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Brownian motion
Beginner
Random jiggling of small particles evidences molecular motion.
Pollen grains in water jitter randomly because molecules hit them unevenly. Einstein's 1905 analysis of this motion gave decisive evidence for atoms and let Perrin measure Avogadro's number.
Smoke particles dancing in a light beam.
The grain, not the molecules, is what we see.
In more detail
Brownian motion is the random jiggling of tiny particles suspended in a fluid, caused by collisions with fast-moving molecules. Einstein's analysis helped confirm the existence of atoms.
Real-world uses
- Pollen grains in water show Brownian motion under a microscope.
- Particle tracking in biology uses Brownian motion to measure diffusion.
- Pollen grains in water jiggle erratically under a microscope, a visible sign of countless molecular collisions.
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Frequently asked questions
Why does the motion never stop?
Because molecular collisions are constant and random.
Who explained it?
Einstein in 1905, linking it to molecular bombardment.
Who explained Brownian motion?
Einstein showed in 1905 that it arises from random molecular collisions, which provided strong evidence for the atomic theory.
Related topics in Kinetic Theory & Statistical Physics
Molecular picture of matter · Ideal gas assumptions · Pressure from molecular collisions · Kinetic interpretation of temperature