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Phys. 369, Fall 2009
Homework #9
 Revised c
(same as b except for extra help on problem 3)
Due: Fri. Nov. 13, beginning of class
100 points total
1. (14 points) Suppose that you have a sample of fused silica beads with a diameter of exactly
0.500 microns. Suppose that you wish to measure the Brownian motion of these beads while
suspended in water at a room temperature. Water has a viscosity
η
= 0.010 g cm
−
1
s
−
1
.
a) Go to
http://wien.cs.jhu.edu/AnnusMirabilis/index.html#common
and navigate your way
from there to an English translation of Einstein’s 1905 paper “Investigations on the theory of the
Brownian movement.” Eq. (7) of that paper gives an expression for the diffusion constant
D
of a
small sphere embedded in a fluid of known viscosity and temperature. Read back through the
preceding pages until you understand the meaning of each symbol in eq. (7). Rewrite eq. (7)
using the notation of the book section by Tipler.
Also, read the section “FORMULA FOR THE MEAN DISPLACEMENT OF SUSPENDED
PARTICLES. A NEW METHOD OF DETERMINING THE REAL SIZE OF THE ATOM” on the last
two pages. This section gives the key result of the paper. You do not need to understand
Einstein’s method of derivation of this result, since it is much more difficult to follow than
Langevin’s method, reproduced in the book section by Tipler.
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This note was uploaded on 11/06/2010 for the course PHY 369 taught by Professor Staff during the Fall '08 term at University of Texas at Austin.
 Fall '08
 Staff
 Thermodynamics, Work

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