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Circuit with Series and parallel resistors, voltage division rule

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data

    Given: The power absorbed by the 15 ohm resistor is 15 W. Find R.

    http://sphotos-a.xx.fbcdn.net/hphotos-snc7/384382_495043340515186_1992745410_n.jpg [Broken]


    2. Relevant equations

    parallel circuit for 2 resistor: Req =R1R2/ R1+R2

    Voltage Division Rule: for series circuit VR2 = VsR2/(R1+R2)

    3. The attempt at a solution
    P = V2/R
    P = 15W, R = 15Ω
    P at 15Ω = 15 W => V at 15Ω = 15V

    => V at R = 25V - 15V = 10V

    So I go around the circuit from the left to right.

    Start at the double 4Ω => Req = 2Ω
    the 2Ω + 6Ω || 24Ω => Req = 6Ω
    6Ω+4Ω || 15Ω => Req = 6Ω

    now the circuit only have R and the 6Ω

    apply VR2 = VsR2/(R1+R2)
    Plug in the number
    solve for R = 4Ω

    Please let me know if this is the right approach or if I have made a mistake.
    Thanks for your time.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 30, 2012 #2

    CWatters

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    Science Advisor
    Homework Helper

    Both the approach and the answer look ok to me.
     
  4. Sep 30, 2012 #3

    rude man

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    Gold Member

    answer is correct.
     
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