Re: Charge stored in Dielectric? Not really - MISCONCEPTION

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 


On Mon, 26 Jan 2004, Tesla list wrote:
 > Original poster: Esondrmn-at-aol-dot-com
 > Dan,
 >
 > I applaud your testing.  Real world testing beats conjecture any day.
 >
 > I am confused by your conclusion however.  This test has been performed
 > countles times in college physics and engineering labs.  The charge must be
 > stored in the dielectric.  It can't be stored in the metal plates,
 > conductors can't store charge.  You can disassemble the capacitor and
 > install new plates and still have a charged cap.

The innards of the dielectric are only polarized, not charged. The
dielectric doesn't even have to touch the metal endplates for this to
happen. Its surface isn't charged.
The actual charge is evenly distributed accross the endplate surfaces
(because the plates aren't shorted together, and there's a potential
difference between them, and the plates are in parallel). When you do
work remove the dielectric, the potential difference rises. Sort of,
like, the space (dielectric etc) between the plates contains all the
energy, but the plates carry the potential and thus charge. If this makes
any sense ;)

Ok yeah a dielectric can build up internal charge from electron
bombardment and so on, but then the material degenerates. For example in
microchips mosfet gate oxides may degenerate this way and ultimately
break down.

The surface of the dielectric can be charged, though, and remain charged
for quite long if there's no path for the charge to bleed off. To
demonstrate this, during a dry winter day, grab a cat and rub it accross
a wool carpet. :)

(i lost the initial thread from Dan so I don't know his low voltage
experiment, but it could be a 'too little energy stored to be easily
measured' thing...)

cheers,
   - jfw

--
****************************************************
  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering
  http://www.hut.fi/~jwagner/ - jwagner-at-cc.hut.fi