Reactance and Impedance

Reactance -- the opposition of $L$ or $C$ alone. Sections 7.5 and 7.6 each produced a relation of the Ohm's-law shape, $i_0=v_0/(\text{some constant})$, for a pure inductor and a pure capacitor respectively. These constants are called reactances, and, exactly like resistance, are measured in ohms ($\Omega$) -- but, unlike resistance, they represent an opposition to current that arises purely from the CHANGING nature of the current, not from any actual dissipation of energy as heat.

Inductive reactance.

$X_L$ is directly PROPORTIONAL to frequency: at very low frequency (approaching DC, $f\to0$), $X_L\to0$, so an inductor offers essentially no opposition to a slowly changing or steady current; at high frequency, $X_L$ grows without bound, so an inductor increasingly BLOCKS rapidly alternating current. This is why an inductor is often called a 'choke' for high-frequency signals.

Capacitive reactance.

$X_C$ varies INVERSELY with frequency: at low frequency, $X_C$ is very large, so a capacitor almost completely blocks a slowly changing current (and blocks true DC entirely, since $f=0$ gives $X_C\to\infty$); at high frequency, $X_C\to0$, so a capacitor offers almost no opposition to a rapidly alternating current, behaving almost like a plain wire. …