# Electomagnetic waves

1. Feb 25, 2005

### electromagnet

What are electromagnetic waves? Why are they so important?

2. Feb 25, 2005

### Vern

Electromagnetics

James Clerk Maxwell first wrote down the equations that describe electromagnetic waves back in the 1800's. They've survived to present without any change that I know about.

They're important in that they are the basis for all communication and play a role in everything else we do. About the best book I have come across that looks concepts that work is "Fundamentals of Photonics", Bahhaa E. A. Saleh and Malvin Carl Teich. It takes you all the way from Ray optics up to the modern concepts of Photons as the mediator of the electromagnetic forces.

3. Feb 25, 2005

### rbj

Imagine that you are standing 3 or 4 meters away from me and we are facing each other. You are holding a positively charged object (maybe approximately 1/500 Coulomb of charge) and I am holding a negatively charged object of about the same amount of charge. Being oppositely charged, these two charged objects have an attraction for each other. We are allowing movement in the up-down direction and in the left-right direction but restricting movement along the direction toward the other person.

So, if you are allowing free movement in the up-down and left-right directions, the position of your charge will adjust to be closest to the position of my charge. If I move my charge up, your charge (if allowed free movement) will move up. If I move my charge down, yours will move down. If I move my charge to my left, yours will move to my left (or your right). etc.

Now I move my charge back and forth repeatedly. What does your charge do? It moves back and forth repeatedly. That's an electromagnetic wave. You can think of my moving charge as a transmitting antenna and your moving charge as a receiving antenna.

In real antennas in our common experience, charge is either being driven to slosh back and forth along the radiating element by some transmitter hooked up to it, if it is a transmitting antenna, and the electromagnetic wave that impinges upon a receiving antenna will cause charge to slosh back and forth which causes a voltage to be generated an one end where the receiver is hooked up.

If I move my charge back and forth a million times per second, you could literally get a standard AM radio and tune it in at 1000 kHz.

If I move my charge back and forth 450 trillion times per second, you could literally see it as a blur of red light.

Electromagnetic waves are simply the interacting of moving charges that attract or repel each other. That interaction is not instantaneous because the speed of electromagnetic propagation (often called the "speed of light") is not infinite. When I move my charge to the left, your charge will follow, but not immediately from a POV of a time-scale of nanoseconds.

They say about 90% of the interaction of matter that we commonly see is due to the electromagnetic force and about 10% is due to gravity.

FWIW.

r b-j