Dielectric

A dielectric is an insulator considered in its capacity to be polarized by an electric field. The two words describe the same kind of material — glass, ceramic, plastic, dry wood, mineral oil, distilled water — but used in different contexts. We call it an "insulator" when we care about it blocking current; we call it a "dielectric" when we care about it polarizing in a field and storing energy.

The defining behaviour of a dielectric is bound-charge response. Apply an electric field, and the atoms and molecules inside polarize: positive nuclei and negative electron clouds shift in opposite directions by tiny amounts, producing a polarization density P. The polarized medium produces its own internal electric field that opposes the applied field, weakening the net field inside the material by a factor κ — the dielectric constant. This is why putting a slab of glass between the plates of a capacitor lets the same charge sit at a lower voltage, equivalently storing more charge at the same voltage.

Real dielectrics are imperfect. Every real insulator has some tiny conductivity (a leakage current that bleeds charge over hours or days) and a maximum field strength it can tolerate before its bound electrons are torn loose and the material breaks down — the dielectric strength, typically a few MV/m for air and 10–100 MV/m for solid insulators. Ferroelectric dielectrics are the spectacular exception to ordinary linear behaviour: they remember a polarization in zero field, like an electric analogue of a permanent magnet. Most of the §02 chapter is about ordinary linear dielectrics; the ferroelectric story shows up in the §02.4 piezo / ferro topic.