Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help understanding capacitors

  1. Apr 7, 2013 #1

    So i'm reading that in a capacitor, an electric field is being manifested between two plates. There are no electrons actually travelling from the negative to the positive plate, correct? There is only an attraction?
  2. jcsd
  3. Apr 7, 2013 #2
    Also, I understand that electrons are being gathered on the bottom plate (connected to the negative terminal), but could someone explain what is occurring to the top plate connected to the positive terminal?
  4. Apr 8, 2013 #3
    The simple view is the electrons depart from the top plate, pass through the battery, and arrive at the bottom plate. Thus the battery itself neither gains nor loses electrons. Its function is to provide the force that moves the electrons. The battery is analagous to a water pump - it makes the water move, but is neither a source, nor a sink, for water.
  5. Apr 8, 2013 #4
    Yes, that's correct. Not through the dielectric medium anyway.

    But there's something strange going on here, because despite the lack of flow of charge (i.e. conduction current) between the plates, the magnetic field (as per Ampere's Law) that you typically associate with an electrical current is still present between the two plates when it charges! In fact, the magnetic field between the plates indicate that the current between the plates is equal in magnitude as that of the conduction current in the connecting wires. It turns out there are two kinds of current, and the the latter, ghost-like, type between the plates is called displacement current and does not involve transportation of charge.

    I hope that doesn't muddy the water for you, but you seem to be moving through the gears pretty quick so hopefully it will appeal to your curiosity more than it will confuse you.

    EDIT: Dear OP, a wave of self-doubt washed over me after I posted this and re-read your OP--for some unknown reason, bringing up displacement current felt like a good idea, but that feeling is now completely gone. Sorry for any confusion.
    Last edited: Apr 8, 2013
  6. Apr 8, 2013 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    At this stage it is probably preferable to stick with Sylvia's simpler concept.

    [STRIKE]Her(?)[/STRIKE] Sylvia's mental model will get you a long way.

    Think of the dielectric as an array of polar molecules. Water is a good example - it is odd shaped with + at one end and - at other... that makes it 'polar'..


    In absence of an electric field they'll be oriented randomly.
    In presence of an electric field they'll twist around and align with the field.
    That twisting into a new orientation takes energy , and that's how energy is stored in the dielectric. They'd like to snap back to random orientation.
    Interesting - it's almost a mechanical phenomenon.

    And that's why a capacitor can store energy.

    Observe that pure water is a good dielectric , its ε about 80.
  7. Apr 9, 2013 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The "simple" view of electrons actually 'travelling' a significant distance during an experiment is very risky. By the time your 'average capacitor' has charged up, the net movement of electrons in a circuit is a tiny fraction of a mm. Best just to talk in terms of 'charge' and, at a stroke, you have avoided the problem which many people have of electron flow being in the opposite direction to conventional current and the worry of how far the electrons may actually get. In a circuit, electrons are faceless - like the links in a bicycle chain.
  8. Apr 9, 2013 #7

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    seconded .

    Electrons in wires move imperceptibly slow*,
    whatever charge is it moves comparable to speed of light irrespective of its sign.

    You'll be in agreement with virtually all textbooks and professors if you speak of current in terms of moving positive charge. So your education will go easier.

    Where I worked, engineers and technicians kept up a running good natured rivalry between "Engineer's Current" and "Real Current'......
    My guys were well aware of the fact electron drift is slow.
    But many of them still preferred to think in terms of negative charge when walking a circuit to write Kirchoff's law.. That was quite natural in vacuum tube days.

    So go with the accepted terminology.
    In most engineering circles you'll be looked at askance if you speak of electron flow.

    There are some circles where folks were trained in negative current, and very competently.
    It's good to be able to swap back and forth for it'll gain you respect both places.

    *(they move fast in beam devices like CRT or electron microscope, but we are discussing circuits.)

    old jim
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook