can capacitor act as active element?

Answer 1

No. You cannot get amplification from a capacitor. It is always a passive element.

Answer 2

Capacitor is used as an active element in power
factor improvement circuits of A.C.

http://www.elec-toolbox.com/usefulinfo/pfcorrection.htm

Recently, due to the popularity of electronic and computer product, as well as the increase awareness of environmental protection issue, therefore it has been listed as a improvement item.
Normally power source (exclude battery) for electronic and computer product comes through from public utility power system. Its power source's circuitry structure is using diode rectifier, filter then pass through current transformation circuitry, as shown in figure 1, even though this method will make the voltage waveform becomes positive waveform rectifying diode and filtering capacitor, the rectifying diode short at the instant where the AC voltage is higher than the voltage on the filtering capacitor, causing intermittent current flow which result in an impulse waveform effect, this is the reason why harmonic current and power factor are below (usually 0.6- 0.7).

http://www.imcpower.com/prodigit/an_307.htm

http://www.search.com/search?q=power+factor+improvement+using+capacitor

Answer 3

d_r_siva is confused.

It doesn't matter what a capacitor is used for, or how it is used, or where it is used. It is still a passive component.