Santhakumar Raja

Ferro-electricity When a dielectric material exhibits electric polarization even in the absence of external field, it is known as ferro-electricity and these materials are termed as ferro-electrics. Ferro-electric Ferro-electrics are anisotropic crystals which exhibit spontaneous polarization, i.e., they exhibit polarization even in the absence of external electric field. Examples (i) Rochelle salt. (ii) Barium titanate

If N be the number of molecules per unit volume and α the molecular polarizability then Total polarization [latex] \quad \mathrm{P}=\mathrm{N} \alpha \mathrm{E}_{\mathrm{int}} [/latex] eqn, (10) [latex] \therefore \quad E_{\text {int }}=\frac{\mathrm{P}}{\mathrm{N} \alpha} [/latex] Further, we know that [latex] \begin{aligned} \mathrm{D} & =\varepsilon \mathrm{E}=\varepsilon_{0} \mathrm{E}+\mathrm{P} \\ \left(\varepsilon-\varepsilon_{0}\right) \mathrm{E} & =\mathrm{P} \\ \mathrm{E} & =\frac{\mathrm{P}}{\varepsilon-\varepsilon_{0}} \end{aligned} [/latex]

When a dielectric material is placed in an external electric field, it exerts a dipole moment in it. The total electric field at the atom site of the dielectric kept in an applied electric field is called the internal field. It is given by the sum of the applied field (E) and the electric fields

When a dielectric material is subjected to an alternating field, the polarisation varies with time. The polarization p(t) as a function of time t is given by P(t) = P[1- exp (-t/tr)] where, P is the maximum polarization attained on prolonged application of a static field and tr is the relaxation time for the particular

When the dielectric material is placed in an electric field, the electric field realigns the dipoles resulting in polarization. Polarization occurs in a dielectric material when it is subjected to an external electric field. There are four polarisation mechanisms. They are Electronic polarization, Ionic polarization, Orientation polarisation and Space charge polarisation. Electronic or Induced Polarisation

Depending on the separation between the centre of gravity of positive charges and the centre of gravity of negative charges, the dielectrics are classified as polar (dipole) dielectrics and non-polar (neutral) dielectrics. Polar molecules The polar molecule is one in which it will not have centre of symmetry. Here the ‘centres’ of positive and negative

The dielectric materials may be classified as solid, liquid and gas dielectrics. Solid form In solid form they may be polymeric such as nylon, pvc, rubber, backlite, asbestos and wool or may belong to the ceramic family such as glass, silica, mica, porcelain, etc. Liquid form form In liquid form they may be mineral insulating