Table of Contents

• Prequiz...
• Just Plain Movement
• 1: Measuring movement
• 2: Directed vs. undirected movement
• 3: Measuring movement using flux
• Introducing Gradients
• 4: Gradients and diffusion
• 5: Watching diffusion happen (applet)
• 6: Visualising the Gradient
• Fick's Laws, Part I
• 7: Fick's First Law
• 8: The gradient in Fick's First Law
• 9: A familiar equation for Fick's First Law
• 10: Graphing Fick's First Law
• 11: How does flux depend on distance?
• 12: A fragrant example
• 13: General transport equations and Fick's First Law
• Review
• Review
• 14: Summary

Gradients and diffusion

Diffusion occurs through the process of random movement. We will see that, based on these random movements, particles demonstrate flux in a predictable direction. However, but this raises a tricky question: if movement is random, how can flux (net movement in a particular direction) occur? The key to understanding this apparent contradiction is the concentration gradient -- namely, a difference in concentration of particles between two areas.

In this next section, we are going to introduce the idea of how random movement and gradients interact to produce a predictable level of flux.