In this video, Professor Bowley from the University of Nottingham discusses how our understanding of Brownian motion has developed from the first recorded observations by scientists such as Robert Brown, to a detailed scientific explanation through the work of Einstein and Perrin.

 

Random force and Brownian motion

The video takes you through a history of our understanding of Brownian motion and also contains a model that shows why pollen grains move randomly on the surface of water. In the second half of the video, the science is more in-depth than is required at KS3. However, it is a great video to extend the more able.

 

Video: Random force and Brownian motion

 

Some questions you could ask your students after they have watched the video are:

 

Why did Robert Brown think pollen molecules were alive?

He used a microscope to observe them on water and saw that they moved.

How did he disprove his hypothesis?

He added ground up sand and dust to the water which are not living (inert). He saw these also moved so the movement of the pollen molecules wasn’t because they were alive.

Could Robert Brown explain his observations? Why/why not?

No, because he did not know that water contained molecules that moved. His microscope wasn’t powerful enough for this! This could be a useful opportunity to get students to discuss historical scientific equipment and the types of microscope scientists used in the past.

How does the model show Brownian motion?

The small brass beads are like the water molecules. The polysytrene bead is like the pollen molecule. When the machine is switched on, the brass beads shake around like the moving water molecules. They bump into the polystyrene bead and move it about until it reaches the side of the container. This imitates water molecules moving and hitting the pollen molecules so that they move around.

 

Further work:

 

To test their understanding of this model of Brownian motion, ask your students to complete the free resource: structured questions.