A capacitor as an open wire that may induce current

Discussion in 'Homework Help' started by pigpen, Jan 29, 2016.

  1. pigpen

    Thread Starter New Member

    Jan 26, 2016
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    Just wondering if there is anything very wrong about my personal idea of capacitors vs their standard definition as devices " consisting of one or more pairs of conductors separated by an insulator (etc)" --

    Well, that's just like a broken wire, isn't it? The insulator / dielectric might be designed according to how much electric field it's required to hold, but unless the capacitor leaks, no current ever passes between the conductors. Rather, the field induces current at the other end upon discharge (etc). So wherever capacitors are in any circuit there are broken wires that upon certain conditions will allow current to be induced...

    Also this thing I often hear about lightning being a natural capacitor -- it's more like lightning is a leak in a natural capacitor with the clouds and ground being the conductors, and air the dielectric.

    This line of thinking has worked for me through further readings -- but I've never heard of capacitors being explained as broken wires designed to induce current pending certain conditions, so I really start to wonder.
     
  2. sailorjoe

    Member

    Jun 4, 2013
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    Find a capacitor and take it apart. It's not built like a broken wire, it's more sophisticated than that, and for good reason. Also, search the term Leyden jar.
    And I've never read about lightning being a capacitor, but yes, the atmosphere can act like a capacitor. Lightning travels from clouds to earth, from earth to clouds, and from clouds to clouds.
    When I lived in a high lightning zone, I found that lightning never finds the lightning rod, though.
     
  3. RBR1317

    Active Member

    Nov 13, 2010
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    Q: What is the difference between a capacitor and a broken wire?

    A: The value of capacitance is marked on the capacitor.

    Q: If no current ever passes between the conductors, how is Kirchoff's current law satisfied when a capacitor is connected to a circuit node?

    A: ......???......
     
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  4. pigpen

    Thread Starter New Member

    Jan 26, 2016
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    Oh current never passes between the conductors (though there can be some leakage current), that's why the capacitor symbol is an open switch. The electrons stop dead within the conducting plates, which causes an electric field to build up in the insulating layer (etc) This stuff is in LIEC.

    When talking about capacitors some people make sure to say "displacement current" because no electrons flow between the 2 conductors. I'm looking for a way to describe what happens inside capacitors simply because I know many many people including 2 technicians who believe what happens inside capacitors is totally inaccesible. Theyve just memorized all the testing methods and types and i feel bad for them....
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What "happens inside capacitors" is precisely the displacement current you seem to disbelieve. It is quite a real current with an associated magnetic field though it does not consist of moving charges. It is due to the varying electric field.

    If it did no exist your radio would not work.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Your "broken wire" thinking is correct in the sense that a capacitor cannot pass a DC, or steady state, current. It is also correct in the sense that even a wire - or any real component - has a small capacitance.

    The big difference comes from the design of the capacitor to purposefully maximize the capacitance. They are able to sustain a relatively large electric field across a distance and over some area. This gives them far more capacitance - the ability to store energy in an electric field - than a "broken wire".

    Lightning is like the breakdown of the dielectric in a large natural capacitor.
     
  7. RBR1317

    Active Member

    Nov 13, 2010
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    And yet in network analysis we say that current flows through the capacitor just like current flows through a resistor, although with a differential equation relating the current and voltage rather than Ohm's Law. Physically it could be said that one capacitor terminal sustains an accumulation current while the other terminal sustains an equal depletion current. So the electrons that enter the capacitor are not the same electrons that leave the capacitor via the other terminal. But the same could be said of a simple length of wire, that because of electron drift velocity, it can be quite some time before the same electrons entering one end of the wire will eventually exit the other end.
     
  8. Papabravo

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    Feb 24, 2006
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    It is important to understand the distinction between AC current and DC current. Capacitors do block DC current, but they pass AC current. Until you can get your head around how AC current can "flow through" a capacitor, you'll keep tying yourself in knots.

    Although the math is above most folks heads, one of the primary results of Maxwell's Equations is the following:
    1. All changing electric fields produce a magnetic field.
    2. All changing magnetic fields produce an electric field.
    In an AC circuit, electric and magnetic fields are constantly being produced and consumed; wash, rinse, repeat.
     
  9. RBR1317

    Active Member

    Nov 13, 2010
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    This must be where engineers have the advantage, because in the complex frequency domain there is no distinction between AC current and DC current.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    Sure there is. Let ω be equal to zero, which it is on the real axis, and you have DC. Not that using the frequency domain or the complex plane to solve DC problems appears to be particularly useful. In control theory we often have poles on the negative real axis which correspond to non-oscillatory exponential decay of transients.
     
  11. RBR1317

    Active Member

    Nov 13, 2010
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    What about DC problems involving DC transients that would, for instance, cause current to flow through a capacitor?
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Then how do you explain one of the most common circuits in the world, one that passes not just amperes but tens of amperes through capacitors - a low-quality, high-power stereo.

    ak
     
  13. pigpen

    Thread Starter New Member

    Jan 26, 2016
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    OK it looks like everybody except for wayneh and ErnieM forgot that there is an insulator between the two metal plates. What is the definition of an insulator?

    ErnieM, of course I believe in displacement current (rereading my TS now I do see how disorganised it is--sorry). What I meant to say was that I appreciate those who take care to use the term "displacement current" instead of "current flow". I wish displacement current could become the standard way to refer to how current is induced via capacitors. This way people will never forget that there is an insulator between the metal plates, that it is the reason why the charges accumulate in the metal plates instead of flowing through, and why the insulator carries an electric field. And it's of course a large part of the reason why capacitors charge and discharge and "block" DC and "pass" AC.

    Yes, thanks, the dielectric breaks down, resulting in a leak.

    Yes but my main concern is the simplest possible analogy -- then add the details later -- because I'm trying to explain capacitors to two guys who believe this stuff is just beyond their ken forever. They are, ironically, adept at metal arc welding.

    That is an excellent point. But you see I'm looking for a description of capacitors that doesn't immediately contradict the fact that there is an insulator between the two metal plates.
     
    Last edited: Jan 30, 2016
  14. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Before i say anything else, note that the current in both leads of the capacitor is a real electron flow current, but electrons can not pass through a dielectric. Also note that when current does flow in the two leads, it is never under a static condition, it is a very dynamic situation that can not happen indefinitely but only for a relatively short time period in real life, and this means that there was a temporary external change that had occurred. The external change gives rise to an acceleration of electrons.
     
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  15. RBR1317

    Active Member

    Nov 13, 2010
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    There is no explanation that avoids the fact that electrons move from one plate to the other; that excess charge accumulates on one plate, and an equal amount of charge is depleted from the opposite plate; that current appears to flow through the capacitor. However, the electric current actually flows from one plate to the other through the external circuit, not through the dielectric.

    You might resort to a hydraulic analogy here. A water storage drum is divided in the center by a flexible membrane through which no water passes; however, the membrane can stretch to accommodate more water on one side and less on the other - but with increased pressure. An external pump can move water from one side to the other, but no water ever passes through the membrane - unless the stretching pressure becomes too great and the membrane ruptures. So just think of the drum as a capacitor, the membrane as dielectric, water as electric charge, and the pump as a voltage source. Works for me.
     
  16. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi again,

    What i was getting to was that if we had a bare wire with temporary constant current say 1 amp and a capacitor with temporary constant current 1 amp we would see the same magnetic field outside each of these devices even though the dielectric does not conduct current. Thus it appears to conduct, and we use that model as the circuit analysis model even though it may not be as true to life as we would like.

    The real question however is what happens when we use a vacuum capacitor which has no dielectric that can contain any bound electrons. The field is the same for that device, yet electrons can not flow through a vacuum freely (with low potentials). Either the plates are supplying the temporary electrons or the electrons are being induced in the opposite plate through the magnetic field. If the magnetic field is not changing however, then they can not come from any induced field.
    The next thing to do then would be to calculate the volume of each plate and the number of electrons that can take part in the movement of charge and see if there is enough volume to allow such a thing for a long enough time before the voltage built up enough to cause a spark over. The point of spark over would be related to the distance between plates, and the number of electrons would be due to the plate volume.

    If you care to try that math/physics experiment that would be interesting too. The plates could be assumed to be copper for example.
    Two main outcomes would be possible, either we run out of electrons before the sparkover or the sparkover occurs before that. It may be related to the overall size of the cap too, such as 1pf, 10pf, etc. There may be an optimal cap construction size also.
    In normal use we never reach this extreme because we usually charge and discharge rapidly, which would never stress the cap in this manner.
    We might prove that we can never build a cap that can "run out of electrons", or we might prove that such a construction is possible.
     
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  17. pigpen

    Thread Starter New Member

    Jan 26, 2016
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    Thank you.
    -------

    2.
    I wonder why I have never seen a comparison made between transformers and capacitors: transformers require a changing field for voltage to be induced, hence they don't work for DC, can only work for AC. Capacitors require a changing field for current to be induced, hence they "block" DC and "pass" AC.


    3.
    I think everyone agrees with this stuff. The problem is more in the basic description... I'm sure there are other newbies out there too who are wondering how current can flow THROUGH the metal plates when there is an insulator between them. Why doesn't the basic description of capacitors say immediately that this insulator prevents current flow BUT is designed to carry a changing field that helps to induce current.... etc. Why do basic definitions of capacitors not focus on the changing field in the insulator?

    Why is it that saying

    ---that current doesn't flow between the metal plates but rather current is induced via a change in the field inside the insulator/dielectric--.

    why is that so controversial? I mean, it is in LIEC. Why are people fighting over "current flow" and why do so many refuse to accept the term "displacement current" or "induced current" when it is a better descriptor of what capacitors do?
     
  18. dannyf

    Well-Known Member

    Sep 13, 2015
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    The thinking is often the other way around: an open wire as a capacitor.
     
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  19. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I have oft seen them described as analogs of each other, as one does in voltage the other does in current.

    Sounds like you ar trying to fix the interweb. Lots of stuff gets posted that is utter crap.

    I would not say it is controversial rather it is plain and simply incorrect. A current does indeed flow between the plates.

    What is this "LIEC" thing?
     
  20. wayneh

    Expert

    Sep 9, 2010
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    Well it is a bit mysterious. With experience and training - a few introductory physics classes - you learn how to describe electric fields, and to predict how they will behave. How charges will move. It's not terribly complex. Static on a balloon is a pretty good demonstration of charge separation.

    But what you won't learn is just how a field can act over a distance, with no medium in between. How exactly does a magnet work, or gravity? You have to study to an advance level to begin to grasp how a simple magnet works, beyond precisely describing what anyone can see.

    Man has wrestled with this for centuries, and the "impossibility" of fields led to the aether hypothesis, that there is some mass in empty space that can propagate waves and fields. It was too hard to grasp how forces can work at a distance across the void.
     
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