A d.c. main supply of e.m.f. `220 V` is connected across a storage battery of e.m.f. 200 V through a resistance of `1 Omega` . The battery terminals are connected to an external resistance ‘R’. The minimum value of ‘R’, so that a current passes through the battery to charge it is:

Options

A. `7 Omega`
B. `9 Omega`
C. `11 Omega`
D. Zero

Answer

C

Explanation

Given emf of cell V= 200V
Internal resistance of cell `=1 Omega`
D.C main supply voltage E= 220V
External resistance R= ?
`r= ((E-V)/(V)) R`
`1 = ((20)/(220)) xx R`
`R= 11 Omega`