A $12\mathrm{.}0$ V battery is connected into a series circuit containing a $18\mathrm{.}0$ resistor and a $3\mathrm{.}10$ H inductor.

$($a$)$ In what time interval $($in s$)$ will the current reach $50\mathrm{.}0\%$ of its final value?

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$($b$)$ In what time interval $($in s$)$ will the current reach $90\mathrm{.}0\%$ of its final value?

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What If$?$ After a very long time, using a switch like that shown in the figure below, the battery is removed and the inductor is connected directly across the resistor.

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$($c$)$ In what time interval $($in s$)$ will the current decrease to $50\mathrm{.}0\%$ of its initial value?

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$($d$)$ In what time interval $($in s$)$ will the current decrease to $10\mathrm{.}0\%$ of its initial value?

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