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RC circuit problems

  1. Mar 29, 2011 #1
    1. The problem statement, all variables and given/known data

    A 2.00μF capacitor with an initial charge of 5.10μC is discharged through a 1.3 kΩ resistor.

    a) calculate the current through the resistor 9.00ms after the resistor is connected across the capacitor

    b) what charge remains on the capacitor after 8.00ms

    c) what is the maximum current in the resistor?

    2. Relevant equations

    Discharging capacitor
    I = (Io-Vo)e^-t/RC
    q = Qe^-t/RC

    C= Q/(Vf-Vi)

    I = V/R
    3. The attempt at a solution

    C= Q/(Vf-Vi)
    V= 5.1/2


    Unable to find Io and have no idea where to go from there.
  2. jcsd
  3. Mar 29, 2011 #2


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    Staff Emeritus
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    What is the voltage across the capacitor with its initial charge?
  4. Mar 29, 2011 #3
    It should be:


    V=2/5.1 = 392mV
  5. Mar 29, 2011 #4
    If you want to find I0, you should know V0. But what is V0?
    Do you know the capacitor relation C=Q/V. Can you use this to find V0?
    So knowing V0 and R you can find I0.

    And can you check your first equation? how the term -V0 exist there?
  6. Mar 29, 2011 #5

    Io = Vo/R
    Io = 301mA

    (Io)e^-1.38461538 × 10-11
    1 * Io
    I = 301mA?
  7. Mar 30, 2011 #6


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    Staff: Mentor

    Check your units. Always check your units!

    Capacitance has units [Coul]/[Volt]. Charge has units [Coul]. So the basic expression is

    C = Q/V giving you V = Q/C .
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