Dielectric and Magnetic Materials

Flashcards for reviewing Dielectric and Magnetic Materials lecture notes.

Dielectric Materials

Insulators that, when placed in an electric field, experience a separation of positive and negative charges, leading to polarization and a dielectric constant.

Dielectric Polarization (P)

The induced dipole moment per unit volume of the dielectric in the presence of an electric field; a vector quantity measured in Cm^-2.

Dielectric Polarisability (α)

The ability of a dielectric to allow its charges to separate in the presence of an electric field; defined as net dipole moment induced per unit electric field (α = μ/E), with units of Fm^2.

Dielectric Susceptibility (χ)

Measures the amount of polarization in a dielectric for a given electric field; defined by the relation P = χ ε₀ E, and is unitless.

Dielectric Constant (εr)

The ratio between the capacitance with a dielectric to the capacitance without a dielectric (air medium); also the ratio of the electric field without the dielectric to the electric field with the dielectric. It is unitless.

Electronic Polarization

Polarization due to the displacement of the electron cloud in an atom in the presence of an electric field; independent of temperature and occurs rapidly (10^-14 to 10^-15 seconds).

Ionic Polarization

Occurs in ionic dielectric crystals due to the displacement of positive and negative ions when placed in an electric field; independent of temperature and occurs in 10^-12 to 10^-13 seconds.

Orientation Polarization

Occurs when molecular dipoles rotate about their axis of symmetry and align with an applied electric field; strongly dependent on temperature.

Lorentz Internal Field

The total resultant electric field acting on an atom inside a polarized dielectric; given by Ei = E + P/3ε₀.

Clausius-Mossotti Equation

Relates the dielectric constant (εr) and the polarizability (α) of a polarized dielectric: (εr - 1)/(εr + 2) = (Nα)/(3ε₀).

Magnetic Dipole Moment (µm)

The product of magnetic pole strength ('m') and the length of the magnet ('2l'), OR the electric current of 'I' ampere flows through circular wire of one turn howing an area of class section Am^2 (µm = IA). Expressed in Am^2.

Magnetisation (M)

Defined as the magnetic dipole moment per unit volume; expressed in Am⁻¹.

Magnetic Field Strength (H)

The force experienced by a unit north pole placed at a given point; expressed in Am⁻¹.

Magnetic Susceptibility (χ)

The ratio of magnetization produced in a sample to the magnetic field strength (χ = M/H); unitless.

Magnetic Flux Density (B)

The number of magnetic lines of force passing through a unit area of cross-section of magnetic material; expressed in Wb/m² or Tesla.

Magnetic Permeability (μ)

A measure of the amount of magnetic lines of force penetrating through a material; the ratio of magnetic flux density in the material to the applied field intensity (μ = B/H).

Orbital Magnetic Moment

Magnetic moment established by the orbital motion of electrons around the nucleus.

Spin Magnetic Moment

Magnetic moment resulting from the spinning of electrons about their own axes.

Bohr Magneton (µB)

A fundamental unit of magnetic moment, equal to 9.27 x 10^-24 Am^2; represents the magnetic moment due to electron spin.

Hysteresis

The lagging of magnetic flux density (B) behind the applied magnetic field (H) when a ferromagnetic material is magnetized and demagnetized.

Retentivity (Br)

The ability of a ferromagnetic material to retain magnetism when the applied magnetic field is reduced to zero.

Coercivity (Hc)

The magnetic field intensity required to reduce the magnetic flux density to zero in a magnetized material.

Soft Magnetic Materials

Ferromagnetic materials that are easily magnetized and demagnetized; characterized by steep hysteresis loops, high susceptibility and permeability, and low retentivity and coercivity.

Hard Magnetic Materials

Ferromagnetic materials that are difficult to magnetize and demagnetize; characterized by broad hysteresis loops, low susceptibility and permeability, and high retentivity and coercivity.

Domain Theory of Ferromagnetism

Weiss's theory explaining ferromagnetism by postulating the existence of domains, which are regions within a ferromagnetic material where all magnetic moments are aligned in the same direction.

Curie-Weiss Law

Relates magnetic susceptibility to temperature, showing that susceptibility is inversely proportional to the difference between temperature and the Curie temperature: χ = C / (T - θ).