Practical takeaway
Impedance is the overall opposition to AC. In radio work, the familiar 50-ohm system is an impedance system, not merely a DC resistance value.
Electricity & Electronics
Impedance
Introduce the broader opposition that AC signals encounter in resistive and reactive circuits.
TECHNICIAN FOUNDATION10 of 16
Technician exam focus
Understand the distinction between resistance and impedance and recognize that reactance changes with frequency.
Resistance is not the whole AC story
Resistance R dissipates energy.
Resistance affects both DC and AC circuits and does not depend on frequency in this simple model.
Reactance changes with frequency.
Capacitive reactance XC decreases as frequency rises. Inductive reactance XL increases as frequency rises.
XL and XC oppose each other.
In a series RLC circuit, the net reactive part is the difference XL − XC. Whichever is larger determines whether the circuit looks inductive or capacitive.
At resonance, XL = XC.
Because they are equal and opposite reactive effects, they cancel in the ideal series circuit. The remaining impedance is essentially R, so current is at its maximum.
STEMulation
Watch frequency change a series R-L-C circuit
This teaching circuit uses R = 50 Ω, L = 100 µH, C = 100 pF and a 10 V AC source.
Near resonance
Resistance R50 Ω
Capacitive reactance XC
Inductive reactance XL
Total impedance |Z|
Circuit current