THE MILLIKAN OILDROP EXPERIMENT
REFERENCES
1.R.A. Millikan, The Electron (photocopied excerpts available at the Resource Centre).
2.Instruction Manuals for LeyboldHeraeus apparatus (available at the Resource Centre).
(2 weights)
INTRODUCTION
This
experiment
is
one
of
the
most
fundamental
of
the
experiments
in
the
undergraduate laboratory.
The experimental
apparatus is patterned after the original
apparatus, made and used by R.A. Millikan to
show that electric charge exists as integral
multiples of “e” the charge on a single
electron.
Historically, this experiment ranks
as one of the greatest experiments of modern
physics.
GETTING STARTED
This method first described in 1913, is
based on the fact that different forces act on an
electrically charged oil drop moving in the homogeneous electric field of a plate capacitor. When
the plate capacitor’s electric field intensity is
E
, the following forces act on a droplet of charge
Q
:
•
gravitational
force
m
oil
g
, where
m
oil
is the mass of the oil drop,
•
buoyant force
m
air
g,
where
m
air
is the
mass of air displaced by the oil drop,
•
electric force
QE
,
•
and, only if the droplet, considered in this case as a sphere, moves against the ambient air:
Stokes’ resistance force. Stokes’ Law states that for a spherical object of radius
r
moving
through a fluid of viscosity
η
at a speed
v
under laminar flow conditions, the viscous force
F
on
the object is given by
≈
F
6
π
r
η
v
. The viscous force always opposes the motion and is, of
course, responsible for the steady terminal velocities observed when a drop falls in air.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
2
We suggest
the following method to find the charge on an oil drop:
(i)
measure
the fall velocity
v
1
of a droplet in the free space (at zero voltage)
and
also
(ii)
the rise velocity
v
2
of a droplet at a definite voltage.
Write down the equation for a charged oil drop falling freely (in zero electric field) in air under
gravity and the equation for the same drop moving in the opposite direction due to an applied
electric field. Eliminate the radius of the drop from these two equations to obtain an expression
for the charge on the drop in terms of measurable quantities and the constants given below.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Summer '11
 Arubi
 Electron, Electric charge, Elementary charge, Millikan chamber

Click to edit the document details