Re: Unusual capacitor dielectrics

• To: tesla-at-pupman-dot-com
• Subject: Re: Unusual capacitor dielectrics
• From: Jim Lux <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
• Date: Thu, 05 Nov 1998 17:36:55 -0700
• Approved: terryf-at-verinet-dot-com

Tesla List wrote:
> 
> Original Poster: Jacob Roberto <jroberto-at-ieway-dot-com>
> 
> I've noticed that pure water has a dielectric constant of 80 (about 40 times
> higher than that of polyethylene). Would it be practical to build capacitors
> that use water as the dielectric? Such a capacitor could probably be 

Water is often used as a dielectric in high energy pulse forming lines
and in pulse forming networks. While water has a very high dielectric
constant, it has zero DC resistance. For fast pulses (nanoseconds)
though, it works fine. The problem is that the H+ and OH- ions gradually
move so a current does flow. If the current is high enough frequency,
then, the ions never get to the other side.

Typical resistivities of deionized water (the stuff at the supermarket)
are in the 2 Megohm cm range. If your capacitor plates were, for
example, 1 mm apart, and they were, for example, 10x10 cm, the
resistance between the plates would be 2E6 * .1/(10 *10) or 200 Kohms. 
The impedance of the same capacitor at 200 kHz would be about 100 ohms =
1/(2*pi*f*c) C = (.088 * 80 * 100/.1 = )7 nF, so it wouldn't be too
lossy from a straight resistivity standpoint.  It is that close plate
spacing and the large surface area of a typical cap that kills you.


-- 
Jim Lux                               Jet Propulsion Laboratory
ofc: 818/354-2075     114-B16         Mail Stop 161-213
lab: 818/354-2954     161-110         4800 Oak Grove Drive
fax: 818/393-6875                     Pasadena CA 91109

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