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Charge in circuit, capacitor - explaining current

  1. Feb 12, 2012 #1
    1. The problem statement, all variables and given/known data

    I've attached a photo of a circuit with a battery and three capacitors. I have a qualitative question about the problem. In the diagram, four points (a through d) have been labeled. The problem asks how many electrons pass through the wire at those points. Is it ok to make the following statements about each point? I'm having a hard time explaining what I think happens, which I think is indicative of my lack of understanding of this topic.

    - The magnitude of the charge that accrues on the equivalent capacitor C123 is equal to the magnitude of the charge that passes through point a.

    - The magnitude of the charge that accrues on capacitor C1 is equal to the magnitude of that charge at point b, since it's right after the capacitor.

    - The magnitude of the charge that accrues on capacitor C2 is equal to the magnitude of the charge at point c, since it's right after that capacitor.

    - The magnitude of the charge that accrues on point d is equal to the magnitude of the charge that accrues on C3, since the charge has to pass through d to get to C3.

    Some of my wording may be really awkward...If it is, please don't hesitate to call me out on it. My wording above is the best way I explain how I currently conceive of charge, capacitance, etc...if it's off, my understanding is probably off too!

    Thanks!

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Feb 13, 2012 #2

    BruceW

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    Homework Helper

    These aren't quite right. There is no charge buildup at points b,c and d. The problem asks how many electrons pass through the wire at those points. So they are asking for the net charge which flows through those sections of the circuit.
     
  4. Feb 13, 2012 #3
    Thanks for your reply. I'm still super confused with this material and apologize for the awkward wording. I see that there is no charge buildup at points a through d. However, is it ok to say that the magnitude of the moving charge is the same as the magnitude of the charges that accrue on the nearest capacitors (or the equivalent capacitor in the case of a)? The answer key for point b, for example, shows that I should find the charge on C1 (by first finding the voltage of C12). That charge is then divided by the charge of an electron to get the number of electrons passing through b.

    I can't visualize the justification for the math without saying that the charge at the labeled locations must be equal to the charge at the nearby capacitors. Is that not right? Sorry again if I'm going around in circles.
     
  5. Feb 13, 2012 #4
    Can you see that C1 and C2 are in parallel so are equivalent to a single capacitor of (C1+C2)
    This equivalent capacitor is in series with C3
    Do you know how to find the charge in these arrangements of capacitors.
    Asking for the number of electrons that pass each point is asking you for how much charge passes each point to charge up the capacitors.
     
  6. Feb 14, 2012 #5

    BruceW

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    the electrons pass through the point b. Then they end up on the capacitor. So what can you say about the total number of electrons that pass through b compared to the number of electrons which have built up on the capacitor?
     
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