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Water as a Dielectric
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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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Update: June 2012
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