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Question:
I noticed that water has a very high dialectric strength. I was wondering if it could be used. I thought it would be a bad dialectric since water is polarized and highly mobile. I thought about it some more to compare water to other dialectrics and came up with the difference that water is liquid and other dialectrics are solid. Would a liquid dialectric (water or otherwise) leak charge from a plate of a capacitor? And could solid water (ice) be used in a useful way as a good dielectric. ie. is the reason water would leak charge only because it is liquid?



Replies:
You seem to be confusing a couple of things here. Water does indeed have a very high dielectric constant - this is BECAUSE the water molecule has a dipole moment and so water can be polarized. That is, under a given electric field, water tends to polarize strongly, nearly canceling out the effect of the field. However, water also conducts electricity (why it is dangerous to play with electrical equipment in the bath for instance) because it always contains ions (OH- and H+ are always there, and also things like Cl- and Na+ are usually present) which are highly mobile. Being conducting means that there is a current present whenever you apply an electric field, and the current means you have charge "leaking". Yes, the reason water conducts well is because it is liquid, so ice is not a good conductor (though ice with a layer of liquid water on the outside would still probably allow quite a bit of current to flow). However, ice also does not have a high dielectric constant - I think! In ice the water molecules have all lined themselves up in a fixed structure where the hydrogen bonds can be maximally satisfied, and so the molecules are no longer free to rotate as they were in the liquid, which was what gave it a good dielectric constant...

Arthur Smith


This needs to amended. The various forms of ice all have higher dielectric constants than does water.

http://skua.gps.caltech.edu/hermann/ice.htm

which states that Ice Ih has a dielectric constant of 96.6. This is due to the mobility of protons.

Thanks to Brian Peterson for this clarification.


Hi,

I sent in an answer to, "Water as a Dielectric", but I see that I made a mistake in my answer. I would like to change my answer to the following.

There is a different dielectric constant for ice at high frequencies (the constant is stated as 3.2 at

http://skua.gps.caltech.edu/hermann/ice.table2.html). This is because ice has a very high dielectric relaxation time compared to water, and at higher frequencies the molecules are not able to realign themselves as fast as the changing electric field. This is why ice does not melt in a microwave as quickly as water comes to a boil.

Sincerely,

Matthew Homola



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