15.2 Maxwell's EMR Theory

In 1865, James Clerk Maxwell summarized four basic relationships between electricity and magnetism that predicted the existence of an electromagnetic wave.  The four relationships were described in mathematical notation beyond the scope of this course but are presented here as general textual descriptions.

• An electric field surrounds any charged object, and an electrostatic force will act on any other charged object that exists within it, exhibiting action at a distance.
  
  
• A magnetic field is a three-dimensional continuous closed loop.
  
  
• An electric current, or a changing electric field, will produce a magnetic field.
  
  
• A changing magnetic field can produce an electric field capable of causing an induced current and potential difference in a conductor within the field.

From these four relationships, two reverse phenomena exist.  A changing magnetic field produces an electric field, and a changing electric field produces a magnetic field.  Based on this knowledge, Maxwell was able to correctly predict that it is possible for changing magnetic and electric fields to travel through empty space in the form of an electromagnetic wave. His prediction also described other properties and characteristics of the electromagnetic wave.  Specifically, he predicted the following:

• The electromagnetic wave consists of perpendicular, oscillating, electric, and magnetic fields in a constant phase relationship.  The direction of propagation is perpendicular to both the magnetic and electric fields.

 

View the animation of a propagating electromagnetic wave.  In this animation, you can view the wave from different angles by moving the horizontal or vertical slider an electromagnetic wave is produced whenever a charge is accelerated.