IGCSE Physics - 2.1.2 Particle Model - Brownian Motion
IGCSE Physics 0625
2.1.2 Particle Model
Objective: Describe and explain Brownian motion in terms of random molecular bombardment
Brownian motion is the random, erratic movement of particles suspended in a fluid (liquid or gas). This phenomenon provides strong evidence for the particle model of matter, which states that all matter is made up of tiny particles in constant, random motion.
Understanding Brownian Motion
The random movement of the particles in the fluid is caused by the constant bombardment of the suspended particles by the much smaller, invisible molecules of the fluid.
Consider a suspended particle in a liquid. The liquid molecules are constantly moving and colliding with the suspended particle from all directions. Although these collisions are tiny individually, the sheer number of collisions, occurring randomly, results in a net, jerky motion of the suspended particle.
Explanation in terms of Random Molecular Bombardment
Random Motion of Fluid Molecules: The molecules in the fluid are in constant, random motion due to their thermal energy.
Collisions with Suspended Particles: These moving fluid molecules collide with the suspended particles.
Uneven Force Distribution: Because the fluid molecules are moving randomly, the force exerted on the suspended particle during each collision is often unevenly distributed.
Net Jerky Motion: The cumulative effect of these unevenly distributed forces results in a net, jerky, and unpredictable motion of the suspended particle – this is Brownian motion.
Factors Affecting Brownian Motion
The degree of Brownian motion depends on several factors:
Size of the Suspended Particle: Smaller particles exhibit more noticeable Brownian motion.
Viscosity of the Fluid: Fluids with higher viscosity (e.g., honey) result in less Brownian motion compared to fluids with lower viscosity (e.g., water). This is because the higher viscosity fluid offers more resistance to the movement of the suspended particles.
Temperature of the Fluid: Higher temperatures mean the fluid molecules have greater kinetic energy and move faster, leading to more frequent and forceful collisions, and thus more pronounced Brownian motion.
Diagram
Suggested diagram: A small particle (e.g., pollen grain) suspended in a liquid, with arrows indicating the random motion of the fluid molecules colliding with the particle.