Dielectric Constant: If the gap between the conductors
of a capacitor is filled with an insulator
other than vacuum, the capacitance is increased because of the
polarization of the insulator.
It has been discovered that almost all insulators simply increase the capacitance
by a factor, 
, called the dielectric constant:
To change a formula that works in vacuum to one that works in a dielectric,
simply replace 
everywhere by 
.
(Note: air is also a dielectric, but its dielectric constant is so close to
1 that we usually ignore it.)
Dielectric Strength: The dielectric strength of an insulator is the critical electric field beyond which the molecules in the dielectric stretch so much that one or more electrons are ripped free from the molecules. When this happens, the free electrons crash into other stretched molecules, knocking other electrons free; the resulting chain reaction is a spark that burns a hole through the dielectric. If this happens to the dielectric in a capacitor, the capacitor is usually ruined. The most spectacular example of this process is lightning, and the most annoying is when your little brother rubs his feet on the carpet and sneaks up from the rear.