Electromagnetic Waves


In which direction points the Electromagnetic Field detector?


The purpose of this question is to see how two objects influence each other when electric charges are considered. The distance between the two objects is large.

Accordingly to the Relativity Theory only the relative speed is important.

In a second part of this exercise three objects are considered.

Description part 1

Consider a Star with one Planet
The Planet moves in a big circle in the plane of the equator around the Star.
The Planet has a positive charge.

In order to detect the electromagnetic field we build a gallows on the north pole of the Star. On the gallows hangs, via a rope, an iron ball. The iron ball has a negative charge.
When there is no electric field, normally, the iron ball will hang in a straight line, towards the direction of the center of the Star.

When there is an electric field the ball will point in the direction of the planet. The ball will move in a circle in the same time as the planet will make one revolution.

To describe this problem more accurate and to include time, let us assume it is possible to modulate the charge of the planet.

At the begin of every hour the charge of the planet is positive for a short time and then zero.

Description part 2

In the first part only 2 objects were involved: One Star and one Planet.

In this part 3 objects are involved: A very massive Star (Black Hole) to simulate our Galaxy, one Star and one Planet.

Our Sun, in this configuration, has a speed of 250 km / sec.


The ball (at the Sun) will not point in the direction where the Planet is now. In stead the ball will point in the direction a time period delta t ago. This time period dt is equal to the distance r between the Sun and the Planet divided by the speed of light. The angle is equal to speed of the planet divided by c.

The following figure shows this.

            <----                Initial Position  
              x                  O = The Sun			
         x         x             P = The position of the 
      x               x              planet now		
     x                 x         p = The position of the
                                     planet time t ago
    x         O         P        
     x             -   x 
      x               p 
         x         x 
The above picture is only correct if you assume that the Sun has no speed.

A correct description is the following:
Suppose at t0 the planet is turned ON and OFF i.e. explodes.
This disturbance propagates in a sphere, and is independent of the speed of the planet.
The Observer will see the explosion when the path of the Sun crosses this sphere.
The direction will be towards the position where the planet was at t0.


9/9/95 : If you want to be "exact", you have to take all the four (speed of our galaxy, sun, earth and moon) into account.

Last modified: 8 September 1997 Back to my home page Contents of This Document