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Problem finding the direction of current (electricity)

  1. May 4, 2010 #1
    1. Find I1, I2 and I3, see attached file.



    2. I1 = I2 + I3, U = R * I



    3. Can't seem to understand why the current is going the way it is at I3 rather than the other way.
     

    Attached Files:

  2. jcsd
  3. May 4, 2010 #2

    berkeman

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    Staff: Mentor

    When you solve for the currents, it may well turn out that I3 turns out to have a negative sign, which would mean that it actually flows the other way. When starting out KVL loop equations, you just assume some direction for each branch current, and then solve. If some of them turn out negative, you can go back and reverse the arrow and change the answer to positive. No big deal.

    Welcome to the PF, BTW!
     
  4. May 4, 2010 #3
    Thanks! Albeit, I still don't understand why it would come out negative, even though the answer says so...
     
  5. May 4, 2010 #4

    berkeman

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    Staff: Mentor

    I didn't work through the problem you posted, but this is just a general part of using KVL to solve circuits. You have to assume some direction for the current in each leg before you can solve the equations. Once you solve the equations, if the current comes out with a negative sign, then that just means that the initial guess at current direction was wrong. No big deal, the process is self-correcting.

    When you use KCL equations, you don't have this issue, because you just label nodes with some voltages like V_1, V_2, etc. When you solve the simultaneous equations, you get whether they are postive or negative (or changing) voltages as part of the solution.
     
  6. May 4, 2010 #5

    berkeman

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    Staff: Mentor

    Oh, and don't call me Albeit. I prefer Shirley. :biggrin:
     
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