Dielectrics and Polarization
Dielectrics polarize in an electric field, producing bound charges that reduce the field and raise capacitance.
Definition
A dielectric is an insulator whose bound charges shift slightly in an applied electric field, producing a polarization; the resulting bound charges and the displacement field, susceptibility, and permittivity describe how the material modifies the field within it.
Scope
This topic covers the polarization of dielectric materials, bound surface and volume charges, the electric displacement field, electric susceptibility and permittivity, the relationship between microscopic polarizability and macroscopic permittivity, energy in dielectrics, and boundary conditions at dielectric interfaces. It includes linear dielectrics and notes ferroelectric behaviour, while detailed solid-state mechanisms belong to condensed-matter physics.
Core questions
- How does a dielectric become polarized in a field?
- How do bound charges and the displacement field relate to the polarization?
- What links microscopic polarizability to the macroscopic permittivity?
Key concepts
- polarization
- bound charge
- displacement field D
- electric susceptibility
- permittivity
- dielectric constant
- polarizability
- ferroelectricity
Key theories
- Polarization and bound charges
- An applied field induces aligned dipoles whose net effect is equivalent to bound surface and volume charges; the displacement field is defined so that its sources are only the free charges.
- Clausius-Mossotti relation
- The macroscopic permittivity is connected to the microscopic polarizability and number density of molecules through the Clausius-Mossotti relation, accounting for the local field acting on each molecule.
Clinical relevance
Dielectric behaviour governs capacitor and insulator design, high-permittivity materials in electronics, dielectric spectroscopy of materials and tissue, and the electromagnetic response of biological membranes and macromolecules.
History
Faraday introduced the concept of the dielectric and measured how materials raise capacitance in the 1830s. Mossotti and Clausius related permittivity to molecular polarizability in the mid-nineteenth century, a relation later grounded in Lorentz's electron theory.
Key figures
- Michael Faraday
- Ottaviano-Fabrizio Mossotti
- Rudolf Clausius
Related topics
Seminal works
- landau1984
- griffiths2017
Frequently asked questions
- What is the dielectric constant?
- It is the relative permittivity, the factor by which a material reduces the electric field for a given free charge compared with vacuum, and equivalently the factor by which it increases capacitance.
- What are bound charges?
- They are the effective charges that appear on the surface and in the interior of a polarized dielectric due to the alignment of microscopic dipoles; they are not free to move through the material.