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Homework Help: Paralel circuits

  1. Mar 3, 2007 #1
    1. The problem statement, all variables and given/known data
    Im aware that you can find the current passing through each resistor in a parallel circuit by using the equation I=v/r but for homework i was asked how to find the current by using the ratios between each resistor.
    eg.
    ---5ohms---- Resistor 1
    - 20ohm - Resistor 2
    - 10ohms - Resistor 3
    ----| - |-----
    I=20AMPS
    If only resistor 1 and 2 exists i can do it by dividing 5 by 5+20 and dividing
    20 by 2+20. Inverting the results so 5/25 is for resistor 2 and vice versa then multiplying the results by 20amps. This gives the amps passing through each resistor.
    I've no idea how to do it if there is 3 or more resistors.

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 3, 2007 #2
    On the right track--we know that current is inversely proportional to resistance, so lets set up the eqn with a little i which is the current flowing thru the 20 ohm resistor: In fact, this makes use of the V=iR eqn at least implicitly, assuming 20 amps total passes thru the branch.

    i+20/10*i+20/5*i=20A=7i. i=20/7, etc.

    Now if 20 amps passes thru say the small resistor, and total current is unknown,

    then (5/10)*20 will pass thru the 10 ohm and (5/20)*20 thru the 20 ohm.
     
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