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Homework Help: Solving RLC Circuit

  1. Feb 14, 2017 #1
    1. The problem statement, all variables and given/known data

    R = 125 ohms, L = 200 mH, C = 5 uF, initial current in L is -0.3A, and initial voltage across C is 25V.

    2. Relevant equations
    v(t) = (Bcos(wt)+Csin(wt))e^(-at)

    3. The attempt at a solution
    I've solved for v(t) t>0 and got 25e^(-800t)(cos(600t)+ (4/3)sin(600t)). The second part is to solve for iL for t>0. In class, we've been solved for iL using iL+iR+iC = 0 →iL = -iR-iC but I want to solve for iL using v(t) = L*di/dt and I can't get the same answer as the other method. Is this not valid for this case? Thanks!
  2. jcsd
  3. Feb 15, 2017 #2


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    Staff: Mentor

    It should work, providing you have v(t) correct. So you're going to integrate v(t). Can you attach your working?
  4. Feb 15, 2017 #3
    Can you show your work?
  5. Feb 15, 2017 #4
    Thanks for the responses.
    I had iL = (1/L)∫[25exp(-800t)*[cos(600t)+(4/3)sin(600t)] ]dt and I used WolframAlpha to evaluate the integral. The answer was in the right form but the coefficients were off. Thanks!
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