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Homework Help: Solving a circuit using Kirchhoff's Rules

  1. Apr 19, 2007 #1
    1. The problem statement, all variables and given/known data

    using kirchhoff's rules, determine the current through and voltage drop across each resistor in the accompanying figure.

    2. Relevant equations
    V=IR, potential voltage ( voltage drop )= current * resistance
    Voltage in series Vt=V1 + V2 + V3...
    Current in series It = I1 = I2 = I3...
    Resistance in series Rt= R1 + R2 + R3....

    Voltage in parallel Vt= V1 + V2 + V3..
    Current in parallel It = I1+ I2 + I3..
    Resistance in parallel 1/Rt= 1/R1 + 1/R2...

    these are the equations weve been using in class

    3. The attempt at a solution

    i wasnt sure how to start this problem since i dont have any similar examples to look at. but what i did was add the two voltages of the batteries to get a total of 25 V. i also assumed that resistors 2 and 3 were in parallel, and together were in series with resistor 1. making the total resistance 10. ( 1/6 + 1/3 = 3/6 ; 1/(3/6) = 2 ; 8+2 = 10) so that gave me a current of 2.5. so i got the current through R1 to be 2.5 amps, thecurrent through R2 to be .833 and the current through R3 to be 1.67.
    the voltage drop across R1 to be 20 V (2.5 amp * 8 ohms), across R2 to be 5 and across R3 to be 5

    im pretty sure i didnt do it right but im not sure how i should set up the problem. in class our teacher gave us an example like this one with two batteries. but i got confused when he was drawing the arrows for the direction of the current. he drew current flowing from A to B, but also from B to A. i tried to figure it out doing that but it seems too complicated. i always end up with current flowing into a battery through the wrong end. can someone please show me how to set it up correctly? thanks
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Apr 19, 2007 #2
    When solving a circuit problem you really only have two ways of solving it: KVL or KCL.

    The voltage sources are NOT in parallel neither are resistors 2 and 3. This is a wye bridge I believe.
  4. Apr 19, 2007 #3


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    Homework Helper

    Alright, when provided with college-level circuits, it's time to take a college-level approach to solving circuit problems. The good old method of adding voltages and resistances together just doesn't work well anymore. While the approach of separating the "batteries" from the "load" works with simple configurations, they quickly become non-applicable to complex circuits.

    Have you been taught about mesh currents or nodal analysis?
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