What time does it take for the current in a circuit made up of a coil and a resistor to reach 0.9 of its stationary value?

Answer

`t=2.3L//R`

Explanation

According to the result of the previous problem we have for `i=0.91_M` an exponential equation `0.9I_(M)` an exponential equationi `0.9I_(M) =I_(M) (1-e^(-t//pi)), or e^(-t//tau)=0.1`
Taking the logarithms, we obtain `-t/tau log e=log 0,1, -t/tau 0.434=-1`
Hence `t=2.3tau`.