EWN - Electromagnetic Waves

Electromagnetic Waves

Introduction

In the mid-1800s, a Scottish physicist named James Clerk Maxwell turned his intellect to the relationship between electricity and magnetism. We have already studied the work of Ørsted, Ampere, and Faraday. Maxwell combined their work with innovative ideas of his own to complete the merging of electricity and magnetism into electromagnetism. Maxwell showed that all electric and magnetic phenomena could be described by four equations. Maxwell's equations are the foundations of electromagnetism in the same way Newton's laws of motion and gravity are the foundation for classical mechanics. Though Maxwell did Newton one better - his equations work with Einstein's relativity. 

Maxwell's Equations

From a mathematical perspective, Maxwell's equations are elegant. However, they require the use of calculus, so we will only discuss them in words for this course.

  1. Gauss's law: relates electric field to its source, electric charge.
  2. Magnetic field law: similar to Gausss law, but for magnetic fields.
  3. Faraday's law: an electric field is produced by a changing magnetic field.
  4. Ampere's law: a magnetic field is produced by an electric current, or by a changing electric field.

That last part of the 4th law was the genius of Maxwell added to the intellect of those whose work he combined and serves as the final piece of the puzzle for describing the production of electromagnetic waves.

Waves, a refresher

A wave is an oscillation that transmits energy. In a previous course you learned that there are two categories of waves: mechanical and electromagnetic. The primary difference is that mechanical waves require a medium in order to transmit energy through space, while the electromagnetic wave does not. An EM wave can propagate itself through the near vacuum of space.

Here is a list of other important wave information you will need to recall for this unit:

  1. transverse wave: a wave whose oscillations are perpendicular to the direction of motion of a wave.
  2. frequency (f): the number of oscillations per second, measured in units of hertz (Hz).
  3. wavelength (λ): the distance a wave travels through the course of one complete oscillation, or the distance between two successive points on a wave. Wavelength is measure in meters (m).
  4. amplitude: the maximum displacement from equilibrium for an oscillation.

Production of Electromagnetic Waves

EWN_ElectromagneticWaves_Image.jpgElectromagnetic waves are produced by an accelerating charged particle. We know that any moving charge will create a magnetic field. An accelerating charge, however, will produce a changing magnetic field. According to Maxwell, a changing magnetic field produces a changing electric field. This changing electric field produces a changing magnetic field and the process continues. If Maxwell's equations are used to do the calculations it can be shown that these interactions create waves of electric and magnetic fields. As can be seen in this diagram, the strength of the electric and magnetic fields oscillate perpendicular to one another and both are perpendicular to the direction of motion of the wave. This is an example of a transverse wave.

Since EM waves are waves of fields, and not waves of matter like mechanical waves, they can propagate through empty space. This explains why we can see light from the Sun (EM waves) but wouldn't be able to hear the explosion of a space-based Death Star (mechanical, sound wave). The following video shows a more practical example. In the video you will hear what happens when the medium for a sound wave (mechanical wave) is removed by a vacuum pump.

This is a demonstration you may have seen in a previous class. But did you notice how it proves the fundamental difference between mechanical and electromagnetic waves? As the air was removed from the container, the sound from the speaker could no longer be transmitted. The mechanical wave's medium had been removed. However, you could still see the speaker in the container. The sight of the speaker is due to the light waves that reflect off of its surface, carrying its image to your eye. So the light waves (electromagnetic waves) were still able to travel through the vacuum allowing us to see what was inside.

VIDEO SOURCED FROM PUBLIC DOMAIN