ABSTRACT
The electromagnetic field in the steady case, that is, when if is time independent, splits into decoupled electrostatic (Chapter 24) and magnetostatic (Chapter 26) fields. The electrostatic field is created by electric charges (Section 24.1) that specify the electric field through the potential; also the electric displacement and polarization that depend on the constitutive properties of the material, viz. the dielectric permittivity and electric susceptibility (Section 24.2). The electrostatic field due to positive (negative) charges is analogous to (Chapter 12) the potential flow due to sources (sinks); the gravity field (Chapter 18) due to masses is analogous to sinks or negative charges; thus the Newton’s law of attraction leads to (Section 24.3) the Coulomb’s law of attraction (repulsion) for charges with opposite (same) signs. The method of images (Chapter 16) applies to charges or electrical multipoles near a plate (Section 24.4) or a corner (Section 24.5) with conducting or insulating walls; it is extended by the first circle theorem (Section 24.8) to the image on a cylinder. The circle theorem specifies (Section 24.7) the effect of introducing a cylindrical conductor or insulator in an arbitrary electric field, for example, the simplest case in a uniform electric field (Section 24.6). The problems of electrostatics with conducting or insulating boundaries may be extended to the interface between two media with distinct dielectric permittivities (Section 24.9). Electrostatics is the simplest model of electric fields and forces in media bounded by insulators, and of the electric charge distributions induced in conductors; it is also a building block to understand unsteady interactions that couple the electric and magnetic fields, for example, in electric circuits and electromagnetic waves.
