MIT dissectable capacitor

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__dan

Senior Member
Watching this video and trying to explain what I see, I'm just babbling at myself, da da dab da da.

This is the MIT dissectable capacitor demo. They charge the Leyden jar and take it apart to show where the charge is actually stored. It's not what I would expect. The charge is either in the glass or on the glass but looking at how the parts are handled ... If the charge is on the glass, it is mobile and self repulsive. It would evenly distribute on the outer surface and be equalized when he touches it.

It looks like the charge is *in* the glass (but then the glass would be conductive). Or, the glass becomes polarized and remembers its polarization.

I have no idea, I just want to be spoon fed.

http://www.youtube.com/watch?v=9ckpQW9sdUg
 

mivey

Senior Member
The electric energy is stored in the field between the surfaces of the copper plates and is in the dielectric. The glass is polarized when the capacitor was charged. When the first copper plate was removed, the surface charges on the plate stayed on the surfaces of the glass next to the polarized glass. When the second plate was removed, the charges again stayed next to the polarized glass.

Assuming you could conduct well enough, reaching across the bottom part of the glass on the inside and outside would have caused charge flow and a release of energy (i.e. "ouch"). Unlike copper, glass does not have a mobile "sea" of electrons on the surface to move charges. If you might recall from the thread where I discussed energy flowing outside the conductor, you have to have both an electric field and a magnetic field present to exchange energy. When the charges move (current), then the energy can be exchanged (the capacitor is discharged).

Add: thread with energy flowing outside conductor
 
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mivey

Senior Member
It might be worth noting that the copper plates also have polarization. This is what I referenced in the other thread when I said a pair of conductors with a voltage between them has a net radial electric field in the space between, not in the wire. That is because the applied electric field is countered by the internal field due to polarization of the conductor (exactly countered at equilibrium).

The point being is that there could actually be a small discharge when the two plates are touched in the video but this energy is small as compared to the energy stored with the dielectric, thus we still get a big spark at the end of the demo.
 

__dan

Senior Member
The electric energy is stored in the field between the surfaces of the copper plates and is in the dielectric. The glass is polarized when the capacitor was charged. When the first copper plate was removed, the surface charges on the plate stayed on the surfaces of the glass next to the polarized glass. When the second plate was removed, the charges again stayed next to the polarized glass.

Assuming you could conduct well enough, reaching across the bottom part of the glass on the inside and outside would have caused charge flow and a release of energy (i.e. "ouch"). Unlike copper, glass does not have a mobile "sea" of electrons on the surface to move charges. If you might recall from the thread where I discussed energy flowing outside the conductor, you have to have both an electric field and a magnetic field present to exchange energy. When the charges move (current), then the energy can be exchanged (the capacitor is discharged).

Add: thread with energy flowing outside conductor
Thank You

I was wondering if the polarity of the copper cups had something to do with this working. It looks like the polarity is such that the excess electrons are on the inside of the jar. As he carefully removes the inside cup, the electrons are trapped inside the jar by the electric field from the charge difference relative to the outside cup, amplified by the glass dielectric.

I was wondering if the ordering of the action was the same and the polarity reversed, the charge would leave the glass with the outside cup as it is disassembled, the electrons are self repulsive and distribute to the outside, making the demo fail. He is showing he can have a jar of electrons. He must also be standing on an insulating mat.
 

mivey

Senior Member
Thank You
You're welcome

I was wondering if the polarity of the copper cups had something to do with this working. It looks like the polarity is such that the excess electrons are on the inside of the jar. As he carefully removes the inside cup, the electrons are trapped inside the jar by the electric field from the charge difference relative to the outside cup, amplified by the glass dielectric.
The polarity is not the key as we are not storing electrons like apples in a basket. The electrons are not stored in the jar but are on the surface of the glass.

I was wondering if the ordering of the action was the same and the polarity reversed, the charge would leave the glass with the outside cup as it is disassembled, the electrons are self repulsive and distribute to the outside, making the demo fail. He is showing he can have a jar of electrons.
I don't think so.

He must also be standing on an insulating mat.
He may be, but that is of little concern. One side of the glass has too many electrons and the other side of the glass has too few electrons. A completed circuit would let the charge imbalance try to equalize the two sides. It has very little to do with the floor.

The reason I say very little is that the charge separation across the glass can cause side effects. An illustration:

Positive Pete is standing on one side of a fence and Negative Norton is on the other. Due to prior events, Pete is covered with holes due a deficit of electrons. Norton is covered with an excess of electrons. The charge imbalance each has balances the other across the fence.

Wearing rubber boots, a neutral monkey named Balanced Barry approaches Pete and notices that electrons begin piling up on his face while his tail develops holes. Curious indeed.

Then Barry goes to the other side of the fence and approaches Norton and notices that holes appear on his face and electrons begin piling up on his tail. If Barry only knew he was being polarized, he might take caution with his tail.

Barry lets his tail touch the ground and some of the extra electrons on his tail begin to spill onto the ground. When Barry is right next to Norton, he picks up his tail. Barry now has a net positive charge. He begins the journey back to Pete's side of the fence and somewhere along the way lets his tail touch the ground and gets a static shock.

On the other side of the fence again, Barry approaches Pete and again notices that electrons begin piling up on his face while his tail develops holes. Barry lets his tail touch the ground and some electrons begin to leave the ground and disperse themselves somewhere along his frame. When Barry is right next to Pete, he picks up his tail. Barry now has a net negative charge. Now bored with the whole thing, Barry leaves the scene but lets his tail touch the ground and gets another static shock as the atoms in his body restore equilibrium. We suspect Barry does not thank Mother Earth for the charge donations.
 
T

T.M.Haja Sahib

Guest
The lesson from the video is this:
Do not handle a power capacitor with terminals open and discharge resistor missing.
 
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