ABSTRACT
Any region in which an electric charge experiences a force is called an electrostatic field. Electric fields, Coulombs law, capacitance and permittivity are discussed in Chapter 6 — refer back to page 39. Summarizing the main formulae:
Electric field strength, E= Vd volts/metre
Capacitance C= Q V
farads
Electric flux density, D= Q A
D E
= ε0εr = ε
Relative permittivity εr = flux density in materialflux density in vacuum The insulating medium separating charged surfaces is called a dielectric. Compared with conductors, dielectric materials have very high resistivities (and hence low conductance, since ρ = 1/σ). They are therefore used to separate conductors at different potentials, such as capacitor plates or electric power lines. For a parallel-plate capacitor,
capacitance C= ε0εrA(n− 1)d
39.2 Polarization
When a dielectric is placed between charged plates, the capacitance of the system increases. The mechanism by which a dielectric increases capacitance is called polarization. In an electric field the electrons and atomic nuclei
of the dielectric material experience forces in opposite directions. Since the electrons in an insulator cannot flow, each atom becomes a tiny dipole (i.e. an arrangement of two electric charges of opposite polarity) with positive and negative charges slightly separated, i.e. the material becomes polarized.
