Practical takeaway
Reactive circuits have memory: capacitors and inductors do not change stored energy instantaneously. Time response and phase are two views of that behavior.
E5B · Amateur Extra question group
Time Constants, Phase & Admittance
Reactive circuits have memory: capacitors and inductors do not change stored energy instantaneously. Time response and phase are two views of that behavior.
Amateur Extra exam focus
The explanation below teaches the ideas behind E5B while keeping the important terms recognizable. The Quiz Me button uses the current question-pool wording for this group.
1. RC and RL time constants set response speed
An RC time constant is resistance times capacitance; an RL time constant is inductance divided by resistance. After one time constant, a first-order response has moved about 63 percent toward its final value.
2. Phase shows timing between sine-wave quantities
In a pure capacitor, current leads voltage; in a pure inductor, current lags voltage. Real circuits combine resistance and reactance, producing intermediate phase angles.
3. Admittance is the reciprocal view of impedance
Admittance expresses how readily AC current flows and is the reciprocal of impedance. Conductance is the real part and susceptance the reactive part, which can simplify parallel-network analysis.
E5B checklist
- RC time constant = R×C; RL time constant = L/R.
- One time constant represents about 63% of the final step change.
- Capacitive current leads voltage; inductive current lags voltage.
- Admittance is the reciprocal of impedance.
Exam scope: NCVEC 2024-2028 Amateur Extra Class (Element 4), effective July 1, 2024 through June 30, 2028, with February 4, 2026 fourth errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool