DOC.
16
129
1.
a
motion
of the
suspended
substance under
the influence
of
the force
K
which
is exerted
on
each
suspended
particle,
2.
a
process
of
diffusion,
which
is
to
be
conceived
as
the result
of
the
random
motions of the particles
due to
thermal molecular motion.
If
the
suspended
particles
are
of
spherical
shape
(where
P
is the
radius of the
sphere)
and
the coefficient of friction of the
liquid
is
k,
then the force
K
imparts to
the individual
particle
the
velocity1
K
and
vK
6
irkP
particles
pass
through
the unit
cross
section
per
unit
time.
Further, if
D
denotes the
coefficient
of
diffusion
of the
suspended
substance
and
u
the
mass
of
a
particle, then
D
¥*-gram
or
-I)
dv
"
li
particles will
pass
through
the unit
cross
section
per
unit
time
due to
diffusion. Since there should
be
dynamic
equilibrium,
we
must
have
(2)
bTep-'rz-0-
From
the
two
conditions
(1)
and
(2)
found
for
dynamic
equilibrium
we can
calculate
the
coefficient of diffusion.
We
obtain
1)
~
RT
1
1Cf.,
e.g., G.
Kirchhoff,
"Vorlesungen
über
Mechanik"
[Lectures
on
Mechanics],
Lecture
26,
§4.
[15]