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Parallel/Series Circuits Problem

  1. Jan 25, 2013 #1
    http://imgur.com/QnyUSp4



    2. V=IR



    3. I tried combining the the two 10kohm resistors furthest away from the source in parallel. Then combined the top 10kohm resistor with the diagonal 3.3 ohm resistor. Then combining the top resistor and the resistor on the right in series then the two remaining resistors in parallel then using V=IR.
     
  2. jcsd
  3. Jan 25, 2013 #2

    gneill

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    Hi eatlsep, Welcome to Physics Forums.

    Combining the rightmost 10K with the bottom 10K is fine, but note that the top 10K is NOT in parallel with the diagonal R.

    The diagonal going from bottom left to top right is a straight wire, so those two nodes at either end of it are really just one node. That means you are free to shorten that diagonal wire as much as you like. So imagine that this wire is shortening, dragging the two top-right corner resistor connections with it until the wire vanishes and the resistors are now connected at the bottom left corner of the square. What parallelism can you take advantage of now?
     
  4. Jan 25, 2013 #3
    Thank you for the response. I'm sorry, I am a little confused what you mean by dragging the top tow resistors as the wire shortens
     
  5. Jan 25, 2013 #4

    gneill

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    Like this:

    attachment.php?attachmentid=55004&stc=1&d=1359158763.gif

    Pull the connections of R2 and R3 down to the other end of the blue wire.
     

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  6. Jan 25, 2013 #5
    So without the node between R2 and R3 would they then be in series?
     
  7. Jan 25, 2013 #6

    gneill

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    Which node would that be? R2 and R3 remain connected together just as before. No connections change simply by rearranging their positions on the drawing.
     
  8. Jan 25, 2013 #7
    Will R1 and R2 be in parallel?
     
  9. Jan 25, 2013 #8

    gneill

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    Yup :smile:

    What else?
     
  10. Jan 25, 2013 #9
    So R1 and R2 are in parallel. R3 and R4 are in parallel. Does it matter where I put the combined resistors?. For example, I combine R1 and R2 and put the combined resistor at R1. Then I combine R3 and R4 and put the combined resistor at R3 so that now R1eq and R are in parallel. Can I do this?
     
  11. Jan 25, 2013 #10

    gneill

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    No, R1 and R were not in parallel before, so they can't be now. That goes for R1eq as well. Remember, no physical change is made to the circuit by moving connections along existing continuous wires.

    If you place the combined resistance R34 = R3||R4 where R3 was originally drawn, then you still have to maintain its connections to the rest of the circuit, no doubt by bending or extending wires. Something like this:

    attachment.php?attachmentid=55006&stc=1&d=1359162218.gif

    Combine what's in parallel, then combine what's in series,... rinse, repeat :smile:

    Probably easier just to place R34 where R4 was...

    attachment.php?attachmentid=55007&stc=1&d=1359162347.gif
     

    Attached Files:

  12. Jan 25, 2013 #11
    Oh I see what you mean by dragging it down. After combining resistors in parallel. Next, are R12 and R34 in series? Then R1234 and R are in parallel?
     
  13. Jan 25, 2013 #12

    gneill

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    No, R12 and R34 are not in series; there's something else connected where they join (it's the bottom of Vs). For two components to be in series, where they connect must be to each other and to nothing else. That is, they must exclusively share a node.

    However, R is in series with one of the newly combined pairs. Can you spot it?
     
  14. Jan 25, 2013 #13
    Doesn't it also mean if the current is the same through the resistors then it is in series? So R and R1 are not in series because more current wants to flow through the path of least resistance. So R and R4 are in series?
     
  15. Jan 25, 2013 #14

    gneill

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    Yes. If the SAME current flows through both components, then they are in series.
    As long as there are alternate paths (no matter what their resistance), then they are not in series; Some current will flow through all available paths so long as they all present some resistance.

    Of course, if one of the paths has zero resistance (a so-called "short circuit"), then all the current with take that path. But that's just something to remember for another time.
    Not as originally drawn. But after combining R2 and R4 into R34, then R and R34 are in series by the definition.

    attachment.php?attachmentid=55008&stc=1&d=1359164092.gif
     

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  16. Jan 25, 2013 #15
    Ok great! im glad I understand those concepts. Now that we combine R34 and R. My next question is where you put the eq resistance. I would guess it goes where R is, but I am not sure why if that is correct
     
  17. Jan 25, 2013 #16

    gneill

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    It's your choice where to put it... remember that the wires are "flexible", so that you don't have to leave them where they're drawn so long as their connection points remain the same. Here it would make sense to redraw the equivalent resistance parallel to R12... :wink:
     
  18. Jan 25, 2013 #17
    Ok. Thank you for all the help. Physics forums is great :D
     
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