Science Practical 10

Brownian Motion

Aim: To observe Brownian Motion of smoke particles in the air.

Apparatus:

Whiteley's Smoke Cell Set

Transformer

Microscope

Procedure:

1. Fill a smoke cell, with smoke from smouldering paper and cover it with a glass slip.

2. Illuminate the smoke particles with a bright source of light.

3. Examine through the lower-powered microscope placed at 90 degrees to the direction of the light and record what is observed.

Observation:

Against a dark background, bright spots of light are observed, and they are in continuous random motion.

Questions:

What are these bright spots actually and why do they appear to be bright?

They are smoke particles and reflect light from the light source.

What causes the 'bright spot' to move in the continuous random manner?

The air particles bombard/collide unevenly on all sides of the smoke particle, causing the continuous random motion of the smoke particle.

Do you think that there will be a difference in the motion of the smokes particles if the temperature inside the smoke cell is increased?

Yes. The smoke particles will move more quickly as the movement depends on the temperature, as the heat energy is converted to kinetic energy.

How can you tell which are the large particles and which are the small ones?

The larger particles are the ones that move slower than the rest of the particles, as the larger particles have more inertia than the smaller particles.

Suggest how you could observe brownian motion in liquids:

Suspend pollen grains/talcum powder in water and observe the movement under a light source and a powerful microscope.