Physics

Class 11

Physics · Class 11 · Chapter 12

Kinetic Theory

13 lessons 94 min 3 simulations

This chapter explains the behaviour of a gas from the motion of its molecules. It first fixes the scales (atoms about 1 Å, spacing in a gas tens of Å, mean free path thousands of Å) and the ideal-gas equation PV = μRT = k_BNT, with Boyle's, Charles' and Dalton's laws as special cases. Treating the molecules as tiny elastic balls hitting the walls gives P = ⅓nm⟨v²⟩; set beside the gas equation it says the average kinetic energy of a molecule is (3/2)k_BT, so temperature measures molecular motion and v_rms = √(3k_BT/m). Equipartition, ½k_BT for every squared term in the energy, then predicts the specific heats of monatomic, diatomic and polyatomic gases and 3R for solids. The chapter ends with the mean free path, which explains why gases mix slowly although their molecules are fast.

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What the exam asks

NEET tests P = ⅓nm⟨v²⟩ and PV = ⅔E, average kinetic energy (3/2)k_BT independent of the gas, v_rms = √(3RT/M₀) and speed ratios √(M₂/M₁), degrees of freedom and equipartition (½k_BT per squared term, k_BT per vibrational mode), C_v, C_p and γ for monatomic (5/3), rigid diatomic (7/5) and polyatomic gases, C_p − C_v = R, the mixture and partial-pressure questions, C = 3R for solids, and l = 1/(√2nπd²). Marks slip on using grams instead of kilograms per mole in v_rms, on °C instead of K, on giving a vibrational mode only ½k_BT, and on thinking heavier molecules carry more kinetic energy at the same temperature.

Lessons

13 lessons · 94 min