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Homework Help: First order RC: Find Unknown DC Circuit, given current graph

  1. Apr 16, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    Tao = R(th) C

    3. The attempt at a solution

    The unknown DC circuit model consists of a V(th) and R(th)

    Tao = (R(th) + 3000) C , total resistance of circuit is R(th) + 3000
    0.004 = (R(th) + 3000) (1x10^-6)
    R(th) = 1 kOhm

    The V(th) should oppose the known source since i(t) goes to 0.
    Doing a KVL around the loop clockwise.

    -10V + 3000( I ) + I (1000) + V(th) = 0, Voltage across capacitor is 0 since at t=0 it is like steady state.
    also, at t=0, I = 0.001 A
    then solving gives V(th) = 6V

    I get the hunch that the working is wrong but the answer is right by coincidence. Would the working be correct?

    Thank you very much.
    Last edited: Apr 16, 2015
  2. jcsd
  3. Apr 16, 2015 #2


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    Science Advisor
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    2017 Award

    It's basically correct. The problem statement doesn't explicitly say the capacitor starts out uncharged, but that's a reasonable assumption if nothing else is given.

    Your wording "at t=0 it's like steady state" isn't all that clear: steady state is clearly not the case at t=0.
    What happens at t=0 follows from a second relevant equation: ##Q = CV## which means that at t=0 the voltage drop over the capacitor is zero (-- provided it's uncharged to begin with!).
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