### diffusion

(the spreading of particles toward an even distribution)

**Diffusion** is the net movement of particles such as atoms or
molecules from areas of higher concentration to areas of lower
concentration.
A **diffusion equation** is an equation describing diffusion
over time. **Fick's laws** include a diffusion equation describing
what happens in many cases, a type of diffusion termed
**Fickian diffusion**. The equation:

dC
J = -D ——
dx

- J - the
**diffusive flux**, the flow of the diffusing substance as **amount of substance**/*unit time* through a *unit area*.
- D - the
**diffusion coefficient** (aka **diffusivity**) of the substance.
- C - the concentration of the diffusing material, as in
**amount of substance** per *unit volume*.
- x - measurement over a direction over which we are describing the diffusion.

**Amount of substance** is a measure of the number of particles
(atoms or molecules) of the substance typically counted in **moles**

(*physics,chemistry*)
**Further reading:**

https://en.wikipedia.org/wiki/Diffusion

http://mathbench.umd.edu/modules/cell-processes_diffusion/page09.htm

**Referenced by pages:**

atmospheric escape

diffusion damping

Eddington approximation

radiation zone

stellar evolution

two-stream approximation

Index