6 - Electromagnetic Waves

Summary

In order to understand the propagation of electromagnetic waves, we need to study the equations that govern electromagnetic phenomena – Maxwell's equations. Rather than dive straight in by writing the equations down, we begin by giving sufficient background material for a reader new to this area. This will only scratch the surface of the subject, and the interested reader should look elsewhere for an in depth introduction, for example, the books by Jackson (1975) and Clemmow (1973). In particular, we will not consider any relativistic or quantum effects.

Electric and Magnetic Forces and Fields

Most matter in the universe is thought to be composed of electrons, with a negative charge, protons, with a positive charge, and neutrons, which have no charge. An attractive force is exerted by an electron on a proton and vice versa. A repulsive force is exerted by an electron on another electron and by a proton on another proton. In a static situation, opposite charges attract, like charges repel due to the electric force between them. The SI unit of charge is the coulomb (C). An electron has negative charge e = 1.6 × 10⁻¹⁹ C and mass m_e = 9.1 × 10⁻³¹ kg. A proton has positive charge e and mass 1839 m_e.

It is found experimentally that, in a vacuum, the electric force between two stationary point charges of magnitudes q₁ and q₂ at x₁ and x₂ acts along the line between the charges, and is proportional to q₁q₂ and ∣x₁ − x₂∣⁻², an inverse square law.