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Leyden jar question

  1. Dec 29, 2003 #1
    Imagine an isolated cilindrical Leyden jar.

    A Leyden jar is something like a big capacitor, and normally, the external plate is grounded as we charge.

    But now, imagine this Leyden jar is not grounded, and you go charging the external plate. Finally, you'll have excess charge on the external plate.

    Now imagine you connect externally the external and the internal plate.

    Will the excess charge hold on the external plate, or will it spread along both plates?
  2. jcsd
  3. Dec 29, 2003 #2


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    It's a bit unclear what you mean by 'where the charge is'. The actual distribution of charge depends on the geometry of an object. For example on a conducting sphere, the charge will be distributed on the surface, since that is the geometry that maximizes the average distance between electrons. A Van der Graf generator exploits this effect to generate high DC voltages.

    So, if you have a leyden jar that has an internal and an external plate, and you connect them, then any net charge in the form of extra electrons would primarily be along the geometrically exterior surface. However, as long as the two are connected, the potential is the same at both locations, so if you used a multimeter to measure the potential between the (connected) inside and ground, you would still see a voltage.
  4. Dec 31, 2003 #3
    so, finally, the Leyden jar external plate will remain charged, although there is an external circuit linking the external and internal plates. Thanks for your answer.
  5. Dec 31, 2003 #4
    I've been wondering something about leyden jars. Say I have a plastic film canister which has tin foil layered on the inside and outside; the cap has a terminal stuck through it which is connected to the foil on the inside via wire or something like that. Now I use something positively charged, like an electrophorus, to draw electrons from the inner foil, causing electrons to gather on the outer foil via the ground wire. Now, how does one ground it in the first place? What to you attach the ground wire to; the plumbing? The literal ground? Also, are there any other ways of charging a leyden jar besides using a positively charged aluminum pie pan; say, using a small van de graff generator? Many thanks.
  6. Jan 2, 2004 #5
    Hello Vosh.

    I think there are a lot of ways to charge a Leyden jar. But all the methods to do that talk about grounding one plate, so one plate charges negatively, and the other positively. I know what happen when you connect the two plates when the Leyden jar is charged like that: there is a discharge spark, and the charges redistribute on both plates to get neutral.

    I was talking on another way of charging the leyden jar, without grounding one terminal. In this case, you are charging only one plate with positive or negative excess charge. I was asking the behaviour of the Leyden jar charged like this: Will exist a discharge spark when you connect the plates?, or will the external plate keep charged whatever the conections?
  7. Jan 2, 2004 #6

    You're glossing over my question, which might be my own fault. You keep referring to "grounding". What is that, I mean, I know what grounding is, but how is that done with your leyden jar? I know there is a wire stuck to the outside plate... Then what? Connect the other end to the plumbing? Shove it in the earth?

    Also, what did the answer to your query turn out to be (because I'm not following the posts very well)? Many thanks.
  8. Jan 3, 2004 #7
    Well, to me "grounding" means to let one of the plates follow the charge changes on the other. This is done by connecting the external plate to a place where it can substract or add charges.

    If you charge the inner plate with an electrophorus grounding the external plate, this will make this plate get opposite charge distribution than the inner one. If you don't ground the external plate, the charges will redistribute on the external plate, but no new charges come into play, so there is an excess of one kind of charges on one plate.

    The question of my post is just to know how the behaviour of the Leyden jar is in this case, when you have a real excess charged device (more charges of one kind than the other), not a balanced charge distribution (the same positive and negative charges, but distributed).
  9. Jan 3, 2004 #8
    Excess charges can occur in a non-grounded system, but, be aware that doing so requires an energy input equal to or exceeding the charge capacity. So, nothing is gained.
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