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Homework Help: Initial Energy/Final energy on 2 capacitors

  1. Mar 27, 2014 #1
    1. The problem statement, all variables and given/known data

    Two capacitors, C1 = 4μF and C2 = 9μF, are connected in series with a 30 V battery. The battery is removed and the plates of like sign are connected. Find the initial and final energies for each capacitor.

    2. Relevant equations

    Ceq = ∑Ci ( don't know how to note it on the computer) --> in parallel
    1/Ceq= ∑1/Ci
    I think that's about it..?

    3. The attempt at a solution
    1) find ceq ( in series) =>2.77uF
    2)Find totaly charge ( same on both cause in series) =>93.077uC => Get initial energy from this (????)
    3)Voltage on C1 => q/c => 20.78 V
    4)Voltage on C2 => 1/c => 9.23 V
    5)After they are reconnected, it needs to reach equilibrium ( I think?) So my total charge is
    2*83.077 = 166.154 uC ??
    Sooooo then my voltage on capacitor 1 would be 166.154/13 => 12.78V ( approx.)
    6)Q1 final => c*v => 4uF*12.78 =>51uC
    7)Q2 final => c*v => 9uF*12.78 =>115.02uC
    9) Energy Final on 1 => q^2/2C => 325.125 J
    10) Energy Final on 1=> q^2/2C=> 735 J
  2. jcsd
  3. Mar 28, 2014 #2


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    Homework Helper

    It is correct, but what are the initial energies?
    You could have used also the formula Energy=U^2*C/2 for the final energies.

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