Water as a dielectric in capacitors

[Indis Nenhrma]

"water" as a dielectric in capacitors

Homework Statement

Why do you think water is not commonly used as a dielectric in capacitors?
K=80.4
*K=dielectric constant for water

thx for any help

 

[Dick]

The dielectric constant looks great for use in a capacitor. What about the conductivity?

 

[Edward G]

Water is occasionally used as a dielectric. There are some large coaxial cables under Imperial College in London that use water as a dielectric, but only for a very short period of time, before it starts to conduct.

 

[Dick]

Water is occasionally used as a dielectric. There are some large coaxial cables under Imperial College in London that use water as a dielectric, but only for a very short period of time, before it starts to conduct.

Really? What for? Now I'm curious.

 

[dynamicsolo]

The dielectric constant looks great for use in a capacitor. What about the conductivity?

I suspect the issue may not be with the conductivity of pure water itself, but the tendency of materials to leach ions into it, creating much more conductive solutions... (The table at this Wiki article -- http://en.wikipedia.org/wiki/Electrical_conductivity -- is instructive.)

 

[mgb_phys]

Really? What for? Now I'm curious.

You use them in very high power applications like pulse X ray or high power laser sources.
You have to use high purity water to reduce the conductivity and they only work for a very short time before they short. They are able to absorb very high powers and they are self healing. And you get the extra useful feature that if things go very wrong the water boils and disconnects the circuit!

 

[Dick]

I suspect the issue may not be with the conductivity of pure water itself, but the tendency of materials to leach ions into it, creating much more conductive solutions... (The table at this Wiki article -- http://en.wikipedia.org/wiki/Electrical_conductivity -- is instructive.)

That's what I was trying to lead the OP to discover. And thanks to mgb_phys for filling me in on the benefits of water as a dielectric.

 

[Indis Nenhrma]

Thanks for your attention and expository answers. :)

 

[dthorn1234]

Why do so many think the plates have to be in physical contact with the water? Capacitors are used as fuel sensors in the A4 Skyhawk aircraft and the dielectric is the fuel. However in this case the plates are insulated from the fuel. The same would logically be the case in a capacitor using water as the dielectric. The plates being insulated from the water the conductivity of the impure water due to ions forming would not be a factor.

 

[Mindscrape]

Yeah, as the others have stated, I don't think there is anything wrong with using ideal, or "pure," water. I wouldn't want to do practically because water is a good solvent, so it has the potential to bring ions into it. I guess people actually do it in the applications that mgb stated, but I wonder about it's leakage (parasitic resistance).

 

[dthorn1234]

Mindscrape, As my previous post says , the plates do not have to be in contact with the water. You can coat the plates in a non-reactive, non-conductive coating then apply the use of water as a dielectric. The conductivity of the water would not be a factor. Also it doesn't have to be pure water. If your water capacitor were going to be mounted outdoors you could add some anti-freeze ( dielectric constant of 40 ) to keep it from freezing.