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Note:
The object is to produce a small number of
individual oil droplets, not a large bright cloud of
them since you only need to select one droplet.
Squeezing the bulb more than once will mean
that there are too many droplets in the
measurement cell, particularly in the area
between the microscope and where it is
focussed. Drops there obstruct observation of
droplets which are in focus. A suitable droplet will
appear as a brightly lit point in the focus of the
measuring microscope.
8.1 Floating method
The voltage U which keeps the drop floating still
is determined along with the velocity v
the drop descends after the voltage is turned off
x
S
v
.
2
t
V t
2
2
t
: Time to descend, S: Separation of scale
2
positions, V: Objective magnification (2x)
The radius r
and charge q
0
determined from the equilibrium level of
electrical force, buoyancy in air, Stokes' friction
in air and the force of gravity:
9
v
2
r
0
2
g
2
1
3
d
2
v
2
q
9
0
U
2
1
η: Viscosity of air, ρ
: Density of oil, ρ
2
of air, g: Acceleration due to gravity, d:
Separation of capacitor plates (3 mm)
Select the polarity of the voltage U, e.g. top
plate "+", bottom plate "–".
If times t
and t
1
2
previously, they can be reset to zero by
pressing "Reset".
Describe, make, observe and select a
suitable droplet as described in section 7.3.
Set the voltage and timer switches U and t
to ON and ignore the running time t
Set up a voltage which causes the selected
oil drop to "float" still at a given scale
marking.
Read off the voltage U from the display and
make a note of it.
Set voltage switch U to OFF. This allows the
observed drop to descend. Measurement of
time t
starts automatically.
2
8. Experiments
at which
2
(1)
of the oil drop are
0
(2)
3
.
(3)
g
: Density
1
have been saved from
.
1
If too much oil has entered the measurement
cell, it will need to be cleaned. If no droplets
appear in the measurement cell, even after the
bulb has been squeezed multiple times, it may
be that the opening in the top capacitor plate
has blocked up. Then it would need to be
cleaned too.
Set timer switch t to OFF as soon as the
drop has reached a scale position decided
in advance. This stops the measurement of
t
.
2
Read off the time t
make a note of it along with the separation
of the two scale positions.
Repeat the measurement as often as
possible for different oil drops, changing the
sign of the voltage U as you do so.
8.2 Rising/falling method
For a selected value of voltage U, the velocity
with which a drop rises v
which it descends v
x
S
v
,
v
1
2
t
V t
1
1
t
: Rise time, t
: Fall time, S: Separation of scale
1
2
position, V: Objective magnification (2x)
The radius r
and charge q
0
determined from the equilibrium level of
electrical force, buoyancy in air, Stokes' friction
in air and the force of gravity:
9
v
2
r
0
2
2
1
d
q
9
v
0
1
U
η: Viscosity of air, ρ
of air, g: Acceleration due to gravity
Select the polarity of the voltage U, e.g. top
plate "+", bottom plate"-".
If times t
and t
1
previously, they can be reset to zero by
pressing "Reset".
Describe, make, observe and select a
suitable droplet as described in section 7.3.
4
from the display and
2
and the velocity with
1
are both determined
2
x
S
.
(4)
t
V t
2
2
of the oil drop are
0
(5)
g
3
2
v
2
v
.
(6)
2
g
2
1
: Density of oil, ρ
: Density
2
1
have been saved from
2
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