A cell of emf `epsilon` and internal resistance r is charged by a current I, then

Explanation

`V_(L)=V_(R)Ee^(-(Rt)/(L))=E(1-e^(-(Rt)/(L))) implies 2e^(-(Rt)/(L))=1, e^(-(R)/(L)t)=(1)/(2), e^((R)/(L)t)=2`
`(Rt)/(L)=ln 2"(or)" t=(L)/(R)ln2`