ac/capacitor theory question

Discussion in 'General Electronics Chat' started by Harmon Rabb, Feb 29, 2008.

  1. Harmon Rabb

    Thread Starter New Member

    Feb 29, 2008
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    Mildly brief intro: I'm a patent attorney with an electrical engineering degree, so I'm not a noob to the field, but I do think I have some holes in my knowledge areas that I'm trying to fill in.

    My question is theoretical. Does ac current actually flow through a capacitor? ie. will an electron on one side of a capacitor flow THROUGH it to the other side? I'm thinking no, because the dielectric inside would breakdown then.

    An alternate way to state my question, I guess, would be how exaclty is AC current not blocked by a capacitor? It seems to me that, since AC current changes directions, it's flowing throughout the circuit everywhere but THROUGH the capacitor itself.

    ??

    I apologize if I sound dumb, but I was working on something yesterday and realize I didn't know the theory behind this and it bothered me.
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    No current does not physically flow through the dielectric.

    Whilst it does not flow through the capacitor dielectric, the idea of AC current in capacitor stems from the notion that the capacitor plates charge and discharge with an alternating voltage. Remember:

    i = C(dV/dt)

    Without a changing voltage across the capacitor (i.e. dV/dt = 0) there is no charging or discharging on the plates and current does not flow to the plates. Where there is the action of charging and discharging there is current flow to the plates:

    i = dQ/dt

    See: http://www.allaboutcircuits.com/vol_2/chpt_4/2.html

    Dave
     
  3. Harmon Rabb

    Thread Starter New Member

    Feb 29, 2008
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    i read that chapter before posting, but now that I'm thinking about it I guess it makes sense. engineering school was a while ago, forgive me :eek:
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    No problems, we all get a little rusty at times. Feel free to clarify any further points you have.

    It may take some time, but you could always read the entire chapter to get a better grasp of the idea and in a variety of contexts (may take you 20 minutes or so to read).

    The idea is fundamentally the same as described above.

    Dave
     
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