NEET Physics Current Electricity Questions Solved

PMT - 2005

Two cells of equal e.m.f. and of internal resistances r1 and r2(r1>r2) are connected in series. On connecting this combination to an external resistance R, it is observed that the potential difference across the first cell becomes zero. The value of R will be 

(1) r1+r2

(2) r1r2

(3) r1+r22

(4) r1r22

(2) Let the voltage across any one cell is V, then

V=Eir=Er12Er1+r2+R

But V = 0

E2Er1r1+r2+R=0

r1+r2+R=2r1

R=r1r2

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