Figure 2.1 Schematic illustration of a dipole.According to Fig. 2.1, it is obvious that magnitude of a dipole is determined by both the separation distance (a) between positive and negative charges and the amount of charges (Q). Thus, a concept of dielectric dipole moment (p) is introduced to measure the magnitude of a dipole, as given in Eq. 2.1. Here, p is a vector from negative charge to positive charge. p = Qa (2.1) External electric field could not only induce the formation of dipoles, but also align the permanent and induced dipoles along the direction of the electric field. This process is called polarization. Polarization is responsible for many dielectric phenomena, such as energy storage and piezoelectricity. There are four types of polarization mechanisms: electronic polarization, ionic polarization, orientation polarization, and interfacial polarization.