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Homework Help: Finding resistance using a series and parallel circuit?

  1. Feb 28, 2007 #1
    1. The problem statement, all variables and given/known data:Two resistors have resistances R1 and R2. When the resistors are connected in series to a 12.0V battery, the current from the battery is 2.00A. When the resistors are connected in parallel to the battery the total current from the battery is 9.00A. Determine R1 and R2.

    2. Relevant equations: V=IR Ohms Law

    3. The attempt at a solution:
    I know that R1 and R2 are the same resistance in both circuits, or R1 and R2 in series = R1 and R2 in parallel.

    I also know that the current remains the same in series but changes in the parralel circuit and that the voltage is not constant in series but the same voltage is applied over the resistors in a parallel circuit.

    Thus V/2.00A=12.0volts/I

    I get confused at this point and am not quite sure how to proceed in finding V or I and thus solve for R1 and R2.
  2. jcsd
  3. Feb 28, 2007 #2
    You're given V and i in this problem. Draw circuits for both cases of series and parrallel circuit. From there you can see how to solve it, then wite out the equations. Hint: Reduce the resistances to one equivalent circuit for both cases. Also why do you think R1 = R2?
    Last edited: Feb 28, 2007
  4. Feb 28, 2007 #3
    I didn't mean to say that R1 = R2 I just meant that R1 and R2 in the series must equal R1 and R2 in the parallel.
  5. Feb 28, 2007 #4
    Is it ok to assume that V/2.00 A = 12.0 Volts/I since R1 in series is equal to R1 in parralel and R2 in series equals R2 in parallel?
  6. Feb 28, 2007 #5
    Yea that works, and I see what you're trying to do. The method I posted in my first post would not need to find V and I.
  7. Feb 28, 2007 #6
    Is it simple algebra from here or am I missing something? I keep getting nonsense answers. Thanks
  8. Feb 28, 2007 #7
    Well I would be careful of the way you're labeling V and i. Like label it i1 and i2, for the currents going through R1 and R2 in the parallel circuit. Same thing for the voltages in the series circuit. Also post up what you have so far, so maybe someone else can help you out. :smile:
  9. Feb 28, 2007 #8
    one resistance is 2 ohm the other one is 4 ohm.

    For parallel:
    R[tot]=V/I and (R[1]*R[2])/(R[1]+R[2]), therefore: V/I=(R[1]*R[2])/(R[1]+R[2])

    For searies:
    R[tot]= V/I and R[1]+R[2], therefore V/I=R[1]+R[2]

    Now you have two unknown and two equations, isolate R[1] or R[2] from V/I=R[1]+R[2] and put into V/I=(R[1]*R[2])/(R[1]+R[2]), then one of the resistance is known and the otherone is easily found.

    Hope this solves the problem
  10. Feb 28, 2007 #9
    :bugeye: :mad: What are you doing??? You just gave him pretty much the entire solution to problem.
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