Day 4: Dielectrics & Their Molecular Description

1. 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

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

3. 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:

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

5. 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:

6. Molecular Description of Dielectrics • An air dielectric capacitor C0, charged to ΔV0, produces charges ±Q on each plate.

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

8. 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

9. 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

10. 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 E0 and the electric field due to the induced charges on the surface of the dielectric Eind

11. 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