Understanding Spark Production in Dissectable Glass Capacitors

[Stryker331]

Homework Statement

1) A flat capacitor is filled with a dielectric slab made out of glass.
2) The capacitor is connected to a generator which sets the difference of potential between the plates.
3) As a consequence of the external electric field, a polarization is established in the dielectric
4) The capacitor is then disconnected and thus the charge is trapped on the plates
5) The capacitor is dismantled and the plates are completely discharged by grounding both of them.
6) The capacitor is reassembled and not connected to any generator. No friction or force has been applied.
7) We connect two plates of the capacitor with a wire and a spark occurs, so the difference of potential is NOT zero.
8) WHY??

Relevant Equations

P =e0 * X * E (epsilon zero, chi, electric field)

I think a high enough voltage may have permanently polarized the glass.

 

[.Scott]

I would suspect a significant surface charge on each surface of the glass plate.
The surfaces can hold a charge - but are not good at conducting a current - so discharging them would take more than just touching them with a grounded wire.

 

[Stryker331]

But only the conductors can be electrified by contact

 

[Steve4Physics]

I think a high enough voltage may have permanently polarized the glass.

Sound plausible. Note that you don't need permanent polarisation. The polarisation might decay (maybe exponentially) but with a relatively long half-life. And perhaps you can have a mix of permanent and decaying polarisations.

But only the conductors can be electrified by contact

Not so. You can you charge (for example) glass by rubbing it with a piece of silk, But you don't always need friction - simple contact between a charged piece of metal and an insulator can directly transfer a small amount of charge. And the bigger the contact area, the bigger the amount of charge transferred.

 

[Stryker331]

But a spark needs a large amount of charge (a large potential difference) to be created

 

[Steve4Physics]

But a spark needs a large amount of charge (a large potential difference) to be created

Some questions to think about.
If you have ever worked with a car battery (12V), can it produce a spark?
A spark needs a strong enough electric field. Why (how is a spark produced)?
What two key factors does the electric field between the ends of 2 wires depend on?