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Circuit question: Find size and direction of current at P

  1. Apr 9, 2016 #1
    1. The problem statement, all variables and given/known data
    O8RYO88.png

    2. Relevant equations
    I = I1 + I2 ... + In
    R = R1 + R2 ... + Rn
    1/R = 1/R1 + 1/R2 ... + 1/Rn

    3. The attempt at a solution
    Let I1 = I2 + I3 where I1 is the current on the top wire, I2 the current on the middle wire and I3 the current on the bottom wire.

    4 = 10I2 + 20I1
    //This is what I started trying but I can kind of tell that this is wrong
     
  2. jcsd
  3. Apr 9, 2016 #2

    cnh1995

    User Avatar
    Homework Helper

    If you assume I1 to be clockwise and I2 to be from right to left, this equation is correct. Assume a direction for I3 and write another equation. Your directions may be wrong, but that will just give you a negative value for the current.
     
  4. Apr 9, 2016 #3
    If 4 = 20I1 + 10I2 is correct then 12 = 10I3 + 10I2 is correct
    I1 = I2 + I3
    4 = 20(I2 + I3) + 10I2
    4 = 20I3 + 30I2
    12 = 10I3 + 10I2 can be made into 24 = 20I3 + 20I2
    Using simultaneous equations: 20 = -10I2
    I2 = -2

    But is this correct?
     
  5. Apr 9, 2016 #4
    dw I figured out where I was going wrong
     
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