Fick's 1st/2nd Law: Explaining the Sign Difference

[asdf1]

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

 

[asdf1]

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

 

[mybsaccownt]

hmm

far from an expert, but here is my best guess

for your first question:

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
for your second question:

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,

 

[asdf1]

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

 

[mbida]

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

 

[mbida]

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