Ch 13 derives macroscopic gas properties (pressure, temperature) from microscopic behaviour (molecular motion). Students learn the ideal gas equation, kinetic interpretation of temperature, and equipartition of energy.
Ideal gas equation: PV = nRT = NkT. Kinetic theory assumptions: large number of molecules, random motion, elastic collisions, no intermolecular forces (except during collision), negligible molecular volume. Pressure: P = ⅓(N/V)mv²_rms = ⅓ρv²_rms.
Kinetic interpretation: T ∝ average KE. ½mv²_rms = ³⁄₂kT → v_rms = √(3kT/m). Degrees of freedom (f): monoatomic = 3 (translation), diatomic = 5 (3 translation + 2 rotation), polyatomic = 6. Equipartition: each degree of freedom gets ½kT energy. Internal energy U = (f/2)nRT. Cp − Cv = R. γ = Cp/Cv = 1 + 2/f.
Download: https://ncert.nic.in/textbook/pdf/keph205.pdf | Part II: https://ncert.nic.in/textbook/pdf/keph2ps.zip
At high pressure, molecules are close together — their finite size and intermolecular forces (attractions and repulsions) become significant, violating the assumptions of negligible volume and no forces. At low temperature, molecules move slowly and attractions can cause condensation into liquid/solid. Real gas behaviour is described by the van der Waals equation.
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