If water has such a good dielectric constant why don't they use it?

Water was used as the dielectric for some inertial confinement fusion research capacitors. The problem with water is keeping it pure enough to not be a mild conductor. Pure water is an insulator but, water being a polar solvent, picks up ions from many things it comes in contact with. These ions cause it to conduct electricity to varying degrees. It's difficult to keep the water pure.

It takes more than a high dielectric constant to make a good capacitor. Conductivity is a show stopper as are nonlinearites of the dielectric constant with voltage stress.

The Er of water drops rapidly with frequency, and leakage/ionic contamination is a problem.

Right, never seen a watercap.

John

Not only that, but pH = 7 and "16.7Mohm" (presumably 16.7Mohm-meter is meant) are two salient figures that limit you. The first, pH, is defined as the negative logarithmic percentage of protons (H+/hydronium H3O+) in the solution. 10^-7 (percent or factor-of-total, I forget) is the degree to which pure H2O ionizes itself, that is, the equilibrium of molecular to ionic components: H2O H+ + OH-. (Since the reaction produces equal parts H+ and OH-, the pOH is also 7.)

And if you remember your chemistry, acidic solutions have more protons (a low pH, which being a negative log means more protonation) while basic solutions tend to release more hydroxyl (OH-) ions (high pH, low pOH).

The other figure, "16.7M" (I think that's right) refers to the resistivity of pure, deionized water. The conductivity comes from the small fraction of H+ and OH- charge carriers naturally present in all water.

An easier way to make water resistive is to freeze it, trapping all ions immobile in a crystalline lattice. Without ions free to move, charge cannot move and it cannot conduct. I think the dielectric constant of ice is still high, so besides water's quirky expansion, ice capacitors may be useful. At least in Alaska, and Wisconsin in the winter. ;)

Tim

-- Deep Fryer: a very philosophical monk. Website:

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.

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Tesla himself used saltwater bottle caps, but the saltwater was only used as the conductor (plate) to the glass, the glass being the dielectric.

Jon

Do a Google search using "water capacitor" to see 646 hits. Here is one example of Panasonic using them in a digital receiver (model SA-XR55):

"A Pure Water Capacitor containing a rayon separator is used as the electrolytic capacitor for each of the seven audio channels. With its high water content this capacitor offers excellent electrical characteristics and low impedance. The clean electric current is delivers to the speakers helps acheive a clear, transparent sound."

Maybe it would be cleaner sound if they put soap in it. (snicker)

I saw references to lasers using water capacitors and there were mentions of saltwater capacitors for use with Tesla coils.

Lots of stuff to read. Enjoy.

John

On Fri, 04 Nov 2005 05:04:26 +0000, John - KD5YI wrote: ...

LOL!

Thanks! Rich