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A 3500-pF air-gap capacitor question

  1. Apr 8, 2015 #1
    A 3500-pF air-gap capacitor is connected to a 32-V
    battery. If a piece of mica is placed between the plates,
    how much charge will flow from the battery?


    2. Relevant equations
    C = K€A/d



    3. The attempt at a solution
    Q1=C1V1
    Q2=C2V2

    V1=V2=V
    C2=7C1(as only diference between C1 and C2 is the mica and it's K is 7 so it's 7 times C1)
    Result: Q2=7.84 x 10^-7Coloumb

    **But solution manual say it's 6.7 x 10^-7Columb BECAUSE it calculates the new K by subtracting 1 from 7 (1 is the K of air) ???!!! Why subtract??? totally lost :(
     
  2. jcsd
  3. Apr 8, 2015 #2

    ehild

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    Homework Helper
    Gold Member

    The question is how much charge is added to the already existing charge to the capacitor, which is Q2-Q1.
     
  4. Apr 8, 2015 #3
    Ahh so by saying "how much charge will flow from the battery?" it means "how much ADDITIONAL/EXTRA charge will flow from the battery?" .. OK I've got got it now but the that part if the question is a bit misty, don't you think?

    Thank you!
     
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