Series problem

Discussion in 'Homework Help' started by Sweetl14, Oct 23, 2012.

  1. Sweetl14

    Thread Starter New Member

    Oct 23, 2012
    The problem I am stuck on is:

    Three resistors are connected in series to a 120V source. The voltage drop across R1 and R2 together is 60V. The voltage drop across R2 and R3 together is 90V. If the total resistance is 12ohms, find the resistance of each resistor.

    How can this be calculated without the voltage for each resistor?

    I have calculated I=E/Rt which equals 10A
    I have drawn the circuit but I am still unsure how to get individual resistors. If R1 + R2= 60V and R2+R3= 90V how can I get the individual voltage to get the individual resistor? Every problem in my textbook gives you the individual resistor. I cannot find a way to manipulate the formulas in Introduction to Electric Circuits 9th Edition to help. There is a question in the back of chapter 7 that is similar but it just gives the answer which is of no help to me. I need to see how it is done step by step. Any assistance would be great!!
    Last edited: Oct 23, 2012
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    If you require help with homework at least show an effort and let us know your thoughts. ;)

    Draw the circuit and look at the voltage drops.
    Sweetl14 likes this.
  3. Sweetl14

    Thread Starter New Member

    Oct 23, 2012
    I have tried and tried and I cannot figure out how to do this one. I have looked at the voltage drops but the 60v and the 90v does not help me get the resistance of each individual least I don't know how. How do you get the resistance of 3 resistors with 2 voltage drops?
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    I give you a tip

    If Rt = 12Ω and I = 120V/12Ω = 10A. And we also know that the voltage drop across R1 and R2 together is 60V so
    R1 + R2 = 60V/10A = 6Ω
    and since Rt = R1 + R2 + R3 ----> R3 = Rt - (R1+R2) = 6Ω
    Simple huh?
    You now should be able to solve this problem
  5. Sweetl14

    Thread Starter New Member

    Oct 23, 2012
    Thank you!! It seems so simple now. Problem is solved!!! R2+R3=9 ohms so R1 and R2 are 3 ohms each. Many, Many many thanks!! :)