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YouTube - MIT Physics Demo -- Dissectible Capacitor
I believe that this intriguing experiment is designed to help in the understanding of how capacitors and indeed dielectrics work and behave.
It is fascinating to watch the Leyden Jar charged up, dissembled, the individual components handled and then rebuilt to find that the jar as a whole is still highly charged.
The following piece of an article is intended to explain the science:
I thought I'd got this cracked until I started to find holes in my logic.
The problem I've got with this experiment, is this. Assuming that corona discharge takes place as the metal plates are removed, so that charges reside on the surface of the glass, presumably held there by the polarised glass, then surely each plate is equally charged - or discharged of electrons. After all one plate should have lost a lot of electrons to the surface of the glass. Yet when the two plates are clinked together, no spark, no apparent exchange of charges... how can this be?
As the glass is a non-conductor, I can see how this can be touched and as long as the charges are not physically all wiped off will remain, but how can the metal plates have no individual charge?
Furthermore, I've read that once rebuilt and discharged, if left for a period of time, the Leyden Jar will recharge to a certain extent by itself and indeed can be discharged again. How can this happen?
To my mind this can only happen if the dielectric to a certain extent remains polarised after the initial discharge. The electric field created by the initial discharge will depolarise the glass to a greater extent, but perhaps not completely. So after a period of time electrons within the plate itself will disperse as a result of the small electric field still remaining in the glass.
But this then brings up another problem. If it were only a case of electrons dispersing within the plates, surely each plate would still be of equal charge, so no current would flow between the two - no spark. But this is not the case!
Which surely must mean that charges are again lost to the surface of the glass dielectric, leaving each plate more or less negative or positive than the other. But if this is the case, when they are taken apart and clinked together, why don't we get a spark between the two metal plates as the charges exchange in order to stabilise?
Curious. What am I missing here?
I believe that this intriguing experiment is designed to help in the understanding of how capacitors and indeed dielectrics work and behave.
It is fascinating to watch the Leyden Jar charged up, dissembled, the individual components handled and then rebuilt to find that the jar as a whole is still highly charged.
The following piece of an article is intended to explain the science:
Do you believe that the energy in a capacitor is trapped permanently in the dielectric? Many people do. Their belief is caused by a famously misleading experiment called "Dissectable Leyden Jar." It's an experiment which involves high voltage and corona discharge. The effect it purports to prove does not occur in capacitors at lower voltages.
First charge up a Leyden jar using a Wimshurst Machine (or other source of high voltage.) Now, carefully remove the inner metal from the jar. Now remove the outer metal. Discharge everything, then hand the parts around the classroom. Next, put the parts together again, connect the two metal cylinders, and BANG!, there is a loud discharge.
Doesn't this prove that the energy in a capacitor is stored in the dielectric? No.
Whenever you take apart a Leyden jar or other high voltage capacitor, there is a corona effect which makes very strange things occur. When you electrify a Leyden jar, and then you pull the inner metal cylinder out of the jar, the capacitance value drops, and this makes the potential difference skyrocket to enormous levels. The potential tries to become huge but it cannot, because instead it creates corona along the metal edges, and and it leaks the excess charge into the air. This corona allows the opposite electrical charges to "paint" themselves onto both sides of the dielectric "jar" surface. So, if you pull a leyden jar apart, the sharp edges of the metal plates sweep along and transfer a large percentage of the separated charges from the metal plates to the glass surfaces. The energy is still there! It's still stored as a field in the dielectric, but those separated charges are not on the metal plates anymore. Instead they are now TRAPPED ON THE GLASS SURFACE! Strange idea, huh? A capacitor with no plates, just a dielectric.
Now reassemble the Leyden jar: momentarily touch each metal plate to ground, and put it back together again. You'll find that it's still strongly electrified! The trapped charges on the glass surface can still induce equal charges on the adjacent metal plates. Touch the two terminals with your fingers and BOOM!, the momentary current in your muscles will throw you across the room.
This strange effect leads many people to claim that the energy in a capacitor is permanently trapped in the dielectric, and that it is not stored in the electric field. This is wrong.
In order to properly perform the take-apart capacitor experiment, you must execute the entire demonstration inside a big tank full of oil. This prevents the corona discharges from spewing charges from the edges of the plate onto the dielectric.
Or, perform the whole experiment at 1.0 volts, not at 10,000 volts. (Use an electrometer to measure the voltage.) You'll find that the dielectric doesn't store energy anymore. In order for the charges to spray onto the dielectric, the voltage must be high.
Or, use high voltage but do this instead: before doing anything, take apart the leyden jar. Now, lay the metal parts on a plastic sheet and use a Wimshurst machine to charge them up. Next, use plastic tongs to assemble the leyden jar. (The voltage across the plates will be very low.) NOW perform the leyden jar dissection. It shouldn't work anymore, since the initial voltage is low enough that it will prevent corona discharges from painting any charges on the dielectric.
First charge up a Leyden jar using a Wimshurst Machine (or other source of high voltage.) Now, carefully remove the inner metal from the jar. Now remove the outer metal. Discharge everything, then hand the parts around the classroom. Next, put the parts together again, connect the two metal cylinders, and BANG!, there is a loud discharge.
Doesn't this prove that the energy in a capacitor is stored in the dielectric? No.
Whenever you take apart a Leyden jar or other high voltage capacitor, there is a corona effect which makes very strange things occur. When you electrify a Leyden jar, and then you pull the inner metal cylinder out of the jar, the capacitance value drops, and this makes the potential difference skyrocket to enormous levels. The potential tries to become huge but it cannot, because instead it creates corona along the metal edges, and and it leaks the excess charge into the air. This corona allows the opposite electrical charges to "paint" themselves onto both sides of the dielectric "jar" surface. So, if you pull a leyden jar apart, the sharp edges of the metal plates sweep along and transfer a large percentage of the separated charges from the metal plates to the glass surfaces. The energy is still there! It's still stored as a field in the dielectric, but those separated charges are not on the metal plates anymore. Instead they are now TRAPPED ON THE GLASS SURFACE! Strange idea, huh? A capacitor with no plates, just a dielectric.
Now reassemble the Leyden jar: momentarily touch each metal plate to ground, and put it back together again. You'll find that it's still strongly electrified! The trapped charges on the glass surface can still induce equal charges on the adjacent metal plates. Touch the two terminals with your fingers and BOOM!, the momentary current in your muscles will throw you across the room.
This strange effect leads many people to claim that the energy in a capacitor is permanently trapped in the dielectric, and that it is not stored in the electric field. This is wrong.
In order to properly perform the take-apart capacitor experiment, you must execute the entire demonstration inside a big tank full of oil. This prevents the corona discharges from spewing charges from the edges of the plate onto the dielectric.
Or, perform the whole experiment at 1.0 volts, not at 10,000 volts. (Use an electrometer to measure the voltage.) You'll find that the dielectric doesn't store energy anymore. In order for the charges to spray onto the dielectric, the voltage must be high.
Or, use high voltage but do this instead: before doing anything, take apart the leyden jar. Now, lay the metal parts on a plastic sheet and use a Wimshurst machine to charge them up. Next, use plastic tongs to assemble the leyden jar. (The voltage across the plates will be very low.) NOW perform the leyden jar dissection. It shouldn't work anymore, since the initial voltage is low enough that it will prevent corona discharges from painting any charges on the dielectric.
The problem I've got with this experiment, is this. Assuming that corona discharge takes place as the metal plates are removed, so that charges reside on the surface of the glass, presumably held there by the polarised glass, then surely each plate is equally charged - or discharged of electrons. After all one plate should have lost a lot of electrons to the surface of the glass. Yet when the two plates are clinked together, no spark, no apparent exchange of charges... how can this be?
As the glass is a non-conductor, I can see how this can be touched and as long as the charges are not physically all wiped off will remain, but how can the metal plates have no individual charge?
Furthermore, I've read that once rebuilt and discharged, if left for a period of time, the Leyden Jar will recharge to a certain extent by itself and indeed can be discharged again. How can this happen?
To my mind this can only happen if the dielectric to a certain extent remains polarised after the initial discharge. The electric field created by the initial discharge will depolarise the glass to a greater extent, but perhaps not completely. So after a period of time electrons within the plate itself will disperse as a result of the small electric field still remaining in the glass.
But this then brings up another problem. If it were only a case of electrons dispersing within the plates, surely each plate would still be of equal charge, so no current would flow between the two - no spark. But this is not the case!
Which surely must mean that charges are again lost to the surface of the glass dielectric, leaving each plate more or less negative or positive than the other. But if this is the case, when they are taken apart and clinked together, why don't we get a spark between the two metal plates as the charges exchange in order to stabilise?
Curious. What am I missing here?
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