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Homework Help: LC circuit analysis problems with 2 voltage sources

  1. Nov 3, 2012 #1
    1. The problem statement, all variables and given/known data

    . In the following transient circuit, assume at t<0, the circuit is at steady state.
    Find v0(t) for t>0
    http://sphotos-b.xx.fbcdn.net/hphotos-ash3/c0.0.354.354/p403x403/546839_509413575744829_42997155_n.jpg [Broken]

    2. Relevant equations

    3. The attempt at a solution
    The first thing that I was suggested to do was to always find iL(t)
    so since we have 2 source of voltage. I will be using superposition method.

    First let use the 12V source only
    Req for the circuit will be: (2||6) + (3+1||3)
    Use Req to find the current source due to the 12V and by using current divider rule we can obtain iL(0-)

    then move on to the 3V source
    at this point, I am having trouble, so when we short out the 12V, will the current just go through the wire where the 12V used to be and skip the right side of the circuit?
    but if it is so then iL(0-) for this source will be 0?, which doesn't seem right to me.

    ok, assume that we actually figured out the value for iL(0-) then to find iL(∞) we only have to deal with the right side of the circuit.
    so iL(∞) will be equal to the current going through the 1Ω ?

    and Rth for t>0 will be 3+(1||3) this can be use to find time constant.

    so suppose we got our equation for iL(t)
    to find Vo(t) we can try to find io(t) from current divider rule
    iL(t)= io(t) X (3/4)

    and once io(t) is obtained, we multiply that by 3Ω to get Vo(t)

    thanks for your time.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 3, 2012 #2


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    Staff: Mentor

    It is correct. The 12V source, situated where it is, effectively isolates the "right hand side" of the circuit from the "left hand side". Nothing that happens to the left of that supply can change the potential across that supply; it's an ideal source and will source or sink ANY current required to maintain that potential.
    Last edited by a moderator: May 6, 2017
  4. Nov 4, 2012 #3

    rude man

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    Homework Helper
    Gold Member

    Right. This is a 'trick' question!
  5. Nov 5, 2012 #4
    The left side of the circuit is isolated from the right side because of the position of the 12V source. The current through the inductor is governed by the right side circuit only, which contains an equivalent resistance (3 + (1||3)).

    Yes. You may want to consider if anything has actually changed by opening the switch.
  6. Nov 5, 2012 #5
    but for t<0, before the switch is open, dont we need to include the 2Ω and 6Ω?

    as for calculating the time constant, then we can just remove everything on the left side.
  7. Nov 5, 2012 #6
    If you cut the circuit in half through the 12V source and separated the left side from the right side, then connected two separate 12V sources to each of the two separated halves, nothing would change.
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