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Homework Help: Resistance/Circuit Help Badly Needed!

  1. Apr 29, 2008 #1
    !Resistance/Circuit Help Badly Needed!

    1. The problem statement, all variables and given/known data

    I am doing a circuit lab with the voltage and resistors shown. I have already measured the currents across each resistor, but now I need to use some of the theory that we have learned in class. Here is an image of the original circuit.


    As you can see I have reduced it to a single equivalent circuit.

    Now I am working backwards to try and find the currents shown. Now keep in mind that the index for the current i may not be the same index as its respective resistor... that is, [itex]i_1[/itex] does not necessarily coincide with [itex]R_1[/itex]. I realize that that is annoying, but I did not type up the lab!

    Now can someone help me to just find [itex]i_4[/itex] for now. I know that the current across [itex]R_{eq}[/itex] is 307.3 mA.

    I also know that (working backwards) that [itex]R_{eq}[/itex] was made up from [itex]R_{6}+R_{12345}[/itex] which were in series with each other.

    So shouldn't the current across [itex]R_{12345}[/itex] be the same as the i_total ? Thus making i_4=i_total=307.3 mA

    Now I am pretty sure that I am not correct in saying this (because my measure currents dictate so) but I am not sure why?

    Also, it says to find the currents sing the "Ladder Method"......I am assuming the above is just that.

    I also have to find the currents using the "Scale Factor Method"..... can someone clue me into how to do that one? I know it has to do with making an assumption and then calculating the currents based on it and this in turn leads to a scale factor.
    Last edited: Apr 29, 2008
  2. jcsd
  3. Apr 29, 2008 #2
    Any ideas on what I am doing wrong?
  4. Apr 30, 2008 #3
    The way i have always done it is through Kirchoffs law.. using I1=I2+I3 then replacing the terms with ohms law V=IR and in the end getting a load nice simultanius equations, one for each loop of the circuit, in your case three.

    I you dont know this method, Ill explain more if needed?

    The last diagram, why have you taken out the resistor?
    Last edited: Apr 30, 2008
  5. Apr 30, 2008 #4
    I am not supposed to use Kirchoff's Law.
    Not sure what you mean. There is clearly a resistor in the last diagram. R_eq.
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