Two sources of equal emfs are connected in series. This combination is connected to an external resistance R. The internal resistances of the two sources are r1 and r2 (r1 > r2). If the potential difference across the source of internal resistance r1 is zero, then the value of R will be:
1
r1 - r2
2
\(\frac{r_1 r_2}{r_1+r_2}\)
3
\(\frac{r_1+r_2}{2}\)
4
r2 - r1
5
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