Making an Electret: A Guide to Creating Electrical Analogues of Magnets

[TeTeC]

Hello,

I've discovered yesterday that there exists such things as electrets, which are some kind of electrical analogues of magnets.

Apparently only few compounds can be turned into electrets. You can of course easily make and electro-electret with an exterior electric field and liquid water for example.

We could imagine an experiment with a bottle of water and a capacitor. If you put the bottle of water between the capacitor plates, the water would become polarized.

I don't think I'm utterly wrong until here.

There is however a question I cannot easily answer. What would happen if I decided to freeze the water with an electric field still active ?

I don't think this would keep the water polarized, transforming it into an electret. However, I cannot explain this last claim. What happens when it freezes ?

A second question, which is not really related to the particular experiment I described above : is there an easy way to make an electret ?

Thank you !

 

[jtbell]

Here's a Web page about making electrets out of wax. It's basically the same idea as you describe, but using wax instead of water.

http://rimstar.org/materials/electrets/

Unfortunately, the Scientific American article that it mentions doesn't seem to be on line any more, at least at the link that it gives, but if you have access to a library, look for the November 1960 issue.

When I was an undergraduate, one of my classmates tried to make an electret using something like this method. I don't remember if she ever got it to work.

Oleg Jefimenko has studied electrets a lot, so if you Google him you might turn up some more information.

Warning: electrets seem to be involved in crackpottish free-energy schemes, so be careful about what you read about them on the Web.

 

[TeTeC]

Great ! Thanks for the information.

I've googled the Scientific American article, and here it is : http://vacuum.ramapo.edu/physics/physics-doc/amsci/AmSci01/1960/11/1960-11-body.html

I still don't understand why it wouldn't work with water. After all, if it works with wax, maybe water will do it.

Another question. If I make a parallelepipedic electret (with my capacitor plates on two opposites sides) my textbook tells me that the electric field produced by the electret, which will roughly be uniformely polarized, is completely equivalent to the field produced by two charged surfaces (with +Q top and -Q down), outside the electret.
To me, that would produce a non-zero electric field only inside the water (in an ideal cas where I neglect any border effects). Consequently I wouldn't be able to test my electret.

So I simply tought about giving it another shape. Something spherical maybe ?

 

[AlephZero]

Try googling "electret microphones" as well. If you want a ready made electret to play with, getting a cheap ($5) electret mike capsule and taking it apart is one way to get started.

 

[jtbell]

If I make a parallelepipedic electret (with my capacitor plates on two opposites sides) my textbook tells me that the electric field produced by the electret, which will roughly be uniformely polarized, is completely equivalent to the field produced by two charged surfaces (with +Q top and -Q down), outside the electret.
To me, that would produce a non-zero electric field only inside the water (in an ideal cas where I neglect any border effects). Consequently I wouldn't be able to test my electret.

Remember that non-ideal (finite width) capacitors have "fringe fields" at their edges.

Start with a nearly ideal capacitor, which has a very narrow plate separation, compared to the plate width, so the field is practically uniform inside and practically zero near most of the outer surfaces of the plates, and has "fringes" around the edge. Imagine reducing the plate width while keeping the separation constant. What should the field look like as the plate width becomes comparable to the plate spacing?

 

[pervect]

Try it (freezing water in an electric field) and let us know if it forms an electret or not.

I don't want to make any definite statements, but the molecular structure of frozen water (a crystal) seems to me to be considerably different from a wax (which I had to look up, it's apparently a long chain). Can the dipole moments of the water molecules be organized in a crystal structure so that they all point in the same direction?

 

[jtbell]

And even if you could get a useful "frozen" dipole moment in a chunk of ice, you'd have to keep your apparatus below 0 C while you work with it... not very practical for a DIY experiment. :yuck:

 

[cesiumfrog]

If the electret's polarity is measured directly after its manufacture, its charge will be just what theory predicts it should be. The negative surface of the electret will be that which made contact with the positively charged polarizing electrode, and vice versa. This agrees with the north-south polarity of a bar of steel magnetized by contact with a permanent magnet. In contrast with the behavior of a magnet, however, the charge on the electret begins to diminish immediately, and in about a week it will have fallen to zero. The charge then begins to build up in opposite polarity to a final value that may be several times as large as the original charge. This may take as long as three months. The negative surface of the stabilized electret will be the face that made contact with the negatively charged polarizing electrode.

Has this "puzzle" (from the article) been explained yet?

 

[TeTeC]

Thanks guys !

Maybe I should try to insert a few numerical values in the formulaes to see what it looks like.

Jtbell : yes, that's true, but I thought it would be easier to find water than wax. I should simply make some kind of petition to open all the fridges in the world and get it a little cooler for my electret. Maybe that will solve another important problem as well.

Seriously, I should find some more information on wax.

I will definitely try the experiment. It's quite easy to set up and it is not too difficult to check whether you get an electret or not (there should be a exploitable potential difference).