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Homework Help: Transient Response RC series circuit

  1. Sep 24, 2013 #1
    Hi, I have been given a question which involves analysis of a series RC circuit. The system in question has a 4Ω resistor in series with a 1000μF Capacitor (the capacitor is to be considered as completely uncharged before t=0). There is a 12V supply in the circuit. At t=0, a switch is closed to complete the circuit. I have been asked to calculate the following and was wondering if someone could confirm whether or not my answers are correct;

    i. - initial (t=0) current in the circuit.
    ii.- initial voltage across the capacitor
    iii- final voltage in the capacitor
    iv- time constant of the circuit
    v - current in the circuit at 6ms
    vi - final energy stored in the capacitor


    i. initial (t=0) current in circuit = V/R = 12/4 = 3.0A

    ii. Initial voltage across the capacitor = 0V (as Capacitor is uncharged before switch is closed at t=0)

    iii. final voltage in capacitor = V = 12V.

    iv time constant of the circuit = ζ = CR = 1000 x 10-6 * 4 = 4mS or 0.004S

    v. current in circuit at t=6ms = i(t) = Ioe(-t/ζ)
    this = 3.0e-(0.006/0.004) = 0.67A

    vi - final energy stored in capacitor = E = (CV2/2) = (1000x10-6 * 122)/2 = 0.072J

    Any feedback would be greatly appreciated! :)
  2. jcsd
  3. Sep 24, 2013 #2
    Your answer looks good. Only kept in mind that capital "S" is reserved for Siemens, electric conductance 1/Ω. For "time" use small "s".
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