1. What Brownian motion shows
The smoke particle moves in a random, zig-zag path. This shows that air molecules are moving
randomly and are constantly colliding with the smoke particle from different directions.
2. Why the smoke particle changes direction
The smoke particle is much larger than an air molecule, but it is hit by many air molecules. The collisions are
not equal on all sides at every instant, so the smoke particle is pushed in different directions.
3. How gas pressure is produced
Air molecules collide with the walls of the container. Each collision exerts a tiny force on the wall.
Because there are many molecules moving quickly and colliding very frequently, the total force
on the wall becomes large. Pressure is force per unit area.
Try this:
Increase the number of air molecules. What happens to the number of wall collisions and the pressure effect?
Increase the molecular speed. Why does the pressure effect increase?
Exam-style answer
(a) The air molecules are moving randomly and continuously.
(b) The air molecules move at high speed and collide with the walls of the container.
Each collision produces a small force on the wall. Since there are a very large number of molecules,
there are many collisions every second, so the total force on the walls is large. Hence, a large pressure is produced.
Teaching note: air molecules are shown here only to help students visualise the idea. In the actual Brownian motion experiment,
the air molecules are too small to be seen with the microscope.