Day 4: Dielectrics & Their Molecular Description Definition of dielectric material Effects of the presence of dielectric material in a capacitor The electric field between capacitor plates in the presence of a dielectric Molecular description of dielectrics

Dielectrics Any insulating material placed between the plates of a capacitor is called a dielectric Ex: paper, Mylar, mica, oil, ceramic

Effects of the Presence of a Dielectric The presence of a dielectric material increases the capacitance of the capacitor is the dielectric constant The energy density stored in the electric is now:

Dielectric Constants of Various Materials The dielectric strength is the maximum electric field before breakdown

Effect of the Electric Field when a Dielectric is Inserted The electric field in the presence of a dielectric is reduced by a factor of: With no dielectric: With a dielectric: The potential difference drops to:

Molecular Description of Dielectrics An air dielectric capacitor C 0, charged to ΔV 0, produces charges ±Q on each plate.

Molecular Description of Dielectrics If the capacitor is isolated and a dielectric is inserted, the charge Q is unchanged

Molecular Description of Dielectrics When the dielectric is inserted, the electric field between the plates causes the molecules of the insulator to become polarized Some of the electric field lines do not pass thru the dielectric but rather end on the induced charges on the surface of the dielectric

Molecular Description of Dielectrics The electric field is weaker than in air by: ΔV across the capacitor is also reduced by: Since capacitance must increase

Molecular Description of Dielectrics The electric field within the dielectric can be treated as the vector sum of the electric field due to the free charges on the plates E 0 and the electric field due to the induced charges on the surface of the dielectric E ind

Consequences of Dielectrics Remember that the electric field between the plates is related to the surface charge density σ The induced charge on the dielectric is always less than the free charge on the capacitor plates