Why the capacitors work on AC only?

Capacitor works on both AC and DC, where a capacitor gives infinite Impedence when connected to a Dc power and gives some moderate value of impedence when connected to a AC source, due to its frequency.

Say a capacitor is connected in an AC single phase Ckt. Now capacitor both side terminal say A & B.

Now switch on the AC supply.

Effect:-

1) in1st half cycle A become Positive & B become Negative.

2) In the2nd half Cycle B become +Ve & A become -Ve.

In a complete  AC cycle once the capacitor got charged & again it got discharged.

That is why Capacitor work in AC only.

Because if We connect a capacitor in DC Ckt,at the time of switching it will charged( DC One directional, or it is not having +ve & -VE half cycle), it will not discharged.

It will only discharged if DC ckt is made off.

Capacitor in AC ckt  used for Power factor improvement, harmonics absorbtion, Smoothing, filtering & in some application Capacitor Stabilizes  Voltage.etc etc

Generally capacitor gives infinite resistance to DC components. It follows the AC components to pass through.