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Homework Help: Capacitors in series and electric potential

  1. Aug 8, 2010 #1
    I was wondering for a series of capacitors connected together (after full charging), is there a voltage difference between the negative plate of one capacitor and the positive plate of another capacitor down the line?
    -------+q||-q(a)-------+q(b)||-q------+q||-q----------
    i.e. between points a and b


    in my opinion, judging from the statement that for a series of capacitors the voltage of each individual capacitor add together to give the total voltage of the charging battery, it must be that from points a to b there is no potential difference change because whatever voltage drop occured in the first capacitor would be cancelled out at least partially as you move up in potential again, furthermore since two capacitors are connected by a wire if there still existed a potential difference between them then current would still exist and the situation wouldn't be equilbrium anymore

    the problem is I can't reconcile the above with the fact that as you go from point a to point b you are going from an area of negative charge (and lower potential) to an area of positive charge (and higher potential), any help?
     
  2. jcsd
  3. Aug 8, 2010 #2
    Your diagram is confussing. If you mean the -ve end of the first cap that connected to the +ve end of the following cap. There is no potential different between the -ve plate of the first cap and the +ve plate of the second cap. Because you assume the connection is ideal and more important when the capacitor chain is fully charged, there is no current flow and even the connection is not perfect, there still be no drop of voltage because there is no current flow through the chain. Remember the D.C. resistance of capacitor is infinite.
     
  4. Aug 8, 2010 #3
    You know that there is no current between points a and b. So, there is no potential difference and you're not going from an area of positive charge to an area of negative charge. You must be careful with + and - signs. In your case, they only indicate which plate of the same capacitor is at higher potential (and which is at lower potential).
     
  5. Sep 6, 2010 #4
    I belierve there is a pd.

    One reply suggested no pd because no current: This is wrong because (as pointed out)the resistance is infinite. So, you can have a pd between points but no current.
     
  6. Sep 6, 2010 #5
    There is no potential difference. Note that potential depends on energy. The question is, how much work must a charge do to move from one plate to the other? It does no work, as the resistance of the wire is negligible. Thus, they must be at the same potential.
    The fact that there is negative charge on one plate doesn't mean it must have negative potential, as the field of the positive charge is also active in that region. The potential on the negative plate is simply (potential on positive plate) - (Q/C)
    I think capacitors cause a potential difference, they are not concerned with the absolute potential values.
    As an analogy, consider two batteries in series, positive connected to negative. There also positive plate of one is connected to negative plate of the other, and they are at the same potential.
     
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