R, yielding a specific heat ratio γ = 1 + (2/f). The mean free path is the average distance traversed by a molecule between successive collisions. It is given by l = 1 / (√2 n π d²), where n is the number density of molecules and d is the molecular diameter. Try each question first. Then tap it to see the answer. E k = (3/2) k B T E k = 1.5 × (1.38 × 10⁻²³) × 300 E k = 6.21 × 10⁻²¹ J 6.21 × 10⁻²¹ J v rms = √(3 R T / M) v rms = √((3 × 8.31 × 300) / (4.00 × 10⁻³)) v rms = √(7479 / 0.004) = √1869750 ≈ 1367 m s⁻¹ 1367 m s⁻¹ C v = (f/2) R = (5/2) R C p = C v + R = (7/2) R γ = C p / C v = 7 / 5 = 1.40 C v = (5/2) R, γ = 1.40 Oscillations . Thermal Properties of Matter . Thermodynamics . The 14 chapters of Class 11 Physics . CBSE does not give marks to one chapter. It prints 20 marks for Units VII-IX together (Properties of Bulk Matter; Thermodynamics; Behaviour of Perfect Gases and Kinetic Theory). Kinetic Theory is part of that group. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Oscillations. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 12 NCERT book - 20 marks, Units VII-IX together - 3 quick checks - Using temperature in Celsius instead of Kelvin in gas equations . Always convert temperatures to Kelvin by adding 273.15 before calculating RMS speed, kinetic energy, or pressure. - Confusing molecular mass with molar mass in the RMS speed formula . Use molecular mass m with the Boltzmann constant k B , or molar mass M (in kg mol⁻¹) with the universal gas constant R. - Counting vibrational modes at room temperature for simple diatomic gases . At ordinary room temperatures, vibrational modes are not active; take 5 degrees of freedom (3 translational and 2 rotational) for a rigid diatomic molecule. - Confusing average speed with root mean square speed . Root mean square speed is the square root of the mean of squared speeds, which is slightly larger than the arithmetic average speed. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 11](https://www.swavid.com/cbse/class-11) - [Physics](https://www.swavid.com/cbse/class-11/physics/chapters) - [Start free →](https://www.swavid.com/signup) - [All 14 chapters](https://www.swavid.com/cbse/class-11/physics/chapters) - [NCERT Physics Part II (Reprint 2026-27)](https://ncert.nic.in/textbook.php) - [Open notes](https://www.swavid.com/cbse/class-11/physics/chapter/oscillations) - [Open notes](https://www.swavid.com/cbse/class-11/physics/chapter/thermal-properties-of-matter) - [Open notes](https://www.swavid.com/cbse/class-11/physics/chapter/thermodynamics) - [Class 11 Physics](https://www.swavid.com/cbse/class-11/physics/chapters) - [Start free →](https://www.swavid.com/signup)