Two cell with the same e.m.f. E and difference internal resistance `r_(1) and r_(2)` are connected in series to an external resistance `R` a value of `R` be selected such that the potential difference as the flow cell E should be zero when

Options

A. `R = r_(1)+r_(2)`
B. `R = r_(1)- r_(2)`
C. `R = r_(1)//r_(2)`
D. `R = r_(1)=r_(2)`

Answer

b

Explanation

Current is circuit,
`I= (E+E)/(r_(1)+ r_(2) +R) = (22E)/(r_(1)+ r_(2) +R) `….(i)
The potential difference first cell is zero,
Then `E - I r_(1) = 0 or I= E//r_(1)`…..(ii)
From (i) and (ii) `(2E)/(r_(1) +r_(2)+R) = (E )/(r_(1))`
On solving `R = (r_(1)- r_(2))`