2.1 Electromagnetic Waves

Before 1800, we knew from Coulomb s experiments in France that static electric charges obeyed a force law similar to Newton s law for gravity, and that magnetic materials seemed to be made of magnetic charge-pairs that also obeyed such a force law. By 1830, Oersted in Denmark had observed that an electric current produced a magnetic force, Amp re in France had quantified its law, and Faraday in England had established the law that a changing magnetic field produces an electric force field. J. C. Maxwell in England put these facts together to conclude that time-varying magnetic and electric force fields generate each other (you can t have one without the other), make waves, and that light is electromagnetic waves. His theory was published in 1873, and in 1888 H. Hertz in Germany demonstrated by experiment the existence of radio waves. Wireless (radio) communication was on its way.

For historical reasons, the symbols used for electromagnetic field quantities have odd names and relations that are not quite intuitive. Two of the symbols, E and B, are force field vectors, but E is called electric field intensity and B is called magnetic flux density. For the other two, D is called the electric displacement vector and H is called the magnetic field intensity vector. We now have a better understanding of what these vectors represent, and so I call E the electric force field vector, B the magnetic force field vector, D is still the electric displacement field vector