1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    upload_2015-4-16_14-29-21.png

    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

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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!).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: First order RC: Find Unknown DC Circuit, given current graph
Loading...