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

Capacitor & Inductor?

  1. Sep 19, 2003 #1
    Q-1: Why capacitor blocks DC and allows AC to flow?

    Q-2: Why inductor blocks AC and allows DC to flow?
  2. jcsd
  3. Sep 19, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    A capacitor consists of 2 conductive plates separated by a non conductive dielectric material. There is no path for DC currents, it is an open circuit as far as DC is concerned. AC is able to pass because electric charge is stored on the plates, energy can be exchanged through the changing E&M fields that exist between the plates.

    An Inductor is merely a piece of wire so it is seen by DC as simply that, a wire. To AC on the other hand the continually changing E&M fields attempt to maintain a constant current, thus it resists the AC changes, imposing an impedance that depends on the frequency of the AC current.

    A Capacitor also presents an impedance to AC which depends on the frequency.
  4. Sep 19, 2003 #3

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    Some clues:

    Think in terms of electrons. In a capacitor, can electrons jump the gap across the plates? Can the electrons exert a force across the gap?

    In an inductor, what must a magnetic field do to induce a "back emf" ? What must happen to the current to make the magnetic field do this?
  5. Sep 20, 2003 #4


    User Avatar
    Science Advisor
    Gold Member

    Think in terms of electrons. In a capacitor, can electrons jump the gap across the plates? Can the electrons exert a force across the gap?

    Of course. You must think in terms of electrons but in a wave form. In the fourth Maxwell's Equation there is a "displacement current" that reffers to this effect. The electrons can be transported inside a conductor, but between the plates too. Think in what it's the real meaning of the electromagnetic force, only an exchanging of some information (virtual photons) between two particles.
  6. Sep 20, 2003 #5
    Regarding your first question:

    In a capacitor there is never any
    flow of current across the plates.
    A capacitor doesn't actually let
    alternating current "flow". What
    happens is that, when the current
    is going in the first direction
    the plates are charged according-
    ly; one side positive, the other

    If the current continued in the
    same direction the capacitor would
    become fully charged and the flow
    would stop. Instead the current
    changes direction. The side that
    was negatively charged is now
    positively charging, and visa

    The illusion that the capacitor is
    allowing alternating current to
    flow across the plates is caused
    by the fact it takes a certain
    amount of time for the capacitor
    to charge. The flow of current is
    actually flowing into "storage"
    so to speak, not across the
    plates. When the current is
    reversed the "full" side is emp-
    tied and the "empty" side is
    refilled. (The "full" side would
    be the "negative" side: it is full
    of electrons which carry what we
    have decided to call a "negative"
  7. Sep 26, 2003 #6
    well thanx to all of you !

    thanx to all of you for quick replies, are those enough answers to my question?Can we go more in depth???

    please suggest me any website where I can read all about Capcitor and Inductors in detail.I am looking for more in depth knowledge.

    Last edited: Sep 26, 2003
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?