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Re: weird electrostatic effect



On Wed, 14 Aug 1996 jmclean@chem.ucsd.edu wrote:

William Beaty says:


Here's a strange thought: I wonder if the electrostatic repulsion effect
in the 3M factory was caused by a gradient in air pressure.

When tape peels from a spool, the tape becomes charged. The opposite
charge is deposited not on the spool, but instead as air ions. Think
about it. If you pull ten feet of tape off a spool, the tape is strongly
charged, but the small spool doesn't become hundreds of times more charged
than the large tape. The opposite charge is mostly ions.


+ + + + + + + + + + + + + + + + + + + + + + + + + +
- - - ---- -- --- - - -
- - - - ------ --- - - - -
- -- - -- ----- ---- ---- - - - -
- - - ---- -- --- - - -
- - - - - ---- - --- - - -
+ + + + + + + + + + + + + + + + + + + + + + + + + +

IONIZED AIR IN A CHARGED PLASTIC TUBE BECOMES PRESSURIZED?


My sense is that the ions attracted to the tent area would not increase the
pressure. They would displace the uncharged molecules, rather than add to
them. If the pressure were to get higher, then the uncharged molecules
would leave, re-establishing the normal pressure.
--
--James McLean


I think I'm visualizing ions differently. I see them behaving not as
individual particles but as a macroscopic substance, like an area of dyed
water existing in clear water. When an e-field is applied to the ionized
air cloud, individual ions do not move much. Instead the whole volume of
ion-filled air is dragged along. Here's an analogy. When water droplets
are added to air, the resulting fog acts as a heavy fluid which can be
poured into cups, flow in streams across a surface, etc. The droplets
respond to a gravity field, but they do not instantly settle out of the
air, instead they drag the air along with them and the combined material
acts like a high-mass gas. (See the TOUCH THE CLOUDS device on my
webpage!)

I'm imagining ionized air as being similar: when an e-field is applied,
individual ions cannot move much because they must fight against viscosity
forces and diffusion. But they *can* drag the whole volume of air along
with them. The resulting behavior would be that of a charged gas where
every molecule carries net-charge.

Of course this all depends on the field strength. By analogy, a water fog
which acts as a dense gas will behave differently if placed in a
centrifuge. The droplets will then behave as individual particles and
move rapidly through the air in response to the immense forces, and the
resulting material acts more like air which is (momentarily) full of heavy
raindrops.

Another analogy: ferrofluids. If a magnet is immersed into a volume
of ferrofluid (colloidial iron), the pressure near the poles of the
magnet is higher than elsewhere in the fluid. If a REALLY STRONG
magnet was used, the iron particles would be swept out of the fluid
and collect on the surface of the magnet, and the leftover fluid would
not have increased pressure. But for weak fields the ferrofluid acts like
liquid iron, even though there is only a small percentage of iron
by volume.

So for air ions and e-fields, there must be a range of low e-fields where
the ions cause the air to act as a charged fluid which moves as a unit.
And there must be a range of high e-fields which causes ions to push
through the air as individual particles which shove the uncharged air
molecules out of their way. What field strength divides these two
behaviors? I suspect that it's pretty high. If ionized air is placed in
the gap between capacitor plates, what value of P.D. will cause the ions
to move independantly through the air with velocity of a few cm/sec? Is
this value of e-field lots higher than the 10 or 20 KV/cm that might exist
inside the "tent?" I have no idea where to start in calculating such a
thing. Perhaps Millikan oildrop labs have some tables or references?

If ions in the tent move fairly slowly with respect to the air, and if
they are being resupplied as fast as they "settle out" onto the plastic
and are removed, then a pressure gradient might exist. In the volume
under the tent, the layer of charged air adjacent to the charged plastic
would be attracted to the plastic, and the layer of air on top of that one
would also be attracted. The outer layers would compress the inner layers
against the plastic. Far from the plastic the field in the tube will have
an axial component (I think!) and so the pressure gradient would also be
partly axial for positions not midway between the ends of the tube.


On 8/15/96, roger haar <haar@soliton.physics.arizona.edu> wrote:
*********************************************

This ion idea has some potential problems:

1.) What keeps the ions for recombining with the
charge on the tape?

1. Good point! The tape is moving at 1000 ft/min, so any ions trapped on
its surface would be rapidly removed and freshly charged tape would move
into view. But there would also be a cylindrical flow of air which might
cause turbulent mixing which keeps some ionized air far from the surface.
Also, the ions are battling viscosity and diffusion forces and *might* be
moving towards the tape only very slowly. And last, a huge quantity of
ionized air is being added to the system constantly, which would replace
any that were lost to the tape surface.

Uh... "potential" problems? :)

2.) On the pressurized tube idea, draw the field lines
in the tube. I think you will find that except at the ends
there is a constant potential inside so the field is zero.
As James McLean said, the neutral gas in the tube, interacts
with the charged ions enough so that the pressure does
not really increase. If enough ions could be poured into
the tube, these ions gain kinetic energy as they approach
the tube and thus slightly increase the temperature
inside the tube.

Thanks
roger haar


2. The field lines in the tube are not those for an empty, charged tube.
Ionized air acts as a virtual capacitor plate, and this negative "piston"
will be pulled into the positive cylinder. I think there would be a large
radial e-field, similar in shape to that of a capacitor made from a tube
containing a charged wire down its center. Although without the central
wire, the field would decrease to zero at the axis.

Interesting air-heating concept. Ion flow represents an electric current,
and ion-filled air is a resistor, so ionized air in an electric field
should experience direct electrical heating. The atmosphere also has a
vertical ion current, and I wonder how the output of this planet-wide
electric heating system compares to solar influx?! Might the temperature
rise in the air between the earth and a thunderstorm be significant or
easily measured? (The e-field below a thunderstorm is WAY higher than the
clear-weather field everywhere else.)


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