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Homework Help: Initially charged capacitor disharged through a resistor

  1. Nov 11, 2008 #1
    1. The problem statement, all variables and given/known data
    A capacitor with initial charge q0 is discharged through a resistor.
    (a) In terms of the time constant τ, how long is required for the capacitor to lose the first one-third of its charge? answer x tau
    (b) How long is required for the capacitor to lose the first two-thirds of its charge?
    answer x tau



    2. Relevant equations
    i = -(q0/RC)e(-t/RC)


    3. The attempt at a solution
    I tried solving for time t and got -RC*ln(q/q0)= t
    I'm lost i don't know what to do
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 11, 2008 #2

    Doc Al

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    Staff: Mentor

    Looks good to me (except for the minus sign). So what's the time when q = q0/3?
     
  4. Nov 11, 2008 #3
    i then get t = RC*ln(1/3) where RC = tau

    is this correct?
     
    Last edited: Nov 11, 2008
  5. Nov 11, 2008 #4

    Doc Al

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    Staff: Mentor

    Yes. But put that minus sign back in. (I didn't see that you had q0 on top! :wink:)
     
  6. Nov 11, 2008 #5
    thanks for the help i figured out everything.
     
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