PX157 - B12a - dielectrics

introduction

in a dielectric (insulator), there are only bound charges if no excess charges have been applied
subjecting a dipole to an electric field, the positive and negative charges in an atom are displaced slightly in opposite directions
surface charges are induced by the external electric field, $\vec{E}$ , which induces an electric field, ${\vec{E}}_{i}$ , that opposes $\vec{E}$

dielectric breakdown

if the external electric field, $\vec{E}$ , is strong enough, it can ionize the atoms $⟹$ free charges $⟹$ no more dielectric
the maximum electric field that a dielectric can withstand is termed the dielectric strength, $E_{m}$
- dry air: $E_{m} = 3 \times 10^{6} V m^{- 1}$
- pyrex glass: $E_{m} = 10^{7} V m^{- 1}$
polarization density $[C m^{- 2}]$ :

\vec{P} (\vec{r}) = \frac{1}{V o l .} \sum_{i} {\vec{P}}_{i}

electric displacement vector $[C m^{- 2}]$ :

\vec{D} = ϵ_{0} \vec{E} + \vec{P}

$\vec{P}$ and $\vec{E}$ are along the same direction