Dielectric

Abstract

We start to understand classification of materials, from the viewpoint of electron migration. Dielectric is classified into insulator, where electron migration is inhibited. In parallel plate capacitor, electrodes are charged in the application of voltage. For a simplicity, instead of charged atom, point charge is introduced. Point charge is mathematical point with negative or positive charge, and its volume is neglected. In electromagnetics, the concept “electric field” is introduced. Electric field around charged plate is represented by Gauss’s law. Under three assumptions: (1) uniform allocation of point charges, (2) uniform electric field, (3) no electric field in perpendicular direction to electrode, electric field within parallel plate capacitor is formulated. By expressing voltage between charged plates using electric field, charge of plate is formulated. Then, vacuum capacitance is defined. Vacuum capacitance is useful index to represent how charge can be stored in a capacitor, though the definition lacks scientific accuracy. Polarisation implies the relative bias of positive and negative charges. In charge neutral system, polarisation vector is expressed using dipole model, where positive and negative point charges are paired. Polarisation vectors for NaCl solid, hydrogen molecule and water molecules are considered. However, as dipole model is not applicable for charged system, general formula of polarisation vector is formulated. Finally, we consider change of capacitance, when dielectric is inserted in parallel plate capacitor. Relative permittivity for dielectric is also defined.