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Condensers connected in series

  1. Feb 23, 2013 #1
    Question 1:

    Two condensers connected in series will be charged such that one gets +Q charge on one of its plates, and the other -Q.

    Schematic: ------(-Q)| |(a)------(b)| |(+Q)-------

    Can somebody prove (or at least show) intuitively to me why the plate "a" gets a +Q charge, and the plate "b" -Q?

    Question 2:

    A dialectic is inserted into a condenser made up of two plates. Afterwards, the condenser acts like it is two condensers connected in series. Why?
    Last edited: Feb 24, 2013
  2. jcsd
  3. Feb 23, 2013 #2
    The plates and wires initially ("uncharged") have + and - charges (electrons and the atoms they would "belong to at zero deg absolute")
    These are all jostling together due to thermal agitation (300 deg above abs zero).
    So at any INSTANT in any volume there IS a charge inbaleance + or -. This is called "Noise"
    But LONG TERM average ther is none - it tends to disappear. "Flow away"
    This is said to be "current flowing away"

    When we apply a voltage, this pushes the charges aside, making one end of a wire or conductor + and the other -.
    + means there is a VERY SMALL % more + charges there then -ve. A charge migration.

    Now look at your diagram how your capacitors are "connected in series"
    The REASON a gets minus the charge b gets (equal and opposite) is a+b=0 total charge is all there is! We cannot CREATE charge - only move it around (ever so slightly)

    Inserting dielectrics between plates and removing them is VERY TRICKY and misleading!
    You DO WORK because of the force needed to remove the dielectric.
    So the VOLTAGE changes
    Also all surfaces have monofilms of WATER on them
    The plates do: the dielectric does
    So where does the charge (voltage: coulombs) REMAIN when the dielectric is removed!!!!
  4. Feb 24, 2013 #3
    You didn't understand my questions.

    1) Why is a=-b=Q charged? Why is the charge of the two inner plates equal to the charge of the outer plates?

    2) If a dialectic of length d is inserted between two parallel condenser-plates distanced D from each-other (d<D), the condensers act like they are connected in series. Why?
  5. Feb 24, 2013 #4
    If the charge on a and b was smaller than Q, the two condensers would each have an excess charge and so they would pull/push electrons in the connecting wire. Only when the amount of positive and negative charge in each condenser is balanced will the external electric field vanish and only then will the current in the wire stop flowing.
    Could you explain your second question in more detail? In which way does the capacitor act like two in series? If you put a dielectric inside the capacity increases.
  6. Feb 24, 2013 #5
    Thanks for answering Q1.

    As for Q2: That's how it is explained in the assignment. It's a weird problem. Look here at the solution-manual, assignment 4b ("Oppgave 4 b", page 4) http://folk.ntnu.no/sveinoll/ov/TFY/4155%20Elektrisitet%20og%20Elektromagnetisme/V11/LF/Ov06los.pdf [Broken].

    The original situation is like A, and the condenser acts like in B.

    Why would the dialectic suddenly be able to provide such a charge-distribution that it acts like a part of a condenser in series?
    Last edited by a moderator: May 6, 2017
  7. Feb 24, 2013 #6
    Is nobody here smart enough to solve my professor's problem? Come on! :tongue:
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