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Equivalent resistance

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  1. Feb 21, 2016 #1
    1. The problem statement, all variables and given/known data
    I'm having trouble trying to find equivalent resistance. Here's a picture of some circuits I am trying to simplify (link)

    38160c3464.png

    For example, in circuit a, I can see that the 84 ohm and 105 ohm resistor are in series, and that can be simplified, but what do the markers A and B do to the circuit? Is the result of combining the 84 and 105 in parallel with the 63? And then what is the relationship of the 42 and the 21? Any tips on how to approach these problems are appreciated, thanks.
     
  2. jcsd
  3. Feb 21, 2016 #2

    cnh1995

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    A and B are the points between which the equivalent resistance is asked. For a, I believe your approach is right. Go on combining two resistors at a time(either in series or parallel, whatever is correct).
     
  4. Feb 21, 2016 #3
    But does it affect the resistance between 42 and 21? Are those in series still?
     
  5. Feb 21, 2016 #4
    The best way to find total resistance in circuits like these is to take it a step at a time and redraw after every step. you have combined the 84 and 105 and noticed that the result is in parallel with the 63. Find your equivalent resistance for those three then redraw. What further simplifications can you make? Eventually you will get to a single resistor.
     
  6. Feb 21, 2016 #5
    Okay, but do the marker points change anything? Is 42 and 21 in series or parallel in diagram a?
     
  7. Feb 21, 2016 #6

    cnh1995

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    They are not in series. Series equivalent of 84 and 105 is in parallel with 63. Their parallel equivalent is?? Can you further simplify?
     
  8. Feb 21, 2016 #7
    Ah okay I think Isee, so after that it is in series with the 42 and that combination is in parallel with the 21?
     
  9. Feb 21, 2016 #8
    The marker points do not change and do not worry about 42 and 21 just yet. Combine the 3 resistors and redraw the circuit. What else can you simplify?
     
  10. Feb 21, 2016 #9

    cnh1995

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    They are neither in series nor in parallel directly. You need to simplify the circuit step by step and find out.
     
  11. Feb 21, 2016 #10

    cnh1995

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    Right.
     
  12. Feb 21, 2016 #11
    Okay, makes sense. Thanks a lot for your help
     
  13. Feb 21, 2016 #12
    You can solve all of the circuits above by simplification alone except for the last one which is why it is a bonus. You will need to do a conversion first to solve that.
     
  14. Feb 21, 2016 #13
    Great, I've been working through them. What type of conversion are you talking about? I can tell it's not the same as the others
     
  15. Feb 21, 2016 #14

    cnh1995

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    Delta to star..
     
  16. Feb 21, 2016 #15
    Hmm okay, I'll definitely look into that. Thanks everyone for the help
     
  17. Feb 21, 2016 #16
    It is not the same in that you have resistors that are neither in series or parrallel
    Star is also referred to as wye
    One person may say half a dozen another may say six. They are the same thing.
     
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