What is the use of dielectric material in capacitor?
Answers
First of all, the capacitor becomes a ‘capacitor' because of the dielectric in between two conductor plates.
Once a DC supply is applied, charges flow in the circuit just to reach the dielectric and get blocked by it. These charges get collected on the conducting plates till the plates reach the their maximum capacity. At this point, the flow of charges comes to a halt.
Initially, you could measure a current in the circuit which ceases when the plates reach their saturation. The stored charges will be such that for every positive charge on one plate, a negative charge will be present on the other. At this time, if you connect a voltmeter across the dielectric you can see the presence of voltage. This capacitor with charged plates is similar to a rechargable battery with a short discharging time.
This is not the case while AC supply is applied. Since it changes direction continuously, the capacitor gets charged during the rising of the ac signal and discharges with the falling of the signal. You can see here that the current flow is similar to that in a normal wire. Hence, for an ac signal capacitor acts just like a short.
Thus with the presence of a dielectric in between two conducting plates, DC current can be blocked and without affecting the ac current in the circuit. It can also be used as a small time battery at times.