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(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:

J = -D ——

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

Further reading:

Referenced by pages:
atmospheric escape
diffusion damping
Eddington approximation
radiation zone
stellar evolution
two-stream approximation