Fick's law of diffusion

1. Oct 24, 2009

jjmclell

Hi there,

I'm trying to wrap my head around Fick's first law of diffusion (for one dimension):

$$J_{x} = -D \frac{\partial \phi}{\partial x}$$

I understand that $$\phi$$ is the concentration in units amount/volume and that $$x$$ is position on the gradient in units length. What I don't understand is why $$-D$$ is in units area/time. If we're talking one dimensional diffusion, why do we bring area into the equation? Or, put another way, why do we square length in the diffusion coefficient?

Thanks,

jjmclell

2. Oct 24, 2009

Andy Resnick

It's in units of area/time because it represents the flux of material through a (control) surface