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Electric Circuit Analysis

  1. Oct 9, 2014 #1
    1. The problem statement, all variables and given/known data
    http://imgur.com/Kq5e4Ka This is the circuit I need to find the Req for, the only things given are the Voltage of 7.1, and the resistance of each circuit.

    2. Relevant equations

    V=IR
    Kirchhoffs laws, the KVL and KCL.


    3. The attempt at a solution
    I have spent some time trying to solve this circuit, however I now realize that using circuits in parallel and in series is not an option due to the 2.3 ohm resistor, my instructor told me to use the KVL and KCL equations to solve, but I am unfamiliar with these as we haven't gone over them in class.

    From what I have tried to do so far, I want to identify all of the loops ( which I didnt do in the picture to leave it easy to view), and all of the junctions to start the equations. I named resistors according to their Ohms.
    There is a loop from the
    11R, 2.1R, and 6.0R resistor,
    11R,7.9R, and 7.7R
    11R, 2.1R, 2.3R, and 7.7R
    11R,7.9R,2.3R, 6R

    There are junctions for
    R11=R2.1+R7.9
    R2.1=R2.3+R6.0
    R7.9=R2.3+R7.7
    And you can say R6.0+R7.7=R11 I believe

    This is where I hit a wall... mathematically I am unsure where to proceed, I know that I have several unknowns and several equations, therefore meaning I need to solve by combination of equations, but when I try to do this I get mixed up with combining the equations and end up with many values I cannot solve for.
    My goal is to find the current coming from the first circuit, and I believe from there for solution will become much easier.

    Some people have mentioned using matrices but this is something completely unknown to me, so I believe algebra is the only option. Am I on the right path to solving this problem, so far?
     
  2. jcsd
  3. Oct 9, 2014 #2

    phinds

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    If you take all the labels off the circuit and just make all the components into wires, do you see 3 rectangles (1 big on the left with 2 smaller next to it on the right) ? THOSE are what you should be using as loops. Draw current loops in each. Write the sum of the voltages around each loop and you will have 3 equations in 3 unknowns. Solve. Write it up neatly on your homework sheet. Go out for pizza.
     
  4. Oct 9, 2014 #3
    So make 11R, 2.1R, 6.0R one loop for the left side and for the right two boxes

    and for the left
    2.1R, 2.3R 7.9R

    2.3R, 6.0R 7.7R?
     
  5. Oct 9, 2014 #4

    phinds

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    Uh ... you think maybe the voltage source should be included?
     
  6. Oct 9, 2014 #5
    Sorry, I should have showed that.. 11R+2.1R+6.0R=7.1
    I'm not sure the same can be said for the other two loops though?
     
  7. Oct 9, 2014 #6

    phinds

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    Uh ... not sure WHAT can be said for the other two loops? That the voltage source be shown? Well it's not IN those loops is it?
     
  8. Oct 9, 2014 #7
    Well.. yes but isn't the voltage for the two second loops going to be less due to the voltage drop from the first resistor?
    and also I'm not seeing how I can get 3 unknowns... I would still have to worry about the 7.9 ohm resistor and 7.7ohm resistor
     
  9. Oct 9, 2014 #8

    phinds

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    reread post #2. Carefully.
     
  10. Oct 9, 2014 #9
    Before you can write equations you need something else: label the currents through each resistor. Like I1, I2, I3...
    Both Kirchoff's rules include currents. So far you did not write meaningful equations. The currents will be the unknowns in the equations.
     
  11. Oct 9, 2014 #10

    phinds

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    This is exactly what I told him to do in post #2 ... draw the current loops and write the voltage equations around the loops. Short of actually redrawing the figure and solving it for him, I don't see what other hints there are to give.
     
  12. Oct 9, 2014 #11
    Sometimes you have to repeat the hints, until they go through. :)
     
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