Physics · 2.1.1 Kinetic particle model · Brownian motion
Brownian Motion. Watch it.
Observe smoke particles (large, visible) jiggling randomly as they are bombarded by fast-moving, invisible air molecules. This is direct evidence for the kinetic particle model of matter.
0625 Topic 2.1.1 — States of matter
Kinetic particle model · evidence
Variables
25
6.0
100
What you observe
Smoke particles
large, slow, visible, random zig-zag
Air molecules
tiny, very fast, invisible
Mean molecule speed
~480 m/s
Effect of heating
faster jiggle
Random motion of the visible smoke particle is caused by unequal bombardment by many fast, light air molecules.
📋 Explanation (Cambridge)
- Smoke particles are large enough to see under a microscope but light enough to be moved by molecular collisions.
- Air molecules are too small to see, but move very fast in random directions.
- At any instant, more molecules strike one side of a smoke particle than the other, so it gets a tiny push — in a random direction. This repeats, giving the jerky, random zig-zag path.
- Heating makes the molecules move faster, so the smoke particles jiggle more vigorously.
- This is experimental evidence that matter is made of tiny, fast-moving particles.
🧪 Apparatus
- Smoke cell with glass cover
- Bright lamp + converging lens to light the smoke from the side
- Microscope
- Smoke source (smouldering straw/taper)
🎯 Syllabus reference (0625)
- 2.1.1 States of matter / kinetic model — describe and explain Brownian motion as evidence for the kinetic particle model; relate the random motion of visible particles to bombardment by fast-moving molecules; describe the effect of temperature on molecular speed.