Dancing Particles
Spot the evidence that particles never stop moving
What this lesson covers
Why it matters
Shine a torch through still water and watch dust specks jitter and zig-zag for no visible reason. They are being knocked about by countless invisible, fast-moving water particles.
Predict first
Why do tiny dust specks in still water jiggle on their own?
This is Brownian motion: water (and air) particles are always moving and colliding, knocking the specks about. It is direct evidence that particles are in constant motion.
- Invisible moving particles keep bumping into them — correct
- The specks are alive
- The water is secretly flowing
What you do
Sort each everyday clue: does it show particles are constantly moving, or not?
Check yourself
Brownian motion is best described as…
Invisible molecules constantly knock the visible specks in random directions.
Brownian motion is evidence that particles are…
Only constantly-moving molecules could keep knocking the specks about.
- the random zig-zag motion of tiny specks bombarded by moving molecules — correct
- particles slowly dissolving
- particles melting
- constantly moving — correct
- completely still
- alive