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Another conductor near a charged conductor

  1. Apr 27, 2010 #1
    I need clarifications on the following points regarding a capacitor in general.
    1.When you bring another conductor near a charged conductor, the capacitance of the latter is said to increase, because the presence of the second conductor near the first one lowers the potential of the former. Otherwise it should imply that the ability of the latter to hold charges should increase. But it is said that the capacitance of a conductor purely depends on its shape and size and the medium but not on the charge or potential. Like the capacity of a water tank depends only on its size but not on the amount of water! If it is so, in this case, how the presence of a neighboring conductor can increase the capacity of the conductor as its shape or medium in which it is placed no where changes?
    2. It is said that the charges in a parallel plate capacitor is stored inside the dielectric portion but not on the plates. How could it be? Is it not the natural choice of the charges to go and stay only on the surface of a conductor?
    3. Since the field inside a closed conductor is zero it is always safe to be inside a closed car rather than under a tree during lightning. OK. But if it is an enclosure made by a nonconductor such as a plastic, then what will happen to a person inside it during lightning? What will be the explanation?
    I will be grateful to the forum members if they share their elaborate views on the above doubts that linger in me for quite a long time.
  2. jcsd
  3. Apr 27, 2010 #2
    Re: capacitors

    To the first, this occours because capcitance is about the total area, and the distance between the plates. So for a given circuit it's about the total area of the plates not each individual component as the circuit acts as "one".

    To the second, the charges don't stay on the surface of the conductor, they stay everywhere, and I mean everywhere in the ciruit. It's just that on the plates they tend to decay as they're are attempting to complete the circuit by transfering to another part of the circuit. In some circumstances, they will be attracted or ripped off by air molecules, (hence why often metal plates may oxidise and you will find a residue appear on them). And be careful when you use the word choice. Suggesting an electron has a consious choice is a dangerous thing to say.

    For your last question I don't quite understand what you mean. Are you asking as if there was a field within the enclosure. If not, then I'd say that nothing will happen, because the lightning won't be attracted to the non-conductive plastic.
  4. Apr 27, 2010 #3


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    Re: capacitors

    The dielectric in a capacitor becomes polarised - all the electrons move a bit to the positive plate (you could say). This is the equivalent of having the plates much closer together but without the possibility of a short circuit.
    If you sit 'inside' an insulator, the field across you will be much smaller than if you were outside because the 'Volts per metre' are higher in the insulator than across you and will limit the current that could flow through you. It's in series with you, effectively.
    On the other hand, a conductor around you will conduct any current through itself and reduce the volts across you because it is, in effect, in parallel - the majority of the current will bypass you.
    And, of course, there is the skin effect, which means the arc current - which comprises some very high frequencies - will never get that far inside your Faraday Cage.

    PS try to avoid anthropomorphism when describing physical processes - it can give you the wrong idea about what's happening. Only humans and a few higher primates, possibly, have much of a 'purpose' in their actions. You can sometimes get away with "Behave as if they want to . . . . . "
  5. Apr 27, 2010 #4
    Re: capacitors

    Thank you all for your kind reply.
    Since I am also having some " writer element" in me, I , perhaps, might have used a word with a little poetic color!
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