Please help setting up random unknown solution...

Discussion in 'Homework Help' started by CADmium, Feb 20, 2014.

  1. CADmium

    Thread Starter New Member

    Feb 20, 2014
    2
    0
    First post, so I do want to say, I've enjoyed this site very much. Very helpful.

    I don't know if I'm just tired or what, but I cannot for the life of me figure out how to set up this problem I was given:

    "Three resistors in parallel with 120-V power line dissipates a total power of 288W. If R2 = 3R1 and R2 = 2R3, calculate the values of R1, R2, R3."

    (R1, R2, R3 being the three different resistors)

    I can find total resistance and Total current, but I'm lost after that.
    Any help with how to set this up and solve it is much appreciated.
    Thanks in advance. :)
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,730
    759
    It's easy.
    But first you said 2 R2's. See above quote.

    Is this a trick question?
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,374
    494
    select one resistor and use it to represent the other 2 resistors.
    Example:
    r2=3r1
    r2=2r3
    since they both equal r2, then:
    3r1=2r3
    therefore:
    r1=2r3/3
    So now you have:
    r3
    r2=2r3
    r1=2r3/3
    all your resistors are represented in terms of r3

    Find equivalent resistor, which you said you can do. Plug it into power formula, solve for r3. Plug r3 into the formula for r2 and r1, BAM! You are done.
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,787
    There's no problem with how it is stated. It is simply relating both R1 and R3 to R2.
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,787
    Q1) In terms of the voltage and the resistor values R1, R2, and R3, what is the power dissipated in R1? In R2, In R3?

    Q2) In terms of the expressions above, what is the total power dissipated in the circuit?

    Q3) What is the term for the power dissipated in R1 become if R1 is written in terms of R2?

    Q4) What is the term for the power dissipated in R3 become if R3 is written in terms of R2?

    Q5) In light of the above, can you get an expression that relates the total power dissipated in the circuit to just the value of R2?

    Q6) What is the value of R2?

    Q7) Given the answer above, what is the value of R1? Of R3?
     
  6. CADmium

    Thread Starter New Member

    Feb 20, 2014
    2
    0
    Sorry it took me so long to get back. Between school and work, I'm suprised I'm wearing pants!

    Just wanted to say thanks!! That was just the help I needed,guys.
     
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