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Homework Help: 2 Resistors in Parallel versus in Series

  1. Aug 19, 2010 #1
    1. The problem statement, all variables and given/known data
    Two resistors when connected in series to a 110 V line use one-fourth the power
    that is used when they are connected in parallel. If one resistor is 2.0 kΩ, what is the resistance of the other?


    2. Relevant equations

    V=IR

    3. The attempt at a solution
    v=ir

    110=i(2000+x)

    110=1/4(2000+x)

    440=2000+x

    x=440-2000

    x=-1560

    snce x= 1560

    the value of another resistance is 1.56 kilo ohm

    Did I do that right?
     
  2. jcsd
  3. Aug 19, 2010 #2
    Not quite. Okay, your series formula is fine. But now you have to get the power for the resistors in series. P=I^2 R and P=V^2/R.

    Now that you have the power for the series resistors, find the power for the resistors if they were in parallel. 110=i*Req again, but different Req this time.
     
  4. Aug 19, 2010 #3
    Can I just cancel the V's out and solve like this:

    R2 + 2000 = 2*(2000*R2)/(2000 + R2)

    Which looks like it would require the quadratic eqn
     
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