dielectric permittivity

History

The concept arose from Michael Faraday's experiments in the 1830s, who noticed that more charge accumulated between the plates of a capacitor with a dielectric. The term 'dielectric permittivity' was introduced by William Thomson (Lord Kelvin). Later, James Clerk Maxwell included it in his equations, linking it with the refractive index.

How it works

An electric field tends to align molecules of the substance, especially polar ones (e.g., water), into chains along the field. These aligned molecules create their own weak field directed opposite to the external field, weakening it. The easier the molecules turn, the stronger the weakening, i.e., the higher the permittivity.

History

The concept arose from Michael Faraday's experiments in the 1830s, who noticed that more charge accumulated between the plates of a capacitor with a dielectric. The term 'dielectric permittivity' was introduced by William Thomson (Lord Kelvin). Later, James Clerk Maxwell included it in his equations, linking it with the refractive index.

How it works

An electric field tends to align molecules of the substance, especially polar ones (e.g., water), into chains along the field. These aligned molecules create their own weak field directed opposite to the external field, weakening it. The easier the molecules turn, the stronger the weakening, i.e., the higher the permittivity.

History

Ideas about dielectric permittivity date back to Henry Cavendish's experiments (late 18th century) and systematic studies by Michael Faraday (1837), who introduced the concept of 'specific inductive capacity'. The term 'dielectric permittivity' was proposed by William Thomson in 1845. James Clerk Maxwell (1862) mathematically linked ε and the speed of light c = 1/√(ε₀μ₀), demonstrating the electromagnetic nature of light. Later developments in quantum theory of polarizability (by Hans Bethe, Lars Onsager) allowed computation of ε from molecular properties.

How it works

An applied electric field E causes displacement of charges in the dielectric: electron shells deform (electronic polarization), ionic lattices shift (ionic polarization), permanent dipoles orient (orientational polarization). Each mechanism creates a dipole moment, and the macroscopic polarization P is the sum of moments per unit volume. Dielectric permittivity ε relates electric displacement D = ε₀E + P = ε₀εᵣE. In alternating fields, a phase shift occurs between displacement and field, leading to dielectric losses. The Lorentz oscillator model or the Debye relaxation model is used for description.

Open problems & fun facts

Creating dielectrics with ultra-low losses for high-frequency applications (5G, radars).; Developing materials with record-high ε for miniaturization of capacitors.; Accurate calculation of dielectric properties of nanostructures and disordered media from first principles.

💡 Some ferroelectrics (e.g., barium titanate) have εᵣ up to 10000, but when heated above the Curie point, they abruptly lose this ability due to a phase transition.