1.3 Source and field quantities

1.3.1 Sources: electric charge and current

The source of every electromagnetic field is electric charge, $q$, measured in coulombs [C]. Charge comes in two kinds, called positive and negative. It is conserved (the net charge of an isolated system never changes) and, in nature, it occurs in integral multiples of the elementary charge

the magnitude of the charge of an electron. Since an everyday amount of charge contains an enormous number of elementary charges, charge can be treated, at the macroscopic level, as being continuously distributed. Depending on the geometry we then speak of a line, surface or volume charge density,

defined precisely in Section 3.1. Charges in motion constitute a current: the current $I$ through a surface is the charge crossing it per unit time, measured in amperes [A] = [C/s], and the vector current density $J$ [A/m²] describes how the current is distributed over a cross-section.

1.3.2 Fields

Four vector fields are used in electromagnetics, two electric and two magnetic (Table 1.3.1). In this course we work mainly with the two field intensities, the electric field $E$ and the magnetic field $H$. The electric flux density $D$ and the magnetic flux density $B$ are related to them through the properties of the medium. In free space the relation is a simple proportionality (Section 1.4), and in material media it contains the response of the material, which we study in Section 3.4. Only the electric quantities $E$ and $D$ are needed in these notes, because the magnetic ones belong to magnetostatics.

Table 1.3.1. Fundamental electromagnetic field quantities and their SI units.

	Field quantity	Symbol	Unit
Electric	Electric field intensity	$E$	V/m
	Electric flux density (electric displacement)	$D$	C/m²
Magnetic	Magnetic field intensity	$H$	A/m
	Magnetic flux density	$B$	T (= Wb/m²)