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Homework Help: Ideal Batteries' Internal Resistance

  1. Feb 28, 2014 #1
    1. The problem statement, all variables and given/known data
    A circuit is constructed with five resistors and one real battery as shown above right. We model. The real battery as an ideal emf V = 12 V in series with an internal resistance r as shown above left. The values for the resistors are: R1 = R3 = 59 Ω, R4 = R5 = 79 Ω and R2 = 140 Ω. The measured voltage across the terminals of the batery is Vbattery = 11.61 V.
    What is r, the internal resistance of the battery?

    2. Relevant equations
    Vb = Vo((R/r)/(1+R/r))
    Vb= voltage across battery terminals
    Vo= Internal voltage
    r= internal resistance
    R= external resistance
    Total ext. resistance in parallel: 1/Rt= 1/r1 + 1/r2
    total ext. resistance in series: Rt = r1 + r2

    3. The attempt at a solution
    Taking total resistance dividing by the amperage.
  2. jcsd
  3. Feb 28, 2014 #2


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    No figure attached.
  4. Feb 28, 2014 #3


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    That would just give you the ideal voltage which is already known.

    Hint: Potential divider
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