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'[OT] Super Caps'
|On Fri, 6 Mar 1998 20:10:40 -0500 Sean Breheny <CORNELL.EDU> writes: shb7
Mostly the third variable in the capaictance equation: surface area.
Lots of surface area. They are made from activated charcoal, one of the
most "porous" materials known. An elecrolytic process creates a very
thin dielectric (of what? I don't know) over this huge surface, and a
very large capacitor is born.
Since carbon/charcoal isn't a very good electrical conductor, a
relatively large internal resistance results compared to something like
the aluminum used in standard electrolytic capacitors. In aluminum
capacitors, the surface is first etched to make a larger active surface
area, then a very thin dielectric (Al2O3?) is formed. I think that
advances in the etching process and controlling the thickness of the
dielectic to be uniformly just greater than the voltage rating are
responsible for the decreasing size of electrolytic capacitors. The
plates are seperated by a fiber soaked in conductive fluid. Since the
fluid is conductive, no electric field occurs across the seperator. It
just keeps metal-on-metal contact from happening and forms a liquid
"plate" right next to the thin dielectric. The capacitance effect
happens entirely across the thin Al2O3 layer. Carbon supercaps have a
similar liquid filling that permeates the carbon.
At 50F 2.5V, there would be
>whopping 125 coulombs of charge on each plate, an unheard of amount of
>charge in usually physics, enough to suck a statically charged comb
>through a solid brick wall from several meters away, if it weren't for
>oppositely charged plate, that is.
125 couloumbs of charge on a comb definitely spells "bad hair day".
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At 11:30 PM 3/6/98 -0500, you wrote:
>On Fri, 6 Mar 1998 20:10:40 -0500 Sean Breheny <CORNELL.EDU> writes: shb7
Sorry, but I still don't get it. I understand that the activated charcoal
has a huge surface area, but how do they get two conductive layers of
charcoal which are separated from each other?
Wow, I don't even want to think about having 125 C on any object close to
me!! Actually, what I meant was that even a normal comb, carrying something
like a few micro-couloumbs of charge, would feel a force of something like
2800 pounds of force at a distance of 3 meters from a 125 Couloumb charged
object! Of course, I realize that the oppositely charged plate in the
capacitor cancels this out, but it is still a scary thing to think about!
| Sean Breheny |
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