Capacitance

Dielectrics are the insulating materials used between the conducting plates of capacitors. Dielectrics increase capacitance or provide better insulation between the plates. Dielectrics materials exhibit very little ability to conduct electric charge. Mylar, paper, mica, and ceramics are commonly-used dielectrics. When extremely-high capacitance is required, a thin film of aluminum oxide on etched aluminum plates is used as a dielectric.

Dielectrics have a property called polarizability. A dielectric placed within an electric field appears to have electric charge on its surfaces even though the insulator remains electrically neutral. Each of the dielectric's molecules is stretched when the electric field causes its negative charges to be pulled toward the positive-charged capacitor plate and the molecule's positive charges are pulled toward the negative plate. This polarization strain causes each dielectric molecule to act as a voltage source. These voltages add in series aiding as do the voltage from several cells making up the battery in a flashlight. A phantom charge appears on each surface of the dielectric canceling much of the electric field produced by the real charges. The greater the polarization developed by a dielectric the larger the quantity of real charge the capacitor must store to develop a given voltage. The capacitance appears to increase as a result of dielectric polarization.

The capacitance multiplier for any dielectric is called its dielectric constant. The dielectric constant of a perfect vacuum is defined as exactly 1. Common dielectrics have dielectrics constants in the range of 2-4. Using a higher quality dielectric increases the capacitance by a factor equal to the dielectric constant.