Brownian motion

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    15-Nov-2014
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Transcript of Brownian motion

  • 1. Suppose you are sitting in a big stadium, watching a game of football, being played between two equally good teams.Suppose the players are invisible: nothing except the ball is visible.Open your eyes once every 15 sec.What will you see?

2. Particle positions were recorded at intervals of 30 sec. 3. History of theBrownian Motion 4. Discovered byScottish botanistRobert Brown in1827 whilestudying pollensof Clarkia(primrose family)under hismicroscope 5. Robert Brownsmain claim to fameis his discovery ofthe cell nucleuswhen looking atcells from orchids20 orchid epidermal cellsshowing nuclei (and 3under his stomata) seen undermicroscopeBrowns original microscopepreserved by the LinneanSociety London 6. And Brownian motion of milk globules in waterseen under Robert Browns microscope 7. How would you understand this observation?(Remember, you are in 1827!) Would you suspect that the pollen is alive? Would you get excited at the thought that youmay have discovered the very essence of life or a latent life force in every pollen? What other experiments would you perform totest your suspicions? 8. He repeated his experiment with other fineparticles including the dust of igneous rocks, which is as inorganic as could be.He found that any fine particle suspended in water executes a similar random motion.This phenomenon is now called Brownian Motion. 9. Sizes (radius or diameter)Suspended particle: a few microns (10-6 m)Atom: 10-10 mWater molecule: somewhat larger Thus the suspended particle is a monster, about10,000 times bigger compared to a watermolecule. 10. Numbers: A spoonful of water contains about1023 water molecules. Speeds: They are perpetually moving in differentdirections, some faster than others. As they move, they keep colliding with eachother, which can possibly change their speedsand directions of motion. 11. Now you can very well imagine the fate of theparticle unfortunate enough to be placed in themad crowd of water molecules. The poor fellow isgetting hit, at any instant, from all sides, bymillions of water molecules. The net force on itkeeps fluctuating in time and it keeps gettingkicks in the direction of the net instantaneousforce. The end result is that its position keepschanging randomly. 12. Is "Brownian" movement more rapid for biggerparticles or smaller particles ? Explain How does Brownian motion depend ontemperature? Explain 13. Worked out aquantitative descriptionof Brownian motionbased on theMolecular-KineticTheory of Heat Published as the thirdof 3 famous three 1905papers Awarded the NobelPrize in 1921 in partfor this. 14. . 15. . 16. The Big Question: What is .the mass of air in theroom? 17. Thank you