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
• 14: Summary

Visualising the gradient

We saw in the last page that diffusion tends to spread things out, making them less "ordered". A central concept of order then is the “gradient”, a fancy way of saying the difference in particle density between two areas. If there is only a slight difference, then the distribution changes only slightly. But if all the molecules are concentrated on one side, then the distribution changes a lot over time.

Another way to say this is that the NET movement of particles is PROPORTIONAL to the difference between the particle density of two sides – or, proportional to the “steepness” of the gradient.

Which gradient is steeper?

Check your answer by pointing with your mouse.

or

 

So, steeper gradient means faster diffusion.