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On capacitors and polarity. Answered

(Asked twice already, but for some reason was not visible.) That is sort of a blanket question. I'm wondering: 1) why only some, and not all or none? 2) what (in their physical structure) causes them to have polarity? 3) why do we make ones that have polarity? Is there something prohibitive, like cost, or safety, making it so in the production? (okay, not safety, as may attest anyone who has seen a reversed capacitor explode.)


Polarized capacitors are commonly called filter caps, because they block DC from being shorted to ground, but will allow small transient ac signals through (another normal use is in speaker and sound systems).   If hooked up backwards, since the polarity comes from a thin doping of material in the cap, the dielectric can break down and the electrolyte can be forcibly ejected from the cap.  

Answer attached to another copy.