Two sources of equal emf are connected to an external resistance R. The internal resistance of the two sources are R1 and R2 (R2>R1) . If the potential difference across the source having internal resistance R2 is zero, then:
A
R=R2 [ (R1+R2)/(R1−R2) ]
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B
R=R2−R1
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C
R=R1R2/(R1+R2)
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D
R=R1R2/(R2−R1)
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Solution
The correct option is AR=R2−R1 In the following figure: Vbc=E−iR2=0. ------------ (1) The current i flowing through the whole circuit is: i=2ER+R1+R2. Using above equation in the first equation we get: E(1−2R2R+R1+R2)=0. Now, E is not equal to zero, so the rest in the bracket must be zero i.e. (1−2R2R+R1+R2)=0 or R=R2−R1.