PROFESSOR: Electrostatic precipitation
demonstrates how air ions generated by a corona discharge
can be used to charge macroscopic particles
and how those particles can then be removed from the air
by the applied electric field.
Although it's made of glass so that we can see through it,
this cylinder is coated on the inside by tin oxide
so that it is conducting.
For the present purposes, it can be regarded
as a perfect conductor.
It's grounded through this microammeter.
This fine wire is stretched along the axis of the cylinder.
It's connected to a positive source of high voltage.
Because we only require modest currents,
we can protect ourselves by having
a series resistance to limit the short circuit current.
The voltage is indicated by this meter.
Because the wire is so fine, raising its voltage
results in ionization of the air,
in a hissing electrical breakdown.
This discharge is confined to the immediate vicinity
of the wire.
With the help of this microphone,
we can hear this corona discharge
as it starts, as the voltage is raised.
[CLICK]
[WHIRRING]
[HISSING]
[CLANK]
It starts at just a little more than 10 kilovolts.
[CLICK]
[WHIRRING]
[HISSING]
[CLANK]
Once charged, the smoke particles
are themselves pulled toward the outer electrode
by the electric field.
Let's see this happen.
With the voltage off and the upper end of the cylinder
closed off, we'll fill it with smoke.
Watch the smoke as the voltage is applied.
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The particles are presumably charged in less than a tenth
of a second.
So the charging time cannot account for the few seconds it
takes to precipitate the smoke on the wall.
[CLICK]
Apparently, the smoke particles do not move through the air
with nearly the velocity of the ions, which
we estimated to be on the order of 20 meters per second
or more.
[CLICK]
What we see at relatively large particle densities
is particle motions that are complicated
by the motion of the air itself, induced by both
the ions and the particles.
This is typical or practical precipitators,
which are used for air pollution control purposes.
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The macroscopic particle motions can be made more evident
by using Styrofoam particles, rather than the smoke.
Watch what happens as they encounter
the combination of radially migrating irons
and radial electric field.
[CLICK]
As long as the field is on, the particles
are electrically pinned to the wall.
But when the field is removed, [CLICK]
so is the pinning force.
As purchased, these particles are good insulators.
[CLICK] They're naturally sufficiently insulating,
that they can contact a conducting
surface like the cylinder wall and still retain
most of their charge.
If the charge were retained in the particles,
we would expect that they would stick to the wall
after the field is removed.
To understand what we've seen, we
need to explain that these particles have been sprayed
with antistatic agent, much as used to prevent electrification
of laundry.
So their charge leaks away after the field is removed.
[CLICK]