**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.

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