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Capacitor in AC
Intermediate
Current leads voltage by 90°.
X_C = 1/ωCCurrent leads voltage by 90°, with reactance X_C = 1/ωC that decreases with frequency. It blocks DC.
C = 10 μF at 50 Hz: 318 Ω.
Opposite behaviour to an inductor.
In more detail
A capacitor in an AC circuit has its current leading the voltage by 90 degrees. Its reactance X_C = 1/(ωC) decreases with frequency, so it passes high frequencies more easily.
Real-world uses
- Capacitors couple AC signals between amplifier stages while blocking DC.
- Power factor correction uses capacitors to offset inductive loads.
- Capacitors in power factor correction supply reactive current that offsets the lag from motors.
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Frequently asked questions
Why does current lead voltage?
Because current charges the capacitor, and voltage builds up later.
Does a capacitor pass DC?
No. At zero frequency its reactance is infinite, so it blocks DC.
Why does current lead voltage in a capacitor?
The current charges the plates, so it is largest when the voltage is changing fastest, which is a quarter cycle before the voltage peaks.
Related topics in AC Circuits & EM Waves
Phasors · Resistor in AC · Inductor in AC · LCR series circuit · Phase angle · Series resonance