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Homework Help: Superposition theorem

  1. Aug 26, 2008 #1
    I was working on this circuit.

    http://img404.imageshack.us/img404/3537/imptm6.jpg [Broken]

    The problem asks to find the current through the R2 branch.....

    Heres my approach and questions....

    Start from the opposite end of the circuit...combine the 2 parallel resistors R2 and R3...the V2 goes to zero because of the superposition.....

    The only question I had was are R1 and XL in parallel?....if so I could combine the R2 and R3 total found with R1..using product sum rule.......

    Finally add XC to (XL)(R1)/(XL + R1).....to get the total impedence

    than from there find the current....(current divider)
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Aug 27, 2008 #2


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    When determining the contribution of V1, R1 and XL are not in parallel, however, XL *is* in parallel with (R1+R2//R3). This combined resistance is then in series with XC. Unless you want to do a source transform on V1 to get a current divider.

    Here's the secret to source transform (or any circuit analysis, for that matter): draw, redraw, and redraw circuits (after every transformation / simplification). Unless you can get to the point where you can just do things by inspection. Also, learn to draw things neatly, otherwise, it won't be worth all the extra paper.

    EDIT: You don't mention this explicitly in your post, so I'll assume you've determined the contribution of V2 in the absence of V1...
    Last edited by a moderator: May 3, 2017
  4. Aug 27, 2008 #3
    ok thanks....now I understand that part....

    I calculated Z total for the first source......now to find the current total for the first source only through branch R2.....I know I can use the voltage divider formula....but I dont remember how........

    I know I have to find the relationships between the branches (except R2)....multiply..divide...and multiply that total by the current total already found.....

    I just dont remember how to apply the voltage divider formula correctly.....

    ( I started finding Z for source 2 and it appears to be the same as source 1?....is that correct?.,...the difference would be in finding the current considering the sources are at different voltages and phases..)
    Last edited: Aug 27, 2008
  5. Aug 27, 2008 #4


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    You need a formula for voltage divider?! :-D The ratio of the potential drop in two resistors (impedances) is the same as the ratio of their resistances (impedances). Or, V_drop_Z1 = V_source*Z1/(Z1+Z2). Remember, that's just the voltage DROP, not necessarily the nodal voltage measured relative to ground (or another reference). See other threads in this forum on that topic if you're still stuck...

    Regarding your other point, superposition says that it's okay if the sources are at different voltages and phases. Their individual contributions just add up (superpose) and the total is what you actually measure when you hook up your oscilloscope.
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