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Re: pure water capacitor?/+Baking soda

On Wed, 16 Feb 2000 06:42:43 -0700, Tesla List wrote:

>  Original Poster: "Mark Broker" <broker-at-uwplatt.edu> 
>  
>  I'm a very new member to the TC community.  I've been interested in
>  building one, or two for quite some time, but have never had the
>  opportunity ($$$, tools, ect) until recently.
>  
>  I'm a senior majoring in Engineering Phyisics (don't ask....) and am
>  involved with the SPS (Society of Physics Students).  The SPS
occasionally
>  gives physics demos to local schools.  We have a POS 250kV 1/2 wave TC
that
>  must be 20 years old.  I thought that a nice medium-sized 1/4 TC might
make
>  an excellent "gift" to the SPS as a going-away gift.
>  
>  I've been trying to do some research regarding design issues, when I came
>  across a list of dielectrics for some materials for use in capacitors. 
The
>  dielectric constant for pure water is around 85 with an infinite
breakdown
>  voltage!  This means that two 12" x 12" plates spaced 1/8" apart will
give
>  a capacitance of .020uF!  I thought it would be a pretty sweet idea: a
pure
>  water cap in a plexiglass (or lexan (?)) box.  But, I have NEVER seen
this
>  mentioned anywhere I've looked.  The only problem I can see is
>  contamination: everything has to be 110% clean before adding water.  This
>  is do-able.
>  
>  Are there any reasons why I shouldn't try to use this?
>  
>  For the record, I'm looking at a 15kV/120mA+ NST.
>  
>  Thanks
>  
>  Mark
>  
Better try freezing it first, in a liquid state it has a low dielectric 
breakdown value, not a high as you seem to believe. The practical dielectric
constant that is achievable is 40, not 80. A 10%
increase of this value is noted on freezing. Distilled water rapidly
absorbes atmospheric gases upon contact with the environment.I made a 4 inch
by 6 in tupperware container of baking soda added to water and when frozen
the aluminum foil frozen in the ends with a 6 inch separation between 2X4
inch plates produced a capacitance of 166 pf. During the time the sample was
in a liquid state the Wavetech LM22A capacitance meter would not give a
reading due to the large leakage.
(it blinks giving different values) I had formerly attemted to measure
capacities in a pure water electrolysisor experiment, and addition of baking
soda seemed to increase the capacity so it interested me to freeze this
baking soda water. This container has been outside during freezing and
melting and some of the baking soda has been washed out from rains. It now
reads 120 pf in freezing conditions lending creedance to the theory that
addition of baking soda increases the capacity of ice. Although this is an
unusual geometry for determining the dielectric constant since this 6 inch
distance between the plates seems great to the meager 8 sq inches of plate
area, nevertheless classical equation predicts a dielectric constant of
around 50 for the 166 pf soda /ice capacitor and therefore less than this
for its watered down version of 120 pf. It might be possibile to increase
the dielectic constant of ice by using a more conventional design where the
area was much larger in relation to the distance between the plates, but of
this I am doubtful. Also is the problem of whether the baking soda decreases
the dielectric breakdown voltage of this special ice as its ordinary use is
a free charge carrier in an electrolysis solution. HDN


Binary Resonant System
http://www.insidetheweb-dot-com/mbs.cgi/mb124201





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