Slide 10 of 32
Notes:
Just so that you don’t sit through the entire talk scratching your head about that 1/r:
In a problems with cartesian geometry, a steady state diffusion problem would lead to a linear concentration gradient, and the flux would be constant with distance.
In a spherical steady-state problem, the same amount of substance must pass through ever-decreasing concentric spheres. This means the flux (per unit area) must increase, and thus the gradient must increase as well.
