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Re: supercaps



These high voltage capacitors that Steve is referring to seem to become
essentially like electrets which are devices that retain a permanent
electric dipole moment even after the external electric field has been
removed. It would seem that the dielectric materials used in these high
voltage capacitors might very well retain a dipole moment even after an
inital discharge. Could this permanent dipole moment also explain the
behavior that Ludwik was observing in supercaps?

One question for Steve. After removing the shorting bar, have you ever
noticed a build up of charge on the terminals even though the terminals are
not connected to anything else?

Roger Pruitt

>I'm still looking at this thread, but I thought I'd throw out a
>comment that may or may not be related.  I remember many years ago
>learning that high voltage capacitors are hazardous because thermal
>processes will cause them to become polarized over time even if they
>are initially "discharged".  The voltage they can reach depends on
>their construction, and can be potentially lethal for units with large
>capacitance designed for operation at many kV.  Such capacitors are
>always stored with their terminals shorted together to prevent such
>spontaneous polarization.  We even have a bunch of them sitting around
>here that were part of a pulsed laser that had been donated many years
>ago.  The smaller ones are 100 uF @ 3 kV, and the large ones are 100 uF
>@ about 20 kV.  Both types are stored with shorting bars between their
>terminals.
>   I have a student doing a senior research project on series and
>parallel combinations of capacitors (a project inspired by discussions
>on phys-l!).  We might look into this problem, too.
>
>Steve Luzader
>Frostburg State University
>