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I want to create a parallel plate capacitor using the glass of a car window as dielectric. My electrodes (the plates) will probably be of the same size and type - but the glass would definitely be of larger surface area. What I am curious about is how would the performance of this parallel-plate capacitor change if the glass is broken into pieces while the area around the parallel plates remain relatively intact?

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The relevant idea here is that finite-size parallel plate capacitors emit fringe fields beyond their edges. These fringe fields will penetrate the dielectric at points not directly between the electrodes, and technically extend quite far outside the plates (though practically, they are negligible beyond a certain distance). If some of the glass around the parallel plates was broken and removed, then the capacitance of the capacitor would probably slightly decrease, as the fringe fields are no longer polarizing the dipoles present in the glass that is no longer there. Whether you can measure such a change depends on how strong the fringe fields are for your capacitor. If the glass was somehow broken while remaining in place, then the fringe fields would still penetrate the glass in basically the same way, and you would see essentially no drop in capacitance.

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  • $\begingroup$ Can I increase the fringe field by increasing the voltage across the parallel plate in order to increase the effect of a broken and removed glass? $\endgroup$ – dackie Aug 1 '18 at 4:12
  • $\begingroup$ @dackie Yes, you will increase both the main electric field and the fringe field by increasing the voltage across the capacitor. However, this will not change the capacitance. $\endgroup$ – probably_someone Aug 1 '18 at 4:44
  • $\begingroup$ good info. I could now proceed with my project. thanks. $\endgroup$ – dackie Aug 1 '18 at 5:09

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