Two cells of emfs 1 V and 2 V and internal resistance 2 and 1 , respectively connected in parallel, gave a current of 1 A through an external resistance. If the polarity of one cell is reversed, then value of current through the external resistance will be 5 A. The value of is .
Answer
3
Solution
[figure]
Two cells in parallel: Eeq= E1/r1+E2/r21/r1+1/r2, req= r1r2r1+r2
Same polarity: Eeq= 12+ 21 12+1= 53 V, req= 2 12+1= 23
I= EeqR+req ⇒ 1= 5/3R+2/3 ⇒ R=1
1 V cell reversed: E'eq= - 12+2 32=1 V; req= 23 (unchanged)
(Reversing the 2 V cell instead gives E'eq=-1 V: same magnitude.)
I'= 11+ 23= 35 A
∴ 5= 35 ⇒ =3
Answer: 3