Answer to Question #99246 in Electric Circuits for NURUL SHAHIRAH NADIA BINTI AHMAD ZUHAIRI

Question

Why the electric field between two parallel plates of a capacitor decreases when a dielectric is inserted if the capacitor is not connected to a power supply , but remains the same when it is connected to a power supply.

Answer 1

No voltage source.

When there's no voltage source the molecules of the dielectric polarize along the lines of the field and thus create their own field opposing the external field. Therefore, the total field is the external field created by the charged capacitor's plates minus the field created by the molecules of the dielectric.

With voltage source.

A power supply means that this is a voltage source, i.e. it provides a constant voltage. As we know, the field voltage per distance between the plates:

$$E=V/d.$$

Voltage in a capacitor depends on its charge and its capacitance:

$$V=Q/C=Qd/(k\epsilon_0A),$$

where $k$ - the dielectric constant. Hence,if we had no voltage source, the voltage of the capacitor would increase. Since we have the voltage source, it increases the charge and the field (the first equation) remains the same.