Practical takeaway
AC circuits add frequency-dependent opposition to current. Resistance, inductive reactance, and capacitive reactance combine as impedance.
G5A · General question group
Reactance, Impedance & Resonance
AC circuits add frequency-dependent opposition to current. Resistance, inductive reactance, and capacitive reactance combine as impedance.
General exam focus
The explanation below teaches the ideas behind G5A while keeping the important terms recognizable. The Quiz Me button uses the current question-pool wording for this group.
1. Reactance changes with frequency
Inductive reactance increases as frequency increases, while capacitive reactance decreases as frequency increases. That opposite behavior lets inductors and capacitors select or reject frequencies.
2. Impedance has magnitude and phase
Resistance dissipates energy; ideal reactance stores and returns energy. In an AC circuit their combination is impedance, so voltage and current may no longer be in phase.
3. Resonance is where reactances cancel
At resonance, inductive and capacitive reactance are equal in magnitude and opposite in sign. A series resonant circuit can have low impedance; a parallel resonant circuit can have high impedance.
G5A checklist
- Inductive reactance rises with frequency.
- Capacitive reactance falls with frequency.
- Impedance combines resistance and reactance.
- At resonance, inductive and capacitive reactance cancel.
Exam scope: NCVEC 2023-2027 General Class (Element 3), effective July 1, 2023 through June 30, 2027, with February 4, 2026 sixth errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool