I was looking at a dielectric chart the other day and notices that
water has a rather high constant. Yet I have never heard about or seen
any water capacitors. Any comments would be greatly appreciated.
Thanks
Because water is "too friendly" - **PURE** water is an excellent
insulator. But even a trace of impurities (dissolved iron or copper,
thousands of different chlorides, peroxides, and hydroxides, various
acids, and a huge list of other things) is enough to turn water into a
conductor whose quality ranges from poor to excellent.
So the answer to your question basically comes down to "they don't use
it in caps because although they can *MAKE* it pure enough (through
distillation or by burning hydrogen gas) they just can't *KEEP* it pure
enough for it to be useful for more than a few seconds at a time."
Some tesla coilers use banks of tinfoil-wrapped, beer bottles filled
with a saturated salt solution as capacitors, but they're using the
water specifically as a conductor - making up one plate of the capacitor
formed by sandwiching the bottle glass between two conductors, with the
foil being the other plate, but that's hardly taking advantage of
water's *INSULATING* properties
Of course, there's also another issue: Hydrolysis. Put voltage through
water, and you get water breaking down into its component H2 + O gasses
- Which means that the thing has a sharply finite lifespan - It's only
going to act as a cap of the specified value for as long as there's
still the proper thickness of water between the plates. Before too long,
there will be no water (And the value of the capacitor will have changed
to that of an cap using air dielectric - Significantly lower), since all
of it will have decomposed into its constituent hydrogen and oxygen
atoms through electrolysis. By that time, unless it's been vented,
there's a third hazard: it's a stochiometrically perfect mix of hydrogen
and oxygen gases, possibly under considerable pressure, just waiting for
any spark to come along and turn it into a rather nice explosion. Since
you're looking for a high dielectric value material to build the cap
with, it seems likely that you're also looking at a cap that's going to
be prone to flashing over due to the high voltages it's seeing. The
first time it does once a gas pocket builds up, it's real likely to go
"boom" in a fairly spectacular fashion.