A dielectric can be placed between the plates of a capacitor to increase its capacitance. The dielectric strength E m is the maximum electric field magnitude the dielectric can withstand without breaking down and conducting. The dielectric constant K has no unit and is greater than or equal to one (K ≥ 1).
Normally, dielectric constant is more than 1. Hence, the capacitance of a capacitor of given dimensions is greater when there is a dielectric material between the plates than when there is a vacuum. The electric field between the plates must also decrease by the same factor:
The capacitance of the capacitor without the dielectric is The battery is then disconnected from the capacitor and the dielectric is inserted between the plates. This is shown in Figure 1.58. The introduction of dielectric between the plates will decrease the electric field. Experimentally it is found that the modified electric field is given by
The energy stored in an empty isolated capacitor is decreased by a factor of κ κ when the space between its plates is completely filled with a dielectric with dielectric constant κ κ. Discuss what would happen if a conducting slab rather than a dielectric were inserted into the gap between the capacitor plates.
The capacitance of an empty capacitor is increased by a factor of κ κ when the space between its plates is completely filled by a dielectric with dielectric constant κ κ. Each dielectric material has its specific dielectric constant.
The electrical energy stored by a capacitor is also affected by the presence of a dielectric. When the energy stored in an empty capacitor is U0, the energy U stored in a capacitor with a dielectric is smaller by a factor of κ. U = 1 2Q2 C = 1 2 Q2 0 κC0 = 1 κU0.