How does an electret retain its charge for years without decreasing?

[arul_k]

How is an electret capable of recharging itself after discharge. I came across an article that stated tha electrets are capable of retaining their charge for years and any number of discharges does not decrease their charge. how does this happen?

 

[GRDixon]

How is an electret capable of recharging itself after discharge. I came across an article that stated tha electrets are capable of retaining their charge for years and any number of discharges does not decrease their charge. how does this happen?

In the case of a bar electret (e.g. one made out of barium titanate) "discharge" might occur when ions in the air are attracted to the ends, thus neutralizing the electret's electric field. This process does not change the bar's permanent, electric polarization. When the electret is "recharged", by stripping away the air ions, the bar's polarization field is restored until "discharge" occurs again. Kept in a vacuum, the electret should remain "charged" indefinitely.

 

[arul_k]

In the case of a bar electret (e.g. one made out of barium titanate) "discharge" might occur when ions in the air are attracted to the ends, thus neutralizing the electret's electric field. This process does not change the bar's permanent, electric polarization. When the electret is "recharged", by stripping away the air ions, the bar's polarization field is restored until "discharge" occurs again. Kept in a vacuum, the electret should remain "charged" indefinitely.

Thanks for your reply. I was thinking in terms of a spark discharge that would occour between the +ve and -ve terminals of the electret, when shorted. The process you suggest seems more like a neutralisation of the electric field.

 

[GRDixon]

Thanks for your reply. I was thinking in terms of a spark discharge that would occour between the +ve and -ve terminals of the electret, when shorted. The process you suggest seems more like a neutralisation of the electric field.

The key to the puzzle may be that the charges responsible for an electret's field are POLARIZATION charges, and are therefore bound charges. As such, shorting the poles as suggested will not result in a loss of the polarization charge. The shorting wire, which does contain conduction electrons, no doubt becomes electrically polarized while in contact (or in close proximity) with the electret's poles. But once removed, it loses this induced polarization and the electret's intrinsic electric field returns to its nominal value. Any conduction charges that "rub off" of the shorting wire, and stick to the electret's positive pole, might be relatively unimportant, as the electret is a dielectric.

But all of the above should be taken with a grain of salt, as I am not experienced with these fascinating devices.