Fundamentals of Electromagnetics

Abstract

Coulomb’s law states that for a given pair of stationary charges, q and Q (which have the unit “coulomb”¹), that are spaced r distance apart, the electric force between them can be expressed as

$${\bf F}={1\over 4\pi\epsilon_{0}}{qQ\over r^{2}}{\bf r}$$

(3.1)

where ε _o = 8.85 × 10^-12 C²/N-m² is a proportionality constant and is the so-called permittivity of free space. Note that F is in the direction of the unit vector r and can be either repulsive or attractive depending on whether q and Q are of the same or opposite signs.

¹Note that along with meter, kilogram and second, coulomb is a fundamental unit. Together they make up the “MKS” system that we will be using throughout the rest of this book. See Appendix A for the conversion between the MKS and the CGS systems and a list of useful physical constants.