Wikijunior:Particles/Brownian motion

Apart from diffusion, there is another interesting phenomenon that can be observed. It is called the Brownian motion, discovered by a scientist called Brown.

Smoke cells and Brownian motion

Before we start on Brownian motion, let's look at an interesting experiment. A smoke cell is a small box full of smoke and air. If you have one (or your teacher gives you one), place it under a microscope. Otherwise, there is a short clip here. As you can see, the bits of smoke move at random directions, unassisted. Or do they?

Smoke particles are larger than air particles. That's why we can see the smoke particles better than the air particles. As you will learn later, gas particles always move at high speed, in random directions. The same things is happening to the air particles. When an air particle bombards a smoke particles, the smoke particle moves to the same direction as the air particle that hit it. When another air particle hits the smoke particle, it changes its direction to that of the second air particle, and so on. This is called Brownian motion.

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  What will happen to the small black particle on top and the large blue particle when they collide?
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  Their directions will change.

The story of pollen grains

A portrait of Robert Brown

The Brownian motion was discovered by, and named after, a botanist called Robert Brown. A botanist is a person who studies plants. In 1827, he was observing some pollen grains when he saw that the pollen grains were moving mysteriously. Do you know why the pollen grains were moving like that? The movement of the water particles causes that of the pollen grains, enabling them to move in random, constantly changing directions. This is what Robert Brown later figured out.

Quiz

1. Brownian motion is named after:
   1. Charlie Brown
   2. Gordon Brown
   3. Robert Brown
   4. John Brown
2. Which of the statements below best describes of movements of particles in a smoke cell?
   1. The smoke particles flow to places of higher concentration
   2. The air particles flow to places of lower concentration
   3. The smoke particles dodge the air particles, thereby causing the smoke particles to rotate
   4. The air particles hit the smoke particles, thereby causing them to move in random directions

Answers:

1. 3
2. 4