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Physics Olympiad question Electric circuit

  1. Nov 11, 2015 #1
    1. The problem statement, all variables and given/known data

    In the Circuit(pic attached) the following resistor combination is found.
    All the resistrs in the figure has resistance R ohms.
    What is the total resistance across AB?
    2. Relevant equations

    V=IR
    3. The attempt at a Solution
    between point A and B there is only one resistor so i thought the answer is R
     

    Attached Files:

  2. jcsd
  3. Nov 11, 2015 #2

    gneill

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    Staff: Mentor

    No, there are multiple paths that current could take from A to B. Look for opportunities to combine series and parallel resistances to reduce the circuit complexity.

    Your Relevant Equations should have included the formulas for combining resistances.:wink:
     
  4. Nov 11, 2015 #3
    I could do the part i) im stuck with the part ii)
     
  5. Nov 11, 2015 #4

    gneill

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    Staff: Mentor

    Show your work for part (i) and what you've tried for part (ii).
     
  6. Nov 11, 2015 #5
    upload_2015-11-11_8-26-24.jpeg
    This is for part i)
    But for part ii) i seriously had no idea.
     
  7. Nov 11, 2015 #6

    gneill

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    Staff: Mentor

    Part i) work is good.

    For part ii), can you not spot any series or parallel connections? What type of connection relationship do resistors AD and DC have?
     
  8. Nov 11, 2015 #7
    yeah i noticed that resistors ad and dc are in series. Then resistors ac is parallel with ad&dc. so the only resistor left out is bc
     
  9. Nov 11, 2015 #8

    gneill

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    Staff: Mentor

    Yes, so can you reduce the whole network down to a single resistance?
     
  10. Nov 11, 2015 #9
    no. I really have no idea how to include resistance ab and bc in the circuit
     
  11. Nov 11, 2015 #10

    gneill

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    Staff: Mentor

    You've already identified that AD is in series with DC and so can be replaced by a single resistor, say "ADC". You also pointed out that this ADC is in parallel with AC, so that pair in turn can be replaced with a single resistor. What's the value of that resistor?
    Fig1.png


    Redraw the circuit with that new resistor in place and the remaining resistors.
     
  12. Nov 11, 2015 #11
    I think i got it 5R/8
    Thank you !!!!!!!!!!!!!!!!!!!!!
     
  13. Nov 11, 2015 #12

    gneill

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    Staff: Mentor

    Looks good.

    You're welcome :smile:
     
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