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If the positive plate on a charged capacitor will pass current to the negative

  1. Dec 11, 2012 #1
    ...plate of a different capacitor, why wont the positive end of a battery (lets say AA) pass current to the negative plate of a different AA battery. Does it have something to do with the chemical reaction that happens inside the battery?
     
  2. jcsd
  3. Dec 11, 2012 #2

    jedishrfu

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    The plates are separated enough and the electrons cant jump from one plate to the next hence no effective current flow. Capacitors hold charge.

    Here's some more info on capacitors:

    http://en.wikipedia.org/wiki/Capacitor
     
  4. Dec 11, 2012 #3
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    lundyjb,

    What do you mean by that statement? Do you mean that the charge from one plate will leak through the capacitor's dielectic, and arrive at the opposite plate (WRONG!)? Or do you mean that if there is an external conduction path from one plate to the opposite plate, the charge imbalance will equalize (RIGHT!)?


    jedishrfu,

    Current flow literally means "charge flow flow". You should just say "current" or "charge flow"

    No, they do not hold charge. They store energy. A capacitor energized to 1000 volts has the same net charge as it did when it was energized to zero volts.

    Ratch
     
    Last edited: Dec 12, 2012
  5. Dec 11, 2012 #4
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    i understood the question, actually there would be current from one battery to the other but only for a short time, because there is no continuity (circuit not formed) between them, the electrons would pile up in the "receiving battery", if you want a continue current to exist you must set a path back from the receiving battery to the first one (both would be connected in series).

    in an extreme scenario you could imagine that the receiving battery would take all electrons from the first battery (there's a finite amount of electrons), because the chemical reaction will always maintain each poles at a constant potencial level, but once electrons pile up in the receiving battery they will create a strong electric field that would reject and stop the incomming electrons.
     
  6. Dec 11, 2012 #5

    jedishrfu

    Staff: Mentor

    Re: If the positive plate on a charged capicitor will pass current to the negative...

    Thanks for your corrections, Ratch. You are correct, although it is common to say current flow.

    With the charge comment, I was really referring to the one plate being more positive (lack of electrons) and the other being more negative (excess of electrons) but you are correct the net charge is zero for the capacitor.
     
  7. Dec 12, 2012 #6
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    Nevertamed,

    Wrong with respect to both a battery and capacitor. According to your reasoning, a bird would not be able to roost on a high voltage wire because of a supposed short transient current it would receive when it first touched the wire. It just doesn't happen that way.

    A battery does not accumulate electrons on either pole, and a capacitor does not imbalance the charge on its plates unless there is a voltage difference between them. Just connecting one plate of a capacitor to a voltage source won't do anything, because there would be no voltage difference between the plates.

    Ratch
     
  8. Dec 12, 2012 #7

    sophiecentaur

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    This actually does happen on the highest voltage transmission cables. Birds stick to Intermediate and Domestic voltage cables. (When I say "stick to", I don't mean glue-like)

    I don't know how they know to avoid them. Perhaps the local field strength causes them to tingle and so they keep clear of such attractive perches.
     
  9. Dec 12, 2012 #8
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    sophiecentaur,

    That has to be caused by high voltage induction (affect without touching due to high electric fields), not conduction. I see plenty of birds roosting on domestic power lines without any ill effects.

    Ratch
     
  10. Dec 12, 2012 #9
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    Actually there's a little current through the bird, since there is finite resistance in the wire, and finite resistance in the bird, it still forms a parralel circuit. of course the wire's resistance would be meaningless compared with the bird's, and we all know the current always "choose" the easiest path, only a tiny amount of current goes through the bird
     
  11. Dec 12, 2012 #10

    sophiecentaur

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    Yes - we all do. It must be a matter of Volts per metre and the capacity and length of the bird's body which limits the Voltage to which they a tolerant. I was referring to High Voltage cables - 132kV and above.
     
  12. Dec 12, 2012 #11

    jim hardy

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    Sophie is correct.

    Birds tend to avoid power lines > about 30KV.
    I'd guess it tickles their feet.

    It'd be the minute capacitive current that charges their body alternately to +/- line peak voltage at line frequency , 50 or 60 hz.

    When working such lines hot, linemen wear a special suit that forms a Faraday cage around them.

    http://www.hubbellpowersystems.com/lineman/accessories/suit.asp
     
  13. Dec 12, 2012 #12
    Re:...

    the thread starter stated " If the positive plate on a charged capacitor will pass current to the negative plate of another capacitor" which is true, because there would be a voltaje between both plates regarless of being part of different capacitors. however there would be only a short current as the voltaje fade away
     
  14. Dec 12, 2012 #13
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    batteries create a constant imbalance of charge, to maintain a DC voltage
     
  15. Dec 13, 2012 #14

    sophiecentaur

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    In fact, there is a small but finite Capacitance between the unconnected end of the capacitor and ground. This will lead to a finite but small charge flow, via the main Capacitor when it's connected. You can regard it as two capacitors in series - one large and one tiny one.
     
  16. Dec 13, 2012 #15
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    Nevertamed,

    Remember, caps do not get charged, they get energized. The positive plate is deficient in electrons. You refer to two capacitors and four plates. So which plates will there be a voltage between? Where is the conduction path for the "short current"? I don't know how to answer your message because I don't know to what you are referencing.

    What does that mean? How much charge for what voltage? If it were a cap, I could multiply the voltage by the capacitance and give you an answer. But a battery is a electrochemical device, and does not use a electrostatic field to maintain its voltage like a cap does. Both a battery and a cap sustain a voltage, but they do it in very different ways. Besides, a battery is an active device, whereas a cap is not.

    sophiecentaur,

    Your description of the above really confuses me. First of all, ground is nothing special, just a common connection point. The capacitance between one end of the capacitor and the other end is the value of the capacitor. Where and how does the second capacitor come into play? Inquiring minds would like to know.

    Ratch
     
  17. Dec 13, 2012 #16

    sophiecentaur

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    OK then. Replace 'ground' with 'the other terminal of the battery'. The effect will then even less but there are still, effectively, two capacitors in series with the battery. (And another small one directly across the battery terminals.) If the battery is supplying an emf, then this network of capacitors will charge up according to Q=CV.
    Explanations in terms of equivalent lumped components are often helpful - antenna theory is often usefully approached this way.


    btw If you want to be fussy about nomenclature, then I think it would be more desctiptive to say that Capacitors become Polarised, if you don't like 'charged'. (I have not read of the term "Energised" in this context - it is used more int the context of Batteries, I think but I don't think it is defined very rigorously, though people do talk of 'energising a coil' when you switch on the current, I suppose). That just means that there is a displacement (imbalance) of net charge from one side to the other. With no dielectric, the charge needs to be 'taken off' one side and 'put into' the other. When there is a dielectric, the additional charge (giving it higher Capacitance) is due to charges being easily displaced within the dielectric material as its molecules become polarised. You need to move more charges in this case for a given Voltage.
     
  18. Dec 13, 2012 #17
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    actually when they get charged they get energized (they get energy from a external source and store it as an electric field between the plates), about the 2 caps and the 4 plates:

    we have the first cap: with one plate full of electrons and the other one lacking (after being charged or energized if you like it)

    we have a second cap: with one plate full of electrons and the other one lacking (after being charged or energized)

    if you set a path (an hypothetical wire) from the first capacitor's full of electrons plate to the second's capacitor lack of electrons plate; there will be a short current until this imbalance disappear (there wuld be a voltage between them, despite the fact the plates involved belong to different caps)

    about your second quote:

    In a way, a capacitor is a little like a battery. Although i know they work in completely different ways, capacitors and batteries both store electrical energy. A battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal, it maintains this imbalance of charge ( charge separation) as the acid keep working in order to offer a constant voltage
     
  19. Dec 13, 2012 #18

    sophiecentaur

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    Re: If the positive plate on a charged capicitor will pass current to the negative...

    I don't understand the need to use a term, other than 'charged' do describe what happens to a battery. Everyone but everyone knows what it means and it's used in every book I have ever read on the subject. The qualifying word 'differentiallly', which might be added in front is omitted because it is such an accepted term.

    Yes, of course Capacitors store Energy but the Energy they 'store' is not the same as the Charge as it depends upon the Capacitance value - the two quantities, Charge and Energy are not synonymous. It is the Charge that is used in the context of most circuit calculations but - horses for courses.

    The answer to the original question lies with the parasitic Capacitances involved.
     
  20. Dec 13, 2012 #19
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    i like and always use "charge"
     
  21. Dec 13, 2012 #20
    Re: If the positive plate on a charged capicitor will pass current to the negative...

    sophiecentaur,

    I still cannot understand what you are saying because I cannot visualize the circuit you are talking about. Perhaps a schematic?

    I don't think "polarization" is a good word to use for energy storage. It sounds too much like a polarized capacitor such as an electrolytic. Energized simply means "charged with energy".

    A capacitor always has a dielectric.

    Just another way of saying the capacitor is more energized. E=1/2*C*E^2 or E = 1/2*(Q^2)/C
     
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