A small signal voltage$\mathrm{V}\left(\mathrm{t}\right)={\mathrm{V}}_0 \sin \mathrm{\omega t}$ is applied across an ideal capacitor C

(4): When an ideal capacitor is connected with an ac voltage source, current leads voltage by ${90}^0$ . Since, energy stored in capacitor during charging is spent in maintaining charge on the capacitor during discharging. Hence over a full cycle the capacitor does not consume any energy from the voltage source.