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Circuit Analysis

  1. Sep 9, 2007 #1
    This is a strange problem that I ran into in my homework. I did not use one of the resistors in the second loop. I don't think this is correct and I am really confused. I have no idea how else I could set this up to solve for the needed variables.

    Find Vs and Is, if V2 = 20V, R1 = 6K, R2 = 10K, and R3 = 2K.

    Here is what I did:

    If it is wrong; how should I set this up correctly?
    Last edited: Sep 9, 2007
  2. jcsd
  3. Sep 9, 2007 #2


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    Staff: Mentor

    Well for one, Vs + V2 does not equal zero because of the presence of R1 and the current Is.

    One should follow the conventions of Kirchoff's voltage and current laws, i.e. the conventions for assigning potential difference across a voltage source and the direction of the current.

    The potential across R2 is the same across R3, therefore they will have different currents passing through them, but the sum must be Is.
  4. Sep 9, 2007 #3
    Oh, ok. I think I'm starting to understand this.

    I redrew the set-up by labeling more currents and labeling other voltages.
    Did I set-up this drawing correctly?

    Would this set-up work?

    "left loop"

    "right loop"
    Last edited: Sep 9, 2007
  5. Sep 9, 2007 #4


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    Staff: Mentor

    Yes, I believe that's correct.

    One then has three equations, but one can reduce it by find the eqivalent resistance to R2||R3 and adding that to R1 = Rtot. The Vs = Is * Rtot.

    Or one can solve for I3 = V2/R3, because V3 = V2.

    I notice that Is and V2 are written in the opposite direction of convention. Normally current flows out of the + terminal of a voltage source (battery), and into the positive potential of the resistor. The + side of the resistor would normally be on the side closest to the + terminal of the voltage source. If one obtains a negative current or voltage, it means that it is opposite of the way it is written.
  6. Sep 9, 2007 #5
    Ah, I see. My book described that as well but the problem given already had the Is pointing into the + side of Vs so I figured I would just follow the original drawing. You are correct, the Vs did come out to be negative due to the odd sign placement.

    Thank you for your help!
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