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Find v(t) and i(t) in the RC circuit

  1. Nov 1, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-11-1_19-30-45.png

    2. Relevant equations
    Voltage Division
    v2 = vs * (r2/(r2+r1))
    KCL - sum of all currents at a given node is 0
    V= i * r

    3. The attempt at a solution

    This is my solution:
    https://scontent-atl3-1.xx.fbcdn.net/v/t34.0-12/14962850_1306124986078874_1009698561_n.jpg?oh=f9056525f1013bcfa49c664feeff07e2&oe=581B4244

    This is my professor's solution:
    upload_2016-11-1_19-34-19.png

    I understand finding v(t) but when it comes to i(t) I'm confused.
    First conceptual question:
    I know the current across an inductor must be continuous at t = 0 but does this also apply to the current through a capacitor at t = 0? I imagine this isn't the case since neither me or the professor's solution point to this.

    Next question, how is 0 mA the answer for t<0 ? If the switch is closed during this time then there should still be a current going down that path.
    Was my approach for finding i(t) when t<0 incorrect? I drew the circuit for t<0, under DC regime a capacitor can be treated as an open circuit, I found Rth and found the current of that circuit. Wouldn't the current of that circuit be i(t) for t<0?

    I know I asked a lot of questions so answering any part of it, if not all, would still be extremely helpful.
    Thanks in advance!
     
    Last edited: Nov 1, 2016
  2. jcsd
  3. Nov 2, 2016 #2

    cnh1995

    User Avatar
    Homework Helper

    No. Capacitor current can change instantaneously, which prevents the voltage across the capacitor to change instantaneously. Capacitor resists sudden changes in the voltage across it.
    I believe you are expected to assume i(t) as the capacitor current. Hence for t<0, it is 0.
     
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