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Homework Help: Solving a 2nd order D.E. of a forced LRC circuit

  1. Aug 10, 2011 #1
    1. The problem statement, all variables and given/known data
    Solve the following forced D.E. (Show work)

    L=10 R=20 C=0.01 x(0)=10 x'(0)=0

    2. Relevant equations
    This is the second order D.E. for a forced LRC circuit


    3. The attempt at a solution

    I calculated ygeneral to be ygeneral = e-tC1cos(3t)+e-tC2sin(3t)

    Now I need to find the particular solution for the DE. I've been trying to use undetermined coefficients, choosing Asin(t)+Bcos(t) as my initial guess but the algebra doesn't seem to work out. Can anyone help me out? Thanks for your time!
    Last edited: Aug 10, 2011
  2. jcsd
  3. Aug 10, 2011 #2
    You'll need to find the particular solution before you can plug in the initial conditions to find the constants.
  4. Aug 10, 2011 #3
    Yeah, the previous problem was to find the solution to the same equation, just without the forcing function at the end, so it was homogeneous. That's how I got the values for the constants and the general equation.
  5. Aug 10, 2011 #4


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    Homework Helper

    This is not the general solution of the homogeneous equation: It has to contain two undetermined constants.

    The "force" function has different frequency from the own frequency of the LCR circuit: so the particular solution is in the form A cos(t) +Bsin(t), as you supposed. Find A and B. Show what you have tried.
    Match the general solution x(homogeneous) +x(particular) to the initial conditions.

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