Particle Size and Diffusion 

Microscopic particles move randomly in a variety of media, with velocities and displacements directly proportional to the absolute temperature and inversely related to medium viscosity .  Their diffusion rates also vary with their size.

In the simulation of the facing page, a small red particle and a much larger blue one are moving randomly.  The history of their diffusion is mapped, respectively, by the red and blue lines.  Start the simulation by moving the cursor into the box, stop it by moving the cursor out, and consider the questions below the simulation.

Of course, the behavior of single particles is nearly impossible to observe, and only rarely do diffusing particles leave trails behind them.  To observe the diffusion of a large number of differently sized particles, move to the next section.