If, in Fig. 34-2, $R=20 \Omega$, $L=0.30 \mathrm{H}$, and $\varepsilon=90 \mathrm{~V}$, what will be the current in the circuit $0.050 \mathrm{~s}$ after the switch is closed?
We are going to use the exponential equation for $i$ given in Eq. (34.9).
The time constant for this circuit is $\mathrm{L} / \mathrm{R}=0.015 \mathrm{~s}$, and $i_{\infty}=\varepsilon / \mathrm{R}=$
$4.5 \mathrm{~A}$. Then
$$
i=i_{x}\left(1-e^{-i /\langle L / R)}\right)=(4.5 \mathrm{~A})\left(1-e^{-333}\right)=(4.5 \mathrm{~A})(1-0.0357)=4.3 \mathrm{~A}
$$