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Homework Help: Capacitor discharge?

  1. Oct 5, 2009 #1
    capacitor discharge???

    1. The problem statement, all variables and given/known data

    A 1.4 uF capacitor, initially uncharged, is connected in series with a 10 k ohms resistor and a 6 V battery of negligible internal resistance.

    How long does it take the capacitor to reach 99% of its final charge?

    this is part b. i already found out part a which asked What is the charge on the capacitor after a very long time?
    answer to this part was 8.4 uC

    2. Relevant equations

    for part b i think i will have to use the equation
    Q=Qf(1-e^(-t/T)) where t is time and T is the time constant defined by T= RC

    3. The attempt at a solution

    i plugged in arbatrary values just to define my 99% i picked 5 but that should not matter

    solve for t
    this will be in seconds i assume and my answer wants it displayed in ms
    i got an answer of around 32 ms but it says im wrong.
    any help would be great!!!
  2. jcsd
  3. Oct 5, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: capacitor discharge???

    First of all, the 5 and 4.95 should be switched around. If 5 is the final charge Qf, then you want to know when Q=4.95. Other than that, your equation is correct.

    I get 1 - e^(-0.032/0.014) = 0.898.., not 0.99 as desired. So you probably made an algebra mistake somewhere in getting the 0.032 sec answer.
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