Dielectrics Experiment: Place dielectrics between plates of capacitor at Q=const condition Observation: potential difference decreases to smaller value with dielectric material relative to air Without dielectric: With dielectric: because V<V 0 C>C 0 K>1: relative dielectric constant

d What happens with the E-field in the presence of dielectric material We know V<V 0 E<E 0 specifically and Definition of the permittivity The surface charge (density) σ on conducting plates does not change but induced charge σ i of opposite sign reduced with dielectric material

Example: K1K1 K2K2 d/2 +Q -Q E0E0 E1E1 E2E2 V

Dielectric breakdown or Dielectric strength Cr 2 O 3 Ground High Voltage Air

Conductor Dielectrics A A A

Example: K rara rbrb r E1E1 E2E2

E E

8 A conductor is an extreme case of a dielectric, since if an electric field is applied to a conductor, charges are free to move within the conductor to set up “induced charges”. What is the dielectric constant of a perfect conductor? A.K = 0 B.K = C.A value depends on the material of the conductor