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Elementary Science Education

# Question regarding Brownian motion. Error in my explanation

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Bernard Nebel
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If we can’t see a desk jiggle, why can we see the milk jiggle when the drop is still thousands (millions?) of atoms? Is it because the fat (or oils) and the water don’t mix? Or is it something else? I’ve read the page several times and watched several videos but I’m just not sure. (Shann Alipour)

Thank you for your question Shann; it illustrates good thinking.

A single unattached atom/molecule travels in one direction until it is hit or hits something (another molecule or hard object) that causes it to bounce off in another direction. Picture balls on a pool table bouncing around in continuous motion (kinetic energy). Now picture a light cardboard box sitting in the middle of the pool table with the balls bouncing around. The box will be jarred around in one direction then another as it is hit randomly by the pool balls traveling in different directions.

This box is your particle of milk fat; the pool balls are the surrounding water molecules. The jiggling of the box is caused by its being hit randomly on its different sides.

Note the jiggling of the particle is caused by its being randomly bombarded, not by jiggling of its own atoms. Yes, if all the atoms in the particle, box, or desk moved the same direction at the same instant, we would see the box/desk jump on its own. However, as you point out, its being made of millions of atoms and all going in different directions the result equalizes out and the object stays still.

Please don’t hesitate to post further questions where you are puzzled.

Dr. Nebel