Brownian motion can be observed by looking at tiny pollen grains suspended in water under a microscope. What causes the pollen grains to move in a random, jerky path?
Direct collisions between the pollen grains themselves.
Convection currents arising inside the water container.
Constant bombardment of the pollen grains by much smaller, rapidly moving water molecules.
Electrostatic forces of attraction and repulsion between the pollen grains.
151 exam-style questions on CIE IGCSE Physics 2.1.2a Particle structure and motion in solids, liquids and gases. Each one has a worked solution and a mark scheme showing where the marks go.