Sec 3 Physics Kinetic Particle Model notes

The properties follow from the model. Solids hold their shape because a lattice of strong attractions locks each particle in place — it can only shiver on the spot. Liquids flow because the particles are still touching but free to swap neighbours, so the material pours while keeping its volume. Gases have almost no cohesion left: the particles are separated by distances far larger than their own size, flying freely until they hit a wall or each other — which is why a gas spreads through every corner of its container and squashes easily when the empty space is squeezed out.

In exam answers, write the triple in order: arrangement (regular/disordered/none), separation (touching/close/far), motion (vibrate/slide/fast-and-random). Three bullets, three marks — it is the most reliably earnable structure in thermal physics.

In 1827 the botanist Robert Brown watched pollen grains suspended in water dance under his microscope — never stopping, never settling into a pattern. The modern school version uses a smoke cell: a glass cell filled with cigarette smoke, lit from the side and viewed under a microscope. The bright specks of smoke twitch in short, jerky, completely random hops, hour after hour.

The explanation needs invisible particles. The air molecules are far too small to see, but they are in continuous random motion, and they bombard each smoke speck (a few thousand molecules across) from every side. At any instant, slightly more molecules might strike one face than another — an unbalanced push — and the speck lurches that way. A moment later the imbalance points somewhere else, so it lurches again. The randomness of the speck’s dance mirrors the randomness of the molecular bombardment: that is what makes the experiment evidence, not just a curiosity.

Warm the cell gently and the dance becomes more violent: higher temperature means faster molecules, harder and more frequent bombardment, bigger kicks. The same reasoning explains why a sugar cube dissolves faster in hot tea — faster water molecules hammer the crystal apart sooner.

Temperature is not a fluid that flows into things — it is a reading of the average kinetic energy of the particles. Heat a solid and its lattice vibrates faster; heat a liquid and its particles jostle faster; heat a gas and its molecules fly faster. Nothing about the particles themselves changes size — "the particles expand" is a classic wrong answer; the spacings grow, the particles do not.

Two consequences follow. First, at any instant the particles have a spread of speeds — temperature reports the average, not a uniform value; that is exactly what lets Brownian kicks be unequal. Second, at the same temperature all substances have particles with the same average kinetic energy, whether solid, liquid or gas — which is why a thermometer in a mixed ice-water bath reads 0 °C however long you wait.

A gas exerts pressure because its molecules are never still. Each molecule that strikes a wall bounces off, delivering a tiny impulse — a micro-push. The wall receives billions of collisions every second over every square centimetre, and the average of all those pushes is the steady pressure a gauge reads.

Two dials control that pressure. Temperature: hotter molecules carry more momentum and arrive more often, so each hit is harder and hits come more frequently — in a fixed volume the pressure rises. Volume: squeeze the container and the molecules cross to the wall in less time, hitting it more often — the pressure rises again. (The full quantitative gas laws sit beyond this syllabus; what is examinable is the particle-level explanation of both effects.)

Diffusion is the spreading of one substance through another by particle motion alone. Perfume molecules leave a bottle in random motion, collide millions of times with air molecules, and wander — on average — from where there are many to where there are few. No breeze is required: random motion is the delivery service. Diffusion is fastest in gases (huge gaps, fast particles), slow in liquids and effectively absent in solids, where particles cannot change places.