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Calculating the resistance between 2 terminals of circuit

  1. Jun 19, 2016 #1
    1. The problem statement, all variables and given/known data
    hqKz4CF.jpg

    2. Relevant equations
    1/Rt = 1/R1 + 1/R2 + ....

    3. The attempt at a solution
    I can do part (i) okay. It is simply 1/12 + 1/12 = 1/6, so R = 6. But I have been trying for ages to figure out part (ii). I tried re-drawing the circuit in different forms but it didn't help me.

    The one that makes most sense to me is 1/12 + 1/12 + 1/6 which gives the total resistance as 3, but the correct answer is 3.75, and I seriously can't figure out why no matter how hard I try.
     
  2. jcsd
  3. Jun 19, 2016 #2

    CWatters

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    Show us your drawing. Label the resistors A,B,C,D etc
     
  4. Jun 19, 2016 #3

    cnh1995

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    Equivalent resistance is asked between Y and Z. Which resistances are in series and in parallel when viewed from terminals Y and Z?
     
  5. Jun 19, 2016 #4
    Why do you figure these things out as soon as you post them? I was honestly at this for ages and just got it.

    Here is my solution for anyone viewing this in the future:
    http://i.imgur.com/nnF8t83.jpg
     
  6. Jun 19, 2016 #5
    Hey TiernanW,

    Try this:

    1. As stated in part (ii), draw a new wire with a new resistor directly between terminals X and Y (along an imaginary diagonal through the middle of the diagram).
    2. Redraw the diagram so that all resistors are oriented in the same direction, so they actually sit parallel to one another in the diagram.
    3. Simplify using the techniques you already know, beginning with summing the resistors in series, and then solving for the three equivalent parallel resistors.

    Alternately, you can redraw the original diagram like this, and then add the new resistor into a parallel configuration with the simplified original - whichever you find easiest.

    I hope this helps - I always found it easier to simplify circuits when all components are arranged this way.

    Best,

    Richard
     
  7. Jun 19, 2016 #6

    cnh1995

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    Looks good!
     
  8. Jun 19, 2016 #7
    Why isn't it just 6 ohms? 2 6 ohm resistors in series =12 ohms and another set of 12 ohms and the two in parallel. Looks like it should be 6 ohms. Sorry, that would be between X and Y. Between X and Z would be a 6 ohm resistor in parallel with an 18 ohm resistor. That would be 6 times 18 divided by 6 PLUS 18 which equals 4.5 ohms. Oops, that would be without the extra resistor.
     
    Last edited: Jun 19, 2016
  9. Jun 19, 2016 #8

    cnh1995

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    That is for question 1. In question 2, equivalent resistance is asked between Y and Z.
    Edit: I see you've already edited the post..
     
  10. Jun 20, 2016 #9
    Because when you sort out your thoughts enought to explain the details you have explained it to yourself.
     
  11. Jun 20, 2016 #10

    SammyS

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    Part (ii) does not make sense as written.

    If a 6Ω resistor is connected between points X and Y, then it is in parallel with the two pairs of 6Ω resistors IF we are considering resistance between points X and Y.

    However, if we are considering resistance between points Y and Z, then the 6Ω resistor connected between points X and Y is in parallel with only one pair of the initial 6Ω resistors.
     
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