2. 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?

4. Particle positions were recorded at intervals of
30 sec.

5. History of the
Brownian Motion

6.  Discovered by
Scottish botanist
Robert Brown in
1827 while
studying pollens
of Clarkia
(primrose family)
under his
microscope

8.  Robert Brown’s
main claim to fame
is his discovery of
the cell nucleus
when looking at
cells from orchids 20 orchid epidermal cells
showing nuclei (and 3
under his stomata) seen under
microscope Brown’s original microscope
preserved by the Linnean
Society London

9.  And Brownian motion of milk globules in water
seen under Robert Brown’s microscope

10.  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 you
may have discovered the very essence
of life or a latent life force in every pollen?
 What other experiments would you perform to
test your suspicions?

11. He repeated his experiment with other fine
particles 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.

12.  Sizes (radius or diameter)
Suspended particle: a few microns (10-6 m)
Atom: 10-10 m
Water molecule: somewhat larger
 Thus the suspended particle is a monster, about
10,000 times bigger compared to a water
molecule.

13.  Numbers: A spoonful of water contains about
1023 water molecules.
 Speeds: They are perpetually moving in different
directions, some faster than others.
 As they move, they keep colliding with each
other, which can possibly change their speeds
and directions of motion.

14. Now you can very well imagine the fate of the
particle unfortunate enough to be placed in the
mad crowd of water molecules. The poor fellow is
getting hit, at any instant, from all sides, by
millions of water molecules. The net force on it
keeps fluctuating in time and it keeps getting
kicks in the direction of the net instantaneous
force. The end result is that its position keeps
changing randomly.

15. ◦ Is "Brownian" movement more rapid for bigger
particles or smaller particles ? Explain
◦ How does Brownian motion depend on
temperature? Explain

16.  Worked out a
quantitative description
of Brownian motion
based on the
Molecular-Kinetic
Theory of Heat
 Published as the third
of 3 famous three 1905
papers
 Awarded the Nobel
Prize in 1921 in part
for this.

17.  .

18.  .

19. The Big Question: What is
 .
the mass of air in the
room?

20. Thank you