Brownian Motion | OCR A Level Physics Revision Notes 2015

Overview of Brownian Motion

• Definition: Brownian motion refers to the random, erratic movement of small particles like pollen or smoke when suspended in a liquid or gas.
• Observation: These movements can be seen under a microscope, providing evidence for the existence of molecules in gases or liquids.
• Random Motion: Particles exhibit a range of speeds and no preferred direction, indicating random motion.

Characteristics of Particles in Brownian Motion

• Particle Size: Observable particles (e.g., smoke) are larger than the unseen molecules causing their movement.
  • Smoke particles: Large and heavy.
  • Air molecules: Small, light, and fast-moving.

Implications of Brownian Motion

• Collisions: Larger particles change speed and direction due to collisions with smaller, faster molecules.
  • Provides insights into molecular behavior in gases, especially related to pressure.
• Momentum Transfer: Small molecules transfer momentum to larger particles during collisions, explaining the random motion.

Significance in Physics

• Brownian motion is a key phenomenon that supports the kinetic theory of gases.
• It demonstrates molecular activity and helps in understanding gas behavior and pressure contributions.

This simple yet profound observation of particle motion under a microscope serves as a fundamental evidence for the kinetic theory, showing how unseen molecules affect visible particles and how gases at a molecular level operate.