Re: weird electrostatic effect

[William Beaty <billb@eskimo.com>]

On Fri, 23 Aug 1996 jmclean@chem.ucsd.edu wrote:

> Regarding ions in air, water droplets in fog, and ferrofluids near
> magnets...
>
> Could there be a confusion here between density and pressure?  Water
> droplets in air can be treated as 'heavy air' with a higher density, but
> the pressure is not increased.  

Yes, the pressure is increased.  It's increased BECAUSE the density is
increased, and because the fluid is in a gravitational field.  The rise is
a tiny fraction of 1 ATM, but is is not zero. Think of it this way: water
is denser than air, and if you put a barometer at the bottom of an empty
swimmingpool and then fill the pool with water, the measured pressure goes
up fast!  Fog-filled air is only slightly denser than air, so the pressure
at the bottom of a swimming pool full of fog would only be slightly high.


> I know next to nothing about ferrofluids,
> but are you sure that near magnetic poles they have a higher pressure?

Yes.  Forces in a fluid are pressures.  Hold an iron object in your hand
and place a powerful magnet on the back of your hand, and you feel a
compression force squeezing your hand.  Replace the iron object with
ferrofluid and you feel the pressure.  Stick your hand into a bowl of
ferrofluid and try to touch an immersed magnet pole.  You'll feel the
magnet's attempt to attract the ferrofluid and expel your finger as an
increasd pressure directed against your finger. 

> In any case, the reason water droplets make 'dense air' is that each
> droplet is quite heavy compared to a air molecule.  Ions, on the other
> hand, have (basically) the same mass as the air molecules, so I wouldn't
> expect a density increase either.

Correct, but I was switching back and forth between analogies.  Water-fog
lying as a pool against a surface in a gravity field will have increasing
pressure with depth.  Charged air lying as a pool against a surface in an
e-field will have increased pressure with proximity to the surface.  I was
using the fog droplets as a mass/g-field analogy to explain my
expectations about charged air and its charge/e-field behavior.  The
charged film should act like a "planetary mass," should attract an
"atmosphere" of charged air to itself, and an intruding object having zero
net-charge should be pushed away from the charged object.  It would be
"bouyant" and float to the "top" of the mass of charged air.

This now has me thinking.  What about induced charge on the human? 
Wouldn't the charged film attract the air AND the human, and so there
would be no perceived repulsion against the human?  Wouldn't it be like
sticking an iron "finger" into the ferrofluid near the magnet pole, and
having the ferrofluid ant the finger attracted equally? Maybe not, because
in the b-field analogy the iron and the ferrofluid are both attracted by
induction, so a field gradient is required.  If there isn't a large
e-field gradient under the "tent" of charged film,, then an uncharged
object (the human) wouldn't be attracted by induction, but the charged
air WOULD be attracted, and the human would be forced to the outside of
the area of charged air.  Pushing your body into the "invisible wall"
would be like pushing styrofoam down into water. 

> How about this for an explanation: as you walk into the ionized region, you
> charge up from contact with the air.  Then there is an electrostatic
> repulsion between your body and the air.

....and an attraction between your body and the charged film.  But the
net repulsion might be significant.

> As far as whether charged air would build up, couldn't we figure that from
> the mean free flight time for an air molecule?  With a guess at the
> electric field, you could figure the acceleration during free flight and
> from that get how fast the molecule is pushed along through the air.  Hmmm,
> where is that thermo textbook....

Right, this should work.  Though I've heard it said that ions quickly
attach to aerosol and smoke particles.  This would give a population of
"slow" ions.

.......................uuuu / oo \ uuuu........,.............................
William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
EE/Programmer/Science exhibit designer        http://www.eskimo.com/~billb/
Seattle, WA 98117  billb@eskimo.com           SCIENCE HOBBYIST web page