h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A worthy start on a large, complex, and difficult subject but not nearly ready for prime time.

The statements "The velocity with which the charged particle moves back and forth is constant for a given medium (eg: a wire or free space). Therefore it is the length of the path which determines the time required for a full cycle of the oscillation"

is not really correct. The velocity of carriers in most conductors is a random distribution and is strongly temperature dependent, and in dielectrics, there is no velocity because it's only the polarizability of the material that changes. What sets the frequency of the electromagnetic wave is the frequency of the force which moves the charges.

In a piece of wire, that force would be the electromotive force from some device which gives rise to an electric current (the aggregate movement of charges with time). The frequency at which the current alternates sets the frequency of any electromagnetic wave that emanates from the wire. The wavelength of such a wave only depends on the propagation speed of the medium which the wave is travelling. You must be very careful about using terminology implying that there are electromagnetic waves within conductors because in general there is no propagating wave solution, only a time varying but non-propagating solution. There are certainly electric and magnetic fields in conductors however and these give rise to most electrical phenomena we deal with in electric circuits.


An electromagnetic wave can also be produced by a time-varying magnetic field.

The statement further down this paragraph leads something to be desired:

"Thus, electrons oscillating in a short piece of wire operate at a high frequency. Within networks of power landlines, such free oscillation must be prevented to avoid fluctuations in the power rating which could lead to black-outs."

Again, the electrons ARE NOT rushing back and forth to the ends of the wire when an alternating current is present in the wire. They only have to move a little bit to let a signal go from one end of the wire to the other. Think of it this way. When we hook the wire up to an electric energy source, the electrons in the wire start moving in response to the electric and magnetic fields created when we made the connection. As the field moves down the wire, the electrons in the wire start to move as a result of these fields, and very quickly the field reaches the end of the wire, and causes the electrons there to move. But these are not the electrons from the start of the wire.

The reason that we want to use lower AC frequencies for power transmission is in large part because of the so-called skin effect. In a conductor carrying a steady-state (DC) current, the electric field is evenly distributed across the cross-section of the conductor. However, if the current varies in time, this induces a magnetic field in the wire, and this field 'pushes' the electric field out towards the outer surface of the conductor. This leaves the center of the conductor wasted as it carries no current. It is this same interrelatedness between electric and magnetic fields which gives rise to the freely propagating electric and magnetic fields which we call the electromagnetic wave.

I would take out the section on photons and take out the energy by wavelength in the table as this adds a lot of confusion.

I would take out the final remarks paragraph. I think it's misleading becasue a conventional antenna is nothing like an atom or molecule. For example, some molecules absorb microwave radiation. This isn't because the radiation is the same length as the molecule, but because the molecule has natural vibrations which occur at the same frequency as the microwave radiation. These vibrations are not the same thing as what happens when a radio wave impinges on a radio antenna.

I hope you don't become discouraged by this wave of negativity
and charge forward with an improved version.