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Problem with open circuit

  1. Jan 17, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the value of v1, v2, vab, vbc, vca. See picture
    HW.png

    2. Relevant equations
    Kirchoff's Law

    3. The attempt at a solution

    Is it true that both current through 3 and 4 ohm resistors are zero?
    The way I think of this is if they aren't zero, there would be charge building up at a and b. Unless there are some way for them to neutralize, such as static electricity.

    Thank You
     
  2. jcsd
  3. Jan 17, 2015 #2

    Dick

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    Yes, it is true that the current through the 3 and 4 ohm resistors is zero.
     
  4. Jan 17, 2015 #3
    Okay. In that case, there's no current in the bottom wire either? (Nodes connecting the negative ends of 7 and 2 ohm resistors)
     
  5. Jan 17, 2015 #4

    Dick

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    Yes, I think the same logic applies. You can assign the potential at point c to be zero and work from there.
     
  6. Jan 17, 2015 #5
    Thank you, Dick
     
  7. Jan 19, 2015 #6

    CWatters

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    Yes zero. Not sure why you think charge will build up.

    With no current flowing through the 3Ohm there will be no voltage drop across the 3Ohm so both sides of the 3Ohm will be at the same voltage.
     
  8. Sep 6, 2016 #7
    So how does one solve for Vab, vbc and vca? Since the current at a, b, and c are zero, that means the voltages at those points are zero too right? (V=IR --> a=0*3, b=0*4). Then wouldn't it just be zero minus zero for all them? (Vab = Va - Vb --> Vab = 0 - 0) Help please?
     
  9. Sep 6, 2016 #8

    Dick

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    No. Having no current through a resistor tells you the voltage at each terminal is the same. It doesn't tell you both are zero.
     
  10. Sep 6, 2016 #9
    So then how would you find the voltage? You cant use KVL because its not looped.
     
  11. Sep 6, 2016 #10

    jbriggs444

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    We are dealing here with a thread that is nearly 2 years old.

    If you have a resistor, Ohm's law applies. E=IR. The potential difference (E) between the two terminals is equal to the current flowing (I) multiplied by the resistance of the resistor (R). If the current is zero, the voltage must be...
     
  12. Sep 6, 2016 #11
    But the guy above me just said it's not zero.
     
  13. Sep 6, 2016 #12

    jbriggs444

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    The guy (@Dick) said that the potential difference between the two terminals is zero. And it is. That does not tell you the potential at both terminals, only that those two potentials are the same.

    Edit: Since the potentials at either end of the 3 ohm resistor are the same and since that resistor carries no current, you can erase it from the drawing and move the label "a" to the black dot where the 3 ohm resistor had been attached. That is a key point that this exercise is driving at -- simplification.
     
  14. Sep 6, 2016 #13
    oh I getcha. Sorry if I annoyed you for bringing up an old thread. Just still confused at how this open circuit works.
     
  15. Sep 6, 2016 #14

    jbriggs444

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    No problem. So are you comfortable with the simplification we made by removing the 3 ohm resistor? Can we proceed with some more simplification?
     
  16. Sep 6, 2016 #15
    Oh yes you bet I am. I just want like the whole solution to the problem lol then discuss the parts I dont get. I got V1 = 14v and V2 = 6v but i dont get how to find the vab, vbc, vca ones.
     
  17. Sep 6, 2016 #16

    jbriggs444

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    So we agree that we can remove the 3 ohm resistor from the drawing without any problems.
    Do you also agree that since there is a wire between "c" and the negative terminal of the 7 ohm resistor that those two points are at the same potential?
     
  18. Sep 6, 2016 #17
    Ok, but what about the negative terminal of the 2 ohm resistor?
     
  19. Sep 6, 2016 #18

    jbriggs444

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    It too is connected by a wire leading to point c, so it too must be at that same potential.
     
  20. Sep 6, 2016 #19
    Oh ok, makes sense. But would that then mean the voltage from a to c is the same as v1? and does the same apply for b to c?
     
  21. Sep 6, 2016 #20

    jbriggs444

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    BINGO!
     
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