Thermal physics · particle model

Brownian Motion and Einstein Diffusion Lab

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Change the model

Temperature293 K Higher temperature means greater average molecular kinetic energy. Tracer radius0.50 µm A smaller suspended particle is disturbed more strongly. Fluid viscosity1.00 mPa s A more viscous fluid produces greater drag. Conceptual magnification0% Reveals the molecular model; this is not a microscope scale. Display speed0.50× Changes playback only, not the calculated diffusion.

Measurement trail

Controlled comparison

Save two sets of conditions, changing one variable if you want a fair test.

Observed tracer

Observe first

Describe before explaining

Watch the visible speck. Its next direction is unpredictable, yet the motion continues.

Look for

Does it travel in a straight line? Does it eventually settle? Could you predict its next movement?

Observation: A visible suspended particle changes speed and direction irregularly.
Inference: Something too small to see is continually transferring momentum to it.

Check your particle model

In a real smoke-cell demonstration, which moving object can the microscope show directly?

What this model simplifies

The molecules and tracer are not drawn to scale. Collision frequency and visual time are exaggerated. The collision view explains the mechanism; the quantitative motion uses a two-dimensional diffusion model and the Stokes–Einstein relationship for a spherical tracer in a fluid.

Brownian Motion and Einstein Diffusion Lab

Mean-square displacement against time

What can this lab do?

Observe

Reveal

Compare

Measure

Observe random motion

Visible suspended particle

Mean-square displacement against time