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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?

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Solution :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`.


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