Lecture on Brownian Motion

Introduction

• Observing dust particles in a room with bright light streaming.
  • Dust particles appear as black circles.
  • They move randomly, not just downwards.

Dust Particles and Gas

• Dust particles are small and supported by a fluid, in this case, a gas.
• Gas particles are too small to see directly.
• Dust moves due to random motion of gas particles.
• Gas presence felt through collision against our hand.

Random Movement

• Air particles move in random directions quickly.
• Frequent collisions with dust particles cause random movement.
• Illustrated as a game of snooker with numerous marbles colliding with larger particles like a ping-pong ball.

Historical Observation

• First observed by Robert Brown.
  • Noticed pollen grains jiggling under a microscope.
  • Initially thought pollen was alive.
  • Realized later that the effect was due to molecular interactions.

Brownian Motion

• Definition: Random motion of particles suspended in a fluid (liquid or gas) resulting from their collision with fast-moving molecules.
• Example: Pollen grains in water.
  • Water molecules are too small to see.
  • Observable effect is the pollen grain's random movement due to many collisions.
• Known as Brownian Motion.

Summary

• Understanding the random motion of particles helps explain phenomena observed in gases and liquids.
• Brownian Motion is a vital concept for explaining the behavior of small particles in fluid environments.