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Resistance & EMF please HELP!

  1. Feb 6, 2008 #1
    1. The problem statement, all variables and given/known data

    [​IMG]

    What is the resistance R? (R1=15.13 Ω, I1=2.77 mA , I2=1.81 mA. Give answer in Ω.)
    What is the emf of the battery?

    2. Relevant equations

    I = E/R

    3. The attempt at a solution

    How do I solve this with the batteries EMF?
     
  2. jcsd
  3. Feb 6, 2008 #2

    berkeman

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    Staff: Mentor

    Write 2 equations with the two unknowns (V and R), then solve for both.

    And shrink that picture!
     
  4. Feb 6, 2008 #3
    Ok let me see what I can come up with...
    How do you shrink it? I did it on tinypic.com
     
  5. Feb 6, 2008 #4
    E = -v1 - V2
    E = Ir1 + Ir2
    R = E/i
     
  6. Feb 6, 2008 #5

    berkeman

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    Staff: Mentor

    With Paint or other figure manipulation/drawing programs, you can choose to "Save As" and specify the resolution.
     
  7. Feb 6, 2008 #6
    Are those the right equations? I am not seeing how to solve this?
     
  8. Feb 6, 2008 #7

    berkeman

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    Staff: Mentor

    Write one equation for the current and voltage in the middle branch, and one equation for the current and voltage in the right branch.
     
  9. Feb 6, 2008 #8
    MIDDLE BRANCH:
    R = E/I

    RIGHT BRANCH:
    R = E/I1 + I2

    That doesn't look right??
     
  10. Feb 6, 2008 #9

    berkeman

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    Staff: Mentor

    Why are you not using the names of the resistors in the figure? There are two resistors in the right branch, correct? And current J1 flows in the left branch, and J2 flows in the right branch. Both branches have the same voltage across them, correct?

    Take your time and think it out. I need to bail -- you'll get it (or more help may stop by if you have more questions).
     
  11. Feb 6, 2008 #10
    MIDDLE BRANCH:
    R = E/I1

    RIGHT BRANCH:
    R + R1= E/I2

    Is that right?
     
  12. Feb 6, 2008 #11
    Ok so I think I got it...I just set them equal to eachother and I get...
    I1 * R = (R + R1) * I2
    But for some reason I don't know how to solve for R!! I can't figure it out...very sad...please help me! I have tried all sorts of things and I am not getting anywhere...
     
  13. Feb 6, 2008 #12
    can anyone help me with this part?
     
  14. Feb 6, 2008 #13

    rl.bhat

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    Homework Helper

    I1 * R = (R + R1) * I2
    Substitute the values of I1, I2 and R1 and solve for R
     
  15. Feb 6, 2008 #14
    Thats what I am trying to do but I can't figure out how to get the R by itself on one side since there are 2 of them. I always end up with R/R which is just 1. Its not making sense to me.
     
  16. Feb 7, 2008 #15
    Do you already know what the answer is suppose to be? We use to get the solution to some of our problems, but had to show the work in getting to it.

    Do you end up with R*(2.77) = R*(1.81) + (27.39) ?
     
  17. Feb 7, 2008 #16
    No I don't know what the answer should be
     
  18. Feb 7, 2008 #17

    berkeman

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    Staff: Mentor

    It's simple algebra. Distribute I2 on the right side into the parenthesis. That gives you one R term on the left and one R term on the right, with the additional R1*I2 term on the right. Just move the R term from the right side to the left (you do know how to do that, right?), and then finish solving for R all by itself.

    Please try again, and show each of your steps here.
     
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