important properties of the series LCR circuit arise because its impedance is lowest at the frequency of resonance (Z =

R) and because it rises rapidly as the frequency is changed. When the applied frequency is below the circuit's resonant

frequency, this impedance consists of resistance plus capacitive reactance. This is because XC is larger than XL.

When the applied frequency is above the circuit's resonant frequency, XL is larger than XC, and the circuit impedance

consists of resistance plus inductive reactance. The impedance curve of a series LCR circuit (R very small) is shown in

figure 1-4A. Figure 1-4B shows the way current changes from leading to lagging as frequency increases.