Dielectrics or insulators

A dielectric is a non-conducting material with no free electrons -- every electron in a dielectric remains bound within its own atom or molecule. Ebonite, glass and mica are common examples. Dielectric materials fall into two categories, depending on their molecular structure. In non-polar molecules, the centre of the positive charge (the nuclei) and the centre of the negative charge (the electron cloud) coincide exactly, so the molecule has zero dipole moment when no external field is present; when an external electric field is applied, however, the field pulls the positive and negative charge centres apart by a tiny distance, inducing a small dipole moment in each molecule, aligned with the applied field. In polar molecules (such as water), by contrast, the positive and negative charge centres are already permanently offset even with no applied field, so each molecule carries its own permanent dipole moment; in the absence of an external field, however, thermal motion orients these permanent dipoles randomly in every direction, so their contributions cancel and the material shows no net polarisation on average. When an external field is applied to a polar dielectric, the field exerts a torque on each permanent dipole (as derived in section 1.4.3), twisting the molecules to align, at least partially, with the field direction; the resulting partial alignment gives the material a net polarisation, even though each individual dipole's magnitude is unchanged, only i …