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Wired Circuits

  1. Apr 20, 2006 #1
    AxxxxxxR1xxxxxxxxxxxxxR2xxxxxxxxR3xxxxxxx
    ooooooooooooooXoooooooooXooooooooooox
    ooooooooooooooXoooooooooXooooooooooox
    ooooooooooooooR4ooooooooR5oooooooooR6
    ooooooooooooooXoooooooooXooooooooooox
    BxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxR7xxxxxx

    For the circuit above the x's represent the wire and R represents the resistance. o's just fill in open space - ignore them. Find the equivalent resistance between points A and B.
    R1=5 ohms
    R2=4 ohms
    R3=6 ohms
    R4=10 ohms
    R5=8 ohms
    R6=7 ohms
    R7=3 ohms

    So i need to find the total resistance in the circuit. I know that R4,5, and 6 are in parallel so their resistance should add up like 1/(1/10+1/8+1/7) to give 2.72 ohms. so the rest of the resistances are in series so you just add them all up right???? when i add them all together with 2.72 ohms, I am not getting the right answer. What am i doing wrong???? please help. thanks.
     
    Last edited: Apr 20, 2006
  2. jcsd
  3. Apr 20, 2006 #2

    andrevdh

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    R3, 6 and 7 are in series. Start by trying to calculate their combined resistance first.
     
  4. Apr 20, 2006 #3

    Ouabache

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    Adding to what andrevdh suggests, a good place to start is the at the end of the circuit. After you've found a combined resistance Rcr for those andrevdh described, you can then work your way towards the right. Notice R5 is now in parallel with your new combined resistance Rcr; you can combine these two to get a new combined resistance. Can you see the process?
     
    Last edited: Apr 20, 2006
  5. Apr 21, 2006 #4
    i do not see how Rcr is in parallel with R5 at all. If it is in parallel with R5 that means that when u add Rcr up, it would have to be at the R6 location. Why cant u place the new Rcr at the R3 location. i am really confused.
     
  6. Apr 21, 2006 #5

    nrqed

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    You could put the combined resistance Rcr (combined from R3, R6 and R7) anywhere in the branch at the extreme right. You could place it wheer R3 is or wher R6 is or where R7 is. In *all* three cases, Rcr is still in parallel with R5
     
  7. Apr 21, 2006 #6
    got it. thank you so much.
     
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