There is another benefit to using a dielectric in a capacitor. Depending on the material used, the capacitance is greater than that given by the equation C = εA d by a factor κ, called the dielectric constant. A parallel plate capacitor with a dielectric between its plates has a capacitance given by C = κε0A d(parallelplatecapacitorwithdielectric).
In many capacitors, there is an insulating material such as paper or plastic between the plates. Such a material, called a dielectric, can be used to maintain a physical separation of the plates. Placing a solid dielectric between the plates of a capacitor serves three functions. See also: Capacitance See also: Electric polarization
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.
We have seen that the capacitance of a parallel-plate capacitor is increased by a definite factor if it is filled with a dielectric. We can show that this is true for a capacitor of any shape, provided the entire region in the neighborhood of the two conductors is filled with a uniform linear dielectric.
The most common application of dielectrics is in capacitors, as one would guess from the figure. How is the capacitance affected by the presence of this substance? Given the same charges on the plates, the polarization charge reduces the electric field between the plates compared to the vacuum case, so the voltage difference is decreased.
(b) A rolled capacitor with an insulating material between its two conducting sheets. A capacitor is a device used to store electric charge. When battery terminals are connected to an initially uncharged capacitor, equal amounts of positive and negative charge, + Q and − Q, are separated into its two plates.