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Diffusion
Intermediate
Spreading of particles from high to low concentration.
Particles spread from high concentration to low by random motion. The mean squared distance grows linearly with time: ⟨x²⟩ = 2Dt.
Perfume spreads slowly across a room by diffusion, much faster by convection.
Diffusion is slow over large distances.
In more detail
Diffusion is the net movement of particles from high to low concentration, caused by random molecular motion. It is slow over long distances but important in biology and chemistry.
Real-world uses
- Oxygen diffuses from the lungs into the blood.
- Perfumes spread across a room by diffusion.
- A drop of food dye spreads through water over time, which is diffusion driven by random molecular motion.
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Frequently asked questions
Is diffusion the same as flow?
No. Diffusion is driven by concentration differences, while flow is bulk motion.
Why is diffusion slow?
Molecules move randomly, so net progress over distance takes time.
Does diffusion need heat?
Diffusion happens at any temperature, but it speeds up when the particles move faster at higher temperature.
Related topics in Kinetic Theory & Statistical Physics
Most probable speed · Average, rms and most probable speeds · Law of atmospheres · Fick's law · Graham's law of effusion · Real gases