diffusion
physics
process resulting from random motion of molecules by which there is a net flow of matter from a region of high concentration to a region of low concentration. A familiar example is the perfume of a flower that quickly permeates the still air of a room.Heat conduction in fluids involves thermal energy transported, or diffused, from higher to lower temperature. Operation of a nuclear reactor involves the diffusion of neutrons through a medium that causes frequent scattering but only rare absorption of neutrons.
The rate of flow of the diffusing substance is found to be proportional to the concentration gradient. If j is the amount of substance passing through a reference surface of unit area per unit time, if the coordinate x is perpendicular to this reference area, if c is the concentration of the substance, and if the constant of proportionality is D, then j = -D1PT(dc/dx); dc/dx is the rate of change of concentration in the direction x, and the minus sign indicates the flow is from higher to lower concentration. D is called the diffusivity and governs the rate of diffusion.
- Francis Meres
- Francis Michael Forde
- Francis of Assisi, Saint
- Francis Of Meyronnes
- Francis of Paola, Saint
- Francis of Sales, Saint
- Francis Otto Matthiessen
- Francis Ouimet
- Francis Parker
- Francis Parkman
- Francis P. Blair
- Francis Picabia
- Francis Pilkington
- Francis Place
- Francis Planté
- Francis Ponge
- Francis Poulenc
- Francis Preston Blair, Jr.
- Francis Preston, Jr. Blair
- Francis P Shepard
- Francis Quarles
- Francis Rawdon Chesney
- Francis Rawdon-Hastings, 1st marquess of Hastings
- Francis Rawdon-Hastings Hastings, 1st marquess of
- Francis Reginald Scott