Class 11 Physics Chapter 12 Kinetic Theory – Revision Notes

Kinetic theory explains gas pressure and temperature as consequences of molecules in constant, random motion, starting from PV=nRT and the pressure formula derived from molecular collisions. Degrees of freedom and the equipartition of energy then connect this molecular picture to measurable temperature.

Last Updated: September 23, 2026

Ideal Gas

• PV=nRT; R=8.314 J/(mol·K).
• P=(1/3)ρv_rms².

Temperature and Energy

• Average KE per molecule=(3/2)k_BT.
• Equipartition: each DOF contributes (1/2)k_BT.

Degrees of Freedom

• Monatomic: 3; Diatomic (moderate T): 5.

One-Line Summary

Kinetic theory explains macroscopic gas behaviour (pressure, temperature) as a result of the random motion and collisions of gas molecules, linking temperature directly to average molecular kinetic energy.

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

What are the basic assumptions of the Kinetic Theory of Gases?
The theory assumes gas molecules are point masses in constant random motion, that collisions between them are perfectly elastic, and that there are no intermolecular forces except during collisions.

How does the Kinetic Theory explain gas pressure?
Pressure arises from the continuous collisions of fast-moving gas molecules with the walls of the container. Each collision transfers momentum to the wall, and the cumulative effect of countless collisions per second produces the pressure we measure.

Written by Satish

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