Make a figure analogous to Fig. $21.4$ for an ideal inductor in an ac circuit. Start by assuming that the voltage across an ideal inductor is $v_{\mathrm{L}}(t)=V_{\mathrm{L}} \sin \omega t$. Make a graph showing one cycle of $v_{\mathrm{L}}(t)$ and $i(t)$ on the same axes. Then, at each of the times $t=0, \frac{1}{8} T, \frac{2}{8} T, \ldots, T$, indicate the direction of the current (or that it is zero), whether the current is increasing, decreasing, on (instantaneously) not changing, and the direction of the induced emf in the inductor (or that it is zero).