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Effective resistance

  1. Nov 11, 2015 #1
    1. The problem statement, all variables and given/known data
    What is the effective resistance of this circuit?

    2. Relevant equations
    The answer given is 3A. so it means the total effective resistance is 4 ohm. But my answer is 4.4 ohm

    3. The attempt at a solution
    My attempt:
    1/(1/6 + 1/4)+2 = 4.4 ohm
     

    Attached Files:

  2. jcsd
  3. Nov 11, 2015 #2

    andrevdh

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    diagram/circuit?
     
  4. Nov 11, 2015 #3
    Thank you. I've just uploaded the picture.
     
  5. Nov 11, 2015 #4
    Try to draw the circuit again, but "split up" the connection in point A. What happens with R3 and the connection at its right handside?
     

    Attached Files:

  6. Nov 11, 2015 #5
    ImageUploadedByPhysics Forums1447232429.736911.jpg

    Is this correct?
     
  7. Nov 11, 2015 #6

    andrevdh

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    It is "shorted out" by the wire forming the corner, so there is in effect "no resitance" due to the wire. That means that you can redraw the circuit without the R3 resistor.
     
  8. Nov 11, 2015 #7
    You missed something. There is another cable/connection from point A to the other and of R3.
     
  9. Nov 11, 2015 #8

    andrevdh

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    Think of electric current flowing like water from the + terminal towards the - terminal of the battery.
     
  10. Nov 11, 2015 #9
    What is the reason that we can omit R3?
     
  11. Nov 11, 2015 #10
    Complete the drawing from your 3rd post with the missing cable/connection. Then you will see.
     
  12. Nov 11, 2015 #11

    BvU

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    Good thread ! You really adapted quickly to the PF culture :smile: !

    In the upper right of the original picture, there is a ' 0 ##\Omega## resistor ' in parallel with the 2 ##\Omega## resistor. Does that help ?

    --
     
  13. Nov 11, 2015 #12
    ImageUploadedByPhysics Forums1447243370.396140.jpg

    Is this correct?
     
  14. Nov 11, 2015 #13
  15. Nov 11, 2015 #14
    Thanks a lot to stockzahn, andrevdh and BvU for helping!
     
  16. Nov 11, 2015 #15

    andrevdh

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    It's a pleasure. The potential difference over a small resistor, or in this case the corner wire, is almost zero. That means that the potential difference over the R3 resistor is also almost zero so that almost no current flows through it. It is thus not contributing to the circuit and can be ignored or eliminated.
     
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