On a sunny day (Sat, 04 Dec 2010 09:57:04 -0500) it happened Phil Hobbs
wrote in
:

Bob Masta wrote:
On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:

snip
I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example...snip

OK, I'll bite: What's wrong with explaining electrets as
permanently polarized capacitance mics? Are you saying
that this is *not* the basic concept, or that it just
doesn't go deep enough to do justice?

If you have a surface with a nonzero net charge density on its surface
(so that it produces an electric field in the air), a small current will
flow due to air ions and surface water films. Therefore no object can
produce an external field forever without a power source.

A current can *only* flow if that object itself is a conductor.


Electrets are just poled piezoelectrics.

Piezo mikes use the BENDING of a piezo crystal to generate voltage,
An electret is in no way related to a piezo mike, if that is what you are implying.

Electret is much more like a capacitor mike, and that also goes for impedance.
The old piezo mikes were not very good, maybe speech only, because the forces needed to bend the crystal.,
non linearity.
The electret can give extremely high quality, as hardly any force is needed to move the membrane.

There were piezo pick up elements for vinyl record players, piezo mikes, piezo 'crystal' earphones,
most of these later replaced by much more 'HiFi' dynamic stuff.


And 'forever' is something that modern electronics tries to avoid, for sales reasons I suppose.
That said I have seem 30 year old electrets working.
Considering the 100 years for most FLASH based firmware, I'd say electrets last forever.

It is like your view on LDRs, you probably have never used one.