Cambridge IGCSE Physics 0625 – Particle Model (2.1.2)
Cambridge IGCSE Physics 0625 – Particle Model
2.1.2 Particle model – Objective
Know that the random motion of microscopic particles in a suspension is evidence for the kinetic particle model of matter.
Key Concepts
The kinetic particle model states that matter is made up of tiny particles (atoms, molecules or ions) that are in constant motion.
In solids, particles vibrate about fixed positions; in liquids they move past one another; in gases they move freely in all directions.
Random (Brownian) motion of particles suspended in a fluid provides direct visual evidence of this continual motion.
Brownian Motion – What is observed?
When a dilute suspension of microscopic solid particles (e.g., pollen, dust) is placed in a liquid or gas, the particles appear to jiggle erratically. This phenomenon is called Brownian motion.
Why Brownian Motion Supports the Kinetic Model
Microscopic particles are too large to be moved by visible currents; the observed motion must come from collisions with much smaller, invisible particles of the surrounding medium.
These invisible particles are the molecules or atoms of the liquid or gas, which according to the kinetic model are in constant, random motion.
Each collision transfers a tiny amount of momentum, causing the suspended particle to change direction randomly.
The cumulative effect of countless collisions produces the erratic path seen under a microscope.
Quantitative Description (Optional)
The mean square displacement ⟨x²⟩ of a particle undergoing Brownian motion in a time interval t is given by
\$\langle x^{2} \rangle = 2 D t\$
where D is the diffusion coefficient, related to temperature T, viscosity η of the fluid, and particle radius r by
\$D = \frac{k_{\mathrm{B}} T}{6 \pi \eta r}\$
Here \$k_{\mathrm{B}}\$ is Boltzmann’s constant. The dependence on temperature and viscosity further confirms that the motion originates from the kinetic energy of the surrounding molecules.
Summary Table
Observation
Interpretation in Kinetic Particle Model
Microscopic particles move randomly in a still fluid.
Invisible molecules of the fluid are colliding with the particles, transferring momentum.
Speed of motion increases with temperature.
Higher temperature → greater average kinetic energy of molecules → more energetic collisions.
Motion slows in more viscous fluids.
Viscosity dampens the effect of molecular collisions, reducing particle displacement.
Suggested Demonstration
Place a drop of milk (or a suspension of fine chalk) in a beaker of water. Observe the suspended particles under a low‑power microscope. Note the erratic, jittery paths they trace.
Suggested diagram: Sketch of a particle in a fluid showing arrows representing collisions from surrounding molecules.
Exam Questions – Practice
Explain why the random motion of pollen grains in water provides evidence for the kinetic particle model.
A student observes that Brownian motion becomes more vigorous when the temperature of the water is increased. Explain this observation using the kinetic model.
Describe how the viscosity of a liquid affects the observed Brownian motion and why.