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Find Resistance Between A and B

  1. Jan 13, 2010 #1
    1. The problem statement, all variables and given/known data

    24m8f7q.jpg

    I have to find the input resistance; the resistance between A and B. I think it's Thevinin's theroem, however it is already an open circuit and there are no voltage sources and I can't combine the resistors to make an equivalent. So what do I do


    Edit: Am I suppose to remove the 4 ohm resistor connecting the two parallels, and find the equivalent resistance of those two and then add back the 4 as if it were in series?....so the final answer is 16/3 ohm? or is that wrong?
     
    Last edited: Jan 13, 2010
  2. jcsd
  3. Jan 13, 2010 #2
    Have you heard of a Y-Delta transform?
     
  4. Jan 13, 2010 #3
    No I don't believe so.
     
  5. Jan 13, 2010 #4

    vela

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    Place a voltage source across A-B and calculate the current that it would supply. The equivalent resistance will be the voltage divided by the current.
     
  6. Jan 14, 2010 #5
    Ok, I tried it like you suggested vela. Made 3 Kirchhoff loops, put in an imaginary voltage source 10V, calculated the current to be 3.5A, so R should be 10/3.5...
     
  7. Jan 14, 2010 #6

    vela

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    That's the same answer I got.
     
  8. Jan 14, 2010 #7
    Sweet, thanks!
     
  9. Jan 15, 2010 #8
    I tried doing the loop thing, I don't think I did it right. Can you explain it please?
     
  10. Jan 15, 2010 #9

    ideasrule

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    What loop thing? You won't get the answer with Kirchoff's laws alone. Instead, use Thevinin's theorem, pretending that there's a voltage V between A and B.
     
  11. Jan 15, 2010 #10

    vela

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    It would be better if you explain what you tried, so we can identify where you're having trouble. It's kind of hard to boil all of circuit analysis down to a forum post.
     
  12. Jan 16, 2010 #11
    Never mind I think I got it. I1=7A/2, R=10V/3.5A.
     
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