APFIVE Battery Internal Resistance and Parallel Resistors
A circuit contains a 12 V battery with internal resistance $r$, connected to a resistor of resistance $R$. When a second identical resistor is connected in parallel with the first, the current from the battery triples. What is the internal resistance of the battery, $r$?
A
-R/4
B
R/2
C
2R
D
R
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