# Fick's 1st/2nd law

1. Sep 18, 2006

### asdf1

Why does Fick's 1st law have a negative sign in the equation, but Fick's
2nd law doesn't?

2. Sep 18, 2006

### asdf1

also one more question:
Why's the D in Fick's 1st law a constant, but a variable in Fick's 2nd law?

3. Sep 24, 2006

### mybsaccownt

hmm

far from an expert, but here is my best guess

in the 1st law, we're looking at steady state diffusion, and how the concentrations vary with respect to position

so, the concentration of the other substance will be decreasing as you go further and furthery away from it, hence the negative sign

in the 2nd law, we are looking at concertration with respect to time and it may be increasing or decreasing

the 1st law is used for steady state diffusion (concentration does not change with respect to time), that's why D is a constant

2nd law is used for non-steady state diffusion, where the concentration does depend on time,

4. Sep 25, 2006

### asdf1

wow! thank you very much!!! i've been thinking about that question for many days...

5. Jan 2, 2010

### mbida

what kind of diffusion constants do you know? what physical parameters does the diffusion constant depend on?

6. Jan 2, 2010

### mbida

what are the boundary condition of the validity of fick's first and 2nd law?