# 6.021/Notes/2006-09-12

### From OpenWetWare

## Review

- Fick's first law:
- Flux is proportional to the concentration gradient

- Continuity equation:
- A flux gradient leads to change in concentration over time

- Diffusion equation:
- Steady state solution: time-invariant ()
- φ(
*x*,*t*) = φ_{0}

- φ(
- Equilibrium: Zero flux + time invariant
- φ(
*x*,*t*) = 0,*C*(*x*,*t*) = constant (uniform distribution)

- φ(
- Impulse response: Gaussian