Visualizing the Gaussian Distribution

Random Walks in Biology, by Howard Berg.

In Chapter 4 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I include a homework problem about the Péclet number.

At the macroscopic level the Péclet number is often large: drift dominates over diffusion. To get an intuitive understanding of how difficult it is to visualize diffusion at the macroscopic scale, consider what Howard Berg says in this excerpt from Random Walks in Biology.

I get the impression he did try it. Berg continues

Diffusion is an effective mechanism to transport molecules over short distances, but it works poorly over long distances. The intuition we develop in our everyday life can sometimes mislead us when we think about small-scale biological processes. We generally live in a large Péclet number world. Cells, bacteria, viruses, membranes, and proteins operate at small Péclet number, where diffusion dominates. We can’t trust our intuition when discussing life at low Péclet number.

Originally published at



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Brad Roth

Brad Roth


Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.