## Walter Lewin's riddle

At 37:00 he charges a conductor with a dielectric inside, removes the conductor plates and discharges them, and in the end puts them back into position.

Then something "remarkable" happens - when he connects the conductor plates with each-other, a spark is produced. Why did this happen?

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Is this because the dielectric is still polarized when he puts back the plates, and thus the plates themselves become charged (by induction) in turn? If not, what is the answer to this riddle?
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 You're on the right track. If you want to put some math on it, consider the simpler case of a plane capacitor, with a dielectric and two gaps. Calculate the field both in the gaps and in the dielectric needed to have a potential difference around 20kV between the two plates, and compare to the breakdown field for air (around 3MV/m)
 Walter Lewin explains how this works here https://www.youtube.com/watch?v=MZOaVXmK5zk#t=32m55s

Recognitions: