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Current electricity

  1. Aug 9, 2012 #1
    1. The problem statement, all variables and given/known data
    Find the resistance between the A and B( image attached)


    2. Relevant equations



    3. The attempt at a solution
    I could only simplify the diagram to this (2 attached pic) and i can't proceed any further since i can't identy find any parallel or series circuit/
     

    Attached Files:

  2. jcsd
  3. Aug 10, 2012 #2

    Simon Bridge

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    You can unfold the first picture ... put points A and B in the corners, then unfold the triangle point A is on so A is outside the big triangle. See how to simplify it now?
    No? Imagine a voltage source VAB between A and B and you have to find the total current drawn ... now do you get it?

    You end up with the resistors forming an H ...
     
    Last edited: Aug 10, 2012
  4. Aug 10, 2012 #3

    CWatters

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    If you redraw it slightly differently you will see that by symetry one R has no voltage across it. So it can be removed without effecting the circuit. Makes it much easier to simplify further.
     
  5. Aug 10, 2012 #4
    resistance in between the parallel line of H.......... how do i combine it with others??:cry:
    (Thanks A LOT for answering!:smile:)
     
  6. Aug 10, 2012 #5
    If you see carefully, there's a wheat stone bridge.
     
  7. Aug 10, 2012 #6

    Simon Bridge

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    Pranav-Arora is correct - it's a Wheatstone bridge. How does one of those work?

    You may understand it better if you add a DC voltage source to the diagram between A and B ... one of the resistors in the H does not carry a current: so you can just replace it with an open circuit.

    If you still don't see it - use Kirkhoffs rules to find the relationship between VAB and the total current drawn, then Ohm's Law gets you the rest of the way.
     
  8. Aug 10, 2012 #7
    sorry-deleting my post
     
    Last edited: Aug 10, 2012
  9. Aug 10, 2012 #8

    Simon Bridge

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    @aziziwi: I'd have let 1/2" explore the situation in terms of existing knowledge more before introducing new ideas.

    Since all the resistors have the same value, there is no need for the Y-Δ transform. Not that it isn't cool to know...

    1/2" is about to discover that the problem only looks complicated. Worst case now is that he does a 3-loop/4-node Kirkhoff analysis ... which is lots of work, but sometimes you have to do something the long way before you realize where the shortcut is. Builds character ;)

    I think we've gone as close as we can to actually doing the problem ... lets wait for 1/2" to get back to us.
     
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