Q.Compare resistance and reactance.

[!TLDR] Resistance is frequency-independent and equally effective for AC/DC, in phase with voltage; reactance ($X_L\propto f$, $X_C\propto1/f$) is frequency-dependent, dissipates no energy itself, and shifts …

Resistance and reactance both oppose the flow of current and share the same SI unit (ohm, $\Omega$), but differ in several fundamental ways (Table 13.1). Resistance R is equally effective at reducing both AC and DC current, and is completely independent of the frequency of the applied AC. Reactance, by contrast, is inherently frequency-dependent: inductive reactance $X_L=2\pi fL$ is DIRECTLY proportional to frequency (zero for DC, growing without bound at high frequency), while capacitive reactance $X_C=\dfrac{1}{2\pi fC}$ is INVERSELY proportional to frequency (infinite for DC, i.e. blocking it completely, and falling towards zero at high frequency).\n\nEnergetically, current opposed by a resistor genuinely dissipates energy as heat (Joule heating, $i_{rms}^2R$), whereas current opposed by a reactance (pure L or pure C) consumes NO net energy over a complete cycle -- the reactance merely reduces the current's amplitude and shifts its phase, without permanently converting any electrical energy to heat; any actual energy loss in a real coil comes only from ITS resistive component, never from the ideal reactive part itself.\n\nFinally, the PHASE relationship differs completely: current opposed by a pure resistor stays exactly IN PHASE with the voltage across it ($\phi=0$). Current …

Systematically compare resistance and reactance along three axes -- dependence on AC/DC and frequency, energy dissipation, and phase relatio …

Treating reactance as if it were simply 'resistance that depends on frequency', while forgetting the two much more fundamental differences: reactance dissipates no net energy at all, and it introduces a genuine $9 …