When an ideal capacitor is charged by a d.c. battery, no current flows. However, when an a.c. source is used, the current flows continuously. How does one explain this, based on the concept of displacement current ?

Explanation

When an ideal capacitor is fully charged by a d.c. battery, the electric field `vecE` between the plates of capacitor does not change with time. Therefore, displacement current is zero.
When an a.c. source is applied between the plates of an ideal capacitor, the electric charge on plates and consequently electric field `vecE` between the plates of capacitor is continuously changing with time. Consequently a displacement current is set up and we say that a current is flowing through the capacitor.