Radio Waves: Spark gaps vs general wireless radio antennas?

Question

I was recently told by a friend who teaches college physics that "in reality radio waves are only generated when an electric current is made to move violently through a conductor". He went on to elaborate how this works with normal radio antennas, WiFi, etc. but honestly it went over my head.

One thing that stood out was that, he explained, early radio transmission (and even some modern) works through "spark gaps" and how antennas are just refined or less visibly "sparky" versions of these.

It was super interesting to me, but I'm not finding details that (to my lay-woman mind) line up with what he explained in terms of something that I could see as "violent". Could someone please explain this to me through a model or visualization and maybe share some key terms that I can use to research this more?

Answer 1

First, your friend wasn't quite correct to say that "radio waves are only generated when an electric current is made to move violently through a conductor" because, for example there is the maser that generates radio waves through molecular processes rather than from electric currents in a conductor.

However, moving current is the most usual way to generate a radio wave. Basically from Maxwell's equations we know that charge produces electric fields, and current produces magnetic fields. So a moving charge produces a changing electric field, and a changing current produces a changing magnetic field.

And a second thing we know from Maxwell's equations is that changing electric fields produce magnetic fields and changing magnetic fields produce electric fields. A self-sustaining wave produced by these processes (in the appropriate frequency band) is exactly what we call a radio or light wave.

Whether you want to call the changing currents or moving charges in the antenna that produces the radio wave "violent" is up to you, though.

Answer 2

By violently moving electric current, I believe your friend meant an accelerating electric charge. An electric charge in constant velocity motion produces an electric current which in turn produces a magnetic field, and an accelerating electric charge, which is in actually a "violent" motion produces an electromagnetic aka radio wave.

This video provides some visualisations, https://www.youtube.com/watch?v=FWCN_uI5ygY