Kinetic Theory
Class 11 · Physics
4 ideas in Class 11 Physics, explained in plain words, with games and quizzes to practise.
Molecules and Gas Laws
Part of: Molecules, Pressure and Energy
Matter packs countable molecules: a mole holds Avogadro's count. Liquids crowd them while gases leave vast gaps. Mixed gases split pressure by mole share, and equal temperatures give every molecule the same average energy whatever its mass.
Why it matters. Counting molecules turns bulk readings into molecular facts. Mixture rules run air, diving and industry.
Formulas
- Partial pressure = mole share x total pressure
- Equal temperatures give every molecule the same average energy.
Key words
- Mole
- : Avogadro count of particles, 6.023 times 10 to the power 23.
- Avogadro number
- : The count per mole, 6.023 times 10 to the power 23.
- Partial pressure
- : One gas's share, what it alone would exert.
- Dalton's law
- : Total pressure is the sum of partial pressures.
Try it in two minutes
- Dissolve sugar and picture the hidden crowd.
- Compare liquid and vapour volumes of water.
- Split a total pressure by mole shares.
Try at home. Weigh a spoon of water and count its molecules.
Pressure and Energy of Gas Molecules
Part of: Molecules, Pressure and Energy
Pressure is wall hits per second: a third of density times mass times mean square speed. That same motion gives each molecule three-halves kT of energy, so temperature is motion in disguise. Faster rebounds off moving walls explain heating by squeezing.
Why it matters. Thermometers read motion; pumps heat by hits. One picture unites pressure, warmth and work.
Formulas
- P = (1/3) rho (mean square speed)
- Average energy per molecule = (3/2) k T
- RMS speed = root(3 R T / M)
Key words
- Kinetic theory
- : Pressure and temperature from molecular hits and motion.
- Mean square speed
- : Squared speeds averaged over molecules.
- RMS speed
- : The root of mean square speed.
- Boltzmann constant
- : The per-molecule gas constant, 1.38 times 10 to the power minus 23.
Try it in two minutes
- Shake beads in a box and feel wall taps.
- Heat a sealed can and predict pressure rise.
- Squeeze fast versus slow and compare warmth.
Try at home. Pump a ball and feel it warm.
Equipartition and Specific Heats
Part of: Sharing Energy and Wandering Molecules
Every squared motion term holds half kT on average: three for flight, plus two spins for dumbbell molecules. Molar heat at fixed volume counts half R per term, and fixed pressure adds one R for push work. Cold can freeze spins out; stiff vibrations stay frozen at room warmth.
Why it matters. Counting motions predicts heat capacities from shape alone. Freezing explains why cold gases hold less heat.
Formulas
- Cv = (f / 2) R for f active terms
- Cp - Cv = R
- Gamma = Cp / Cv
Key words
- Degree of freedom
- : One independent squared motion term.
- Equipartition
- : Each term holds half kT on average.
- Molar heat at constant volume
- : Half R per term for one mole.
- Gamma
- : The ratio of the two molar heats.
Try it in two minutes
- Count motions of a bead, disc and dumbbell.
- Assign half R to each counted motion.
- Compare monoatomic with diatomic predictions.
Try at home. Spin a pen and list its distinct motions.
Mean Free Path and Diffusion
Part of: Sharing Energy and Wandering Molecules
Molecules fly a mean free path between hits, longer in thin gas. Light gases leak faster through pores by the root of mass ratio, so enrichment repeats many passes. Walls that move trade energy with rebounds: advancing walls heat, retreating walls cool.
Why it matters. Vacuum, separation plants and engines all trade on these wanderings. One root law sorts fast from slow.
Formulas
- Mean free path = 1 / (root(2) pi d^2 n)
- Graham law: rate related to 1 / root(M)
Key words
- Mean free path
- : The average flight between hits, longer in thin gas.
- Diffusion
- : Spreading by molecular motion, faster for light gases.
- Graham's law
- : Leak rates follow the inverse root of mass.
- Enrichment
- : Repeated passes raising one isotope's share.
Try it in two minutes
- Weave through a crowd slowly versus quickly.
- Compare leak rates of light and heavy scents.
- Push and pull a syringe to feel warm and cool.
Try at home. Smell cooking from near and far and time the spread.
What your child will practise
- Equipartition and Specific Heats: Space Quick Fire (Quick Fire)
- Mean Free Path and Diffusion: Ocean Puzzle (Clue Puzzle)
- Mean Free Path and Diffusion: Ocean Quick Fire (Quick Fire)
- Molecules and Gas Laws: Jungle Quick Fire (Quick Fire)
- Pressure and Energy of Gas Molecules: Candy Puzzle (Clue Puzzle)
- Quiz: Equipartition and Specific Heats (8 questions)
- Quiz: Mean Free Path and Diffusion (8 questions)
- Quiz: Molecules and Gas Laws (8 questions)
- Quiz: Pressure and Energy of Gas Molecules (8 questions)