Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When an external field is present, the charges in the material shift or orient, reducing the effective field inside and enabling the storage of electrical energy. The response of a dielectric is described by its permittivity. The relative permittivity, or dielectric constant, ε_r, is the ratio of the material’s permittivity ε to the vacuum permittivity ε0, with the absolute permittivity ε = ε0 ε_r. In many linear, isotropic dielectrics, the electric displacement D and the field E are related by D = ε E = ε0 ε_r E.

Polarization in dielectrics arises from electronic, ionic, and orientational processes. Electronic polarization involves displacement of electron clouds, ionic polarization from relative motion of ions, and orientational polarization from the alignment of permanent dipoles. Dielectric losses occur when part of the energy is dissipated as heat, described by the loss tangent tan δ. The dielectric response can depend on frequency and temperature, and many materials exhibit dielectric relaxation as these processes lag behind changing fields.

Dielectric strength, or breakdown field, is the maximum electric field the material can withstand before irreversible conduction occurs. Values vary widely by material and conditions. Dielectrics are used to insulate and to store energy in capacitors, in cables and other electrical equipment, and as substrates in high-frequency devices.

Common dielectric materials include gases (air, SF6), liquids (transformer oil), and solids (ceramics, glasses, polymers such as polyethylene and polyimides). Some dielectrics are ferroelectric with large ε_r and nonlinear permittivity, useful in tunable components and memory. Key properties for applications are permittivity, dielectric loss, breakdown strength, and frequency/temperature dependence. Energy stored per volume in a dielectric under a field is roughly U = 1/2 ε E^2.