physics//electromagnetism//electrical insulator

An electrical insulator is a material whose charges cannot move freely through it, so it blocks current while still letting an electric field pass, and it is used to keep current where it belongs: around wires, between the plates of a capacitor, on the towers of a power line. When the role is to carry the field between two conductors rather than just to separate them, the same material is called a **dielectric**.

An electrical insulator is a material whose charges cannot move freely through it, so it blocks current while still letting an electric field pass, and it is used to keep current where it belongs: around wires, between the plates of a capacitor, on the towers of a power line. When the role is to carry the field between two conductors rather than just to separate them, the same material is called a dielectric.

Why the field passes and the charge does not comes from how electrons are bound. In a metal the outer electrons are shared by the whole crystal and drift under any push; in glass, plastic, ceramic or air every electron belongs to an atom or a molecule. A field cannot pull them away, but it can tug them a little: each molecule stretches, its negative charge shifting slightly against its positive charge, which is polarization. Those billions of tiny shifts carry the field across the material and partly weaken it, while no charge actually crosses. Why some solids have loose electrons and others none is a quantum matter (the gap between the energy levels the electrons fill and the next free ones), but the result is enough to design with. A semiconductor sits between the two: its electrons are bound, but heat, light or doping can free them, which is why it conducts better warm and worse cold, the reverse of a metal.

A dielectric lets the field through and stops the charge.

That is the whole trick of a capacitor, and of every cable: the field does its work across the insulation, the current stays in the copper.

A dielectric multiplies capacitance. Because its polarization partly cancels the field, the plates of a capacitor can hold more charge at the same voltage; how much more is the material's permittivity, about 1 for air and from 2 to several thousand for plastics and ceramics.

Every insulator has a ceiling: above a certain field it fails by dielectric breakdown.

Living tissue uses the same physics. A cell membrane is a thin lipid insulator between two salty, conducting solutions, and the myelin wrapped around fast axons is extra insulation laid on top (developed in Lobeworks/17, saltatory conduction and membrane capacitance).