If the electric field between the plates becomes too strong, the air between them can no longer insulate the charges from sparking, discharging, between the plates. In order to keep this from happening, a dielectric is often inserted between the plates to reduce the strength of the electric field, without having to reduce the voltage being placed across the plates.

 

A dielectric is a polar material whose electric field aligns to oppose the original electric field already established between the plates. The dielectric is measured in terms of a dimensionless constant, K ≥ 1, whose value is usually referenced from a table. If this insulating material is insufficient the capacitor can leak allowing current to flow between the plates. If this occurs, the electric device "smells as if something is burning."

 

K = Eoriginal/Edielectric K = Eo / Ed K = Cdielectric/Coriginal K = Cd / Co   When the battery is removed, the dielectric will decrease the electric field strength and the voltage between the plates while it increases their capacitance.   E = V/d