What effect capacitor has on alternating current ac?

In the basic configuration, a capacitor is constructed with two parallel conductor plates with a layer of insulating material in between. When the cap is hooked up to the AC power supply, the voltage (v) across the plates and the charge (q) induced on the plates follow this capacitance expression:

C = dq/dv or i = C dv/dt, where C is determined by the properties of the insulating material and the geometry of the cap (in the case of the parallel plates, the separation between the two electrodes (t).

For the parallel plates, C can be written as (dielectric constant * plate area / t). Electrically, the change in the charge induced on the plates (dq), is directly related to the change in voltage difference (dv) between the two plates, since C is a constant. Theoretically, no energy is lost by charging and discharging the cap with an AC current. When the cap absorbs electrical energy from the power supply, it stores the energy in the electric field in the insulator. When discharging, the cap gives the stored energy back to the circuit -- hence, no energy loss.

In a circuit, we use the cap to prolong/smoothen/resist any voltage change in time or to absorb a sudden energy surge (electrostatic discharge and power-line glitches, for example).