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Homework Help: Unknown Capacitance

  1. Apr 11, 2015 #1
    1. The problem statement, all variables and given/known data
    A 197-pF capacitor is connected in series with an unknown capacitor, and as a series combination they are connected to a 25.4-V battery. If the 197-pF capacitor stores 143 pC of charge on its plates, what is the unknown capacitance?

    2. Relevant equations
    Q = C*V (charge stored on each capacitor)
    Total Capacitance = 1/C1+1/C2
    Total Charge Stored = QTotal = CTotal*VTotal

    3. The attempt at a solution
    I don't even know where to start. It seems like I should just be able to use one of the equations above but just rearrange it so I am solving for the unknown capacitor.
  2. jcsd
  3. Apr 11, 2015 #2
    I know that the charge of capacitors in series is equal.
  4. Apr 12, 2015 #3
  5. Apr 12, 2015 #4


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    Staff: Mentor

    Capacitors in series don't add like C1 + C2. That's why your attempt at a solution went awry and you came up with a negative value.

    But your observation that the charge on capacitors in series are equal is valid, and will prove to be useful here.

    Since you're given the capacitance and charge on one of the capacitors you should be able to determine the voltage across that capacitor. What then is the voltage on the other capacitor? What's the charge? What then is its capacitance?
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