You're in a darkened room, the air is still and the bright lights are streaming through the window. What do you notice about the dust particles? The black circles as they pass through the beam of light. Pause the video and think about this.
Press play once you've made up your mind. The answer is that the dust particles are moving randomly. Did you see it?
The dust particles are not always moving downwards, but are moving in all directions. The dust particles are very small and are supported by a fluid. In this case, that fluid is a gas. You are unable to see the gas particles directly because they are too small, but you can see the larger dust particles.
The reason that the dust particles move randomly are because of the gas. We know that the gas is there because every time we move our hand through the air we fill them against our hand. Try it now.
Continue when ready. The particles in the air move in random directions quickly. This means they will have many collisions or will bump into the dust particles many times.
Each collision between air particles very small colored circles show how the dust particle, the black circle, moves off in a new random direction. It's a bit like a game of snooker, where there are millions of marbles which collide with larger, observable particles, like a ping-pong ball. This effect was first observed by scientist Robert Brown.
He was looking at pollen grains suspended in water under the microscope, and he wondered why they jiggled around. He thought that the pollen was alive. It was later that scientists realized the importance of his discovery. Now, here's a challenge for you. Thinking about what we know about dust particles, can you explain why the pollen grains appear to be jiggling randomly?
Pause the video and continue when ready. In summary, the fluid is no longer a gas, but is a liquid, water. The pollen grain is colliding with many water molecules.
Each water molecule is too small to see, but we can see the combined effect of their collisions on the pollen grain as it moves randomly. This effect is... known as, you guessed it, Brownian Motion.