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What is the resistance?

  1. Mar 20, 2012 #1
    1. The problem statement, all variables and given/known data

    http://kpopgames.net/res.jpg [Broken]

    In the circuit shown, R1 = 1 ohms, R2 = 10 ohms, I1 = 3.5 A and I4 = 2 A. The battery has negligible internal resistance. If the rate at which R4 is dissipating electrical energy is 20W:

    a) What is R4
    b) What is R3
    c) What is I3
    d) What is the emf of the battery?


    2. Relevant equations



    3. The attempt at a solution

    I solved for a:
    P = (I4)R R4
    20 W = 4 R4
    R4 = 5 ohms.

    Now Im stuck there. Honestly, I dont know what the R3 illustration means. I know that R2 and R4 are parallel to each other. But the R3 which is like a diagonal confuses me. What does that R3 mean? And how do I go about to solve this?

    Does the R3 mean that it connects the R2 and R4, which makes them in series? Im reallly confused and I dont have a book to consult this to.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Mar 20, 2012 #2

    gneill

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    Staff: Mentor

    Don't let diagonal lines fool you; Connections are connections no matter how the lines are drawn on paper.
     
  4. Mar 20, 2012 #3
    I still dont get it. The answer to R3 should be 20 ohms. But I cant think of any solution that will lead to that.
     
  5. Mar 20, 2012 #4

    gneill

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    Staff: Mentor

    Start by labeling your diagram with everything you know or can determine so far. That would include indicating given currents and component values, as well as any voltage drops you can determine from the given resistances and currents. Find the current that must be flowing through R3 and the voltage that must be across it.
     
  6. Mar 20, 2012 #5
    If it helps to imagine that R3 is straight up and down then do so, the resistors R2, R3, and R4 are all in parallel and no matter how it is drawn the laws governing them remain the same.
     
  7. Mar 20, 2012 #6
    I really cant understand this.. I tried this:

    If R3 is parallel to R2.

    R23 = R2R3 / R2+R3 = 10R3 / 10+R3
    I23 = I2 + I3 = 1 + I3
    V23 = V2 = 10V = V3

    Is that right? What should I do next?
     
  8. Mar 20, 2012 #7

    gneill

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    Staff: Mentor

    Did you label your diagram with all the currents you know? Can KCL help you to find I3?
     
  9. Mar 20, 2012 #8
    Oh in that case, I found I3 to be 0.5A.

    Thus, R3 = V3 / I3
    R3 = 10 / 0.5 = 20 ohms

    Thanks!!
     
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