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Homework Help: Irregularly shaped DC circuit?

  1. Jan 24, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img144.imageshack.us/img144/9724/tricircuitxv4.jpg [Broken]
    http://g.imageshack.us/img144/tricircuitxv4.jpg/1/ [Broken]

    2. Relevant equations

    Re = R1 + R2 + R3

    3. The attempt at a solution

    I don't really understand this problem. I'm not sure how to approach it. I thought 5 = Rb + Rc, and I did that for all the rest of the resistors to solve the system but that was wrong.

    Can someone just help me understand what the problem is asking and what it gives?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 24, 2009 #2


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    I would interpret it to mean that

    Ra + Rb = 6


    3 equations - 3 unknowns
  4. Jan 24, 2009 #3
    That's exactly what I did, and got a nice rational number - 3.5 (the answer is almost never rational). It's wrong.
  5. Jan 24, 2009 #4


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    I see what's wrong.

    Ra + Rb = 6 || (5 + 6)

    The equivalent resistance from one to the other includes the alternate series pair in parallel with the direct one.
  6. Jan 24, 2009 #5
    So you're saying Ra + Rb = (1/6 + 1/11)^-1?

    Isn't that saying the total resistance of the first circuit equals only the sum of the resistances Ra and Rb?
  7. Jan 24, 2009 #6


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    Yes that's the calculation.

    No, it's how the resistors are taken. Note the circuit is not symmetrical. You get pair-wise different equations.

    There's 6 || 11 and 6 || 11 and 5 || 12
  8. Jan 24, 2009 #7
    Ah ok, thank you.
  9. Jan 24, 2009 #8


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    This is known as the Y-delta transform (though this question is probably to get you guys to derive the transformation to be covered in class!):
    Last edited by a moderator: May 3, 2017
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