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Can someone help me with 2 circuit problems please

  1. Jun 28, 2004 #1
    Can someone help me with 2 circuit problems...please!!!

    1. The equivalent resistance between points A and B of the resistors shown below is 38 ohms. Find the value of the resistance R.

    The resistor looks like this
    55 Ohms

    _____\/\/\/\/_____
    | |
    12 Ohms | |
    A | |
    .____/\/\/\__ | |______. B
    | |
    | |
    |____\/\/\/\/_____|
    R

    I have no idea what to do...please help.


    2. Determine the magnitudes and directions of the currents through R1 and R2, where R1 = 36 ohms and R2 = 10 ohms. I already found the directions. they are both from right to the left, I just need the magnitudes. The Bs represent batteries, and the + next to them indicates the positive side of the battery. I am also really lost on this one can anyone help?

    V1=9.0V R1
    --+B--/\/\/\-------
    | |
    | R2 |
    .-----/\/\/\/-------.
    | |
    | |
    |----------B+------|
    V2=6.0V
     
  2. jcsd
  3. Jun 28, 2004 #2

    AKG

    User Avatar
    Science Advisor
    Homework Helper

    I would suggest using [ code ] and [ / code ] tags:
    Code (Text):

                                 55 Ohms
                           
                          _____\/\/\/\/_____
                          |                 |
           12 Ohms        |                 |
       A                  |                 |
       .____/\/\/\_______ |                 |______. B
                          |                 |
                          |                 |
                          |____\/\/\/\/_____|
                                   R

                               
                                V1=9.0V     R1
                                   --+B--/\/\/\-------
                                   |                  |
                                   |         R2       |
                                   .-----/\/\/\/-------.
                                   |                  |
                                   |                  |
                                   |----------B+------|
                                              V2=6.0V
     
    EDIT: :yuck: Well, it looks best in the editor window, slightly better (but not great) in the code tags, and unreadable without those tags.

    SECOND EDIT: looks fine now

    ------------------------------------------------------------

    For the first question, you should know how to calculate net resistances when you have resistors in series and parallel. To find the total resistance between A and B, it is:

    [tex]R_{total} = 12 \Omega + (55 \Omega || R)[/tex]

    [tex]38 \Omega = 12 \Omega + (\frac{1}{55 \Omega} + \frac{1}{R})^{-1}[/tex]

    [tex]26 \Omega = (\frac{1}{55 \Omega} + \frac{1}{R})^{-1}[/tex]

    [tex](\frac{1}{26 \Omega} - \frac{1}{55 \Omega})^{-1} = R[/tex]

    [tex]R \approx 49.310 \Omega[/tex]

    [tex]R = 49 \Omega[/tex]

    -----------------------------------------------------------

    Solve question 2 using the principle of superposition. That is, find the voltages and currents with the circuit first by replacing the top battery with a short circuit, then find the voltages and currents with the top battery, replacing the bottom with a short circuit. Add the numbers from the first calculations to the second calculations, and make sure you take direction/sign into account.
     
    Last edited: Jun 28, 2004
  4. Jun 28, 2004 #3
    Thank you so much, I got them both right and 100 on the assignment :) :) :)
     
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