Capacitive reactance decreases as frequency increases. A capacitor passes high frequencies easily (low Xc) but blocks low frequencies (high Xc). This property is used in filters, coupling circuits, and bypass applications.
⚠Xc is purely reactive (imaginary impedance). Real capacitors also have equivalent series resistance (ESR) that adds real power loss. Xc alone does not dissipate power.
What is Capacitive Reactance?
Capacitive reactance is the opposition to current flow presented by a capacitor in an AC circuit. Unlike resistance, it varies with frequency. It is the imaginary part of impedance and causes a 90° phase shift where current leads voltage. Understanding Xc is essential for filter design and AC circuit analysis.
Frequency Dependence
Xc drops as frequency rises. Low freq = high Xc (blocks). High freq = low Xc (passes). DC = infinite Xc (open).
Phase Relationship
In a purely capacitive circuit, current leads voltage by 90°. The reactance causes energy to be stored/released each cycle.
Filter Applications
High-pass filter: capacitor in series (passes high freq). Low-pass filter: capacitor to ground (shorts high freq). Cutoff at fc = 1/(2πRC).
Unit Conversion
1 F = 10⁶ μF = 10⁹ nF = 10¹² pF. 1 kHz = 10³ Hz. Common: μF @ kHz range gives Xc of ohms to kohms.
Teaching Example: C=1μF at f=1kHz.
Xc = 1/(2π×1000×1e-6) = 1/(0.006283) = 159.15Ω.
At f=100Hz: Xc = 1/(2π×100×1e-6) = 1591.5Ω (10× higher). At f=10kHz: Xc = 15.92Ω (10× lower).
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