WHAT, EXACTLY, IS AN ELECTROMAGNETIC WAVE?

 

 

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The previous article listed types of electromagnetic waves:  radio, microwave, infrared, visible light, ultraviolet, X ray, gamma ray.  All of these types do behave like waves; they act as if they are some sort of disturbance in a medium carrying energy from one place to another.  But they can travel through space from, for example, the sun to the earth.  Since there does not seem to be any kind of medium out there, it always seemed difficult to figure out how they traveled.  At various times in history people invented an imaginary medium called the “ether” for these waves to disturb.  There were, however, various ways to show that nothing like this ether could exist starting with the fact that nobody could see or feel it.

 

 

ELECTRIC CHARGES AND FORCES

 

Eventually electromagnetic theory seemed to provide an answer to the question in the title of this article.  There are such things as electric forces and magnetic forces.  Most people are familiar with them to some extent:  rub a balloon on your leg and it will stick to a wall due to the electric charge that it picks up, and magnets attract bits of iron.

 

Electric forces are forces by which one electric charge pushes or pulls on another.  There are two kinds of charge, which have been named positive and negative.  It is fairly well known that positive charges repel other positive charges; negative charges repel other negative charges; and the two types of charge attract one another.   The most common positively charged particle in the atom is the proton, which exists in the nucleus.  The most common negatively charged particle in the atom is the electron, which exists in the space around the nucleus.  The mass of the electron is much less than the mass of the proton; therefore electrons can move around from atom to atom fairly easily. 

 

 

Velocity
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 


The two positive charges shown here repel one another with the forces represented by the red arrows.  If they did not move, that would be all that happened.  But if they do move, other forces appear between them.  If the velocities are in the directions shown above, these extra forces, represented by the blue arrows, repel the charges a little more.  These are magnetic forces; such magnetic forces come about because of moving charges.  If the details of the motion differ from what is shown in the figure above, then the magnetic forces can point in other directions.  Working out the directions can become a little involved, and it does not need to concern us here.

 

In a permanent magnet, with a north magnetic pole and a south magnetic pole, the moving charges that produce the magnetism are spinning electrons.  These electrons tend to line up in certain magnetic materials such as iron, nickel, and cobalt thus adding their magnetic effects to form observable permanent magnetism. 

 

 

 

 

 

 

 

 

 

 

 


Electric forces between ordinary objects are usually not observed because there are just as many protons present as electrons.  That makes all of the electric forces balance out to zero.  It is only when some object gains some extra electrons or loses some electrons that electric forces are obvious.  Electrons can be rubbed off on one object onto another, such as with the balloon.

 

 

Ordinary matter with balanced plus and minus charges
 
 

 

 

 

 

 

 

 

 

Some electrons have been transferred thus charging both objects.
 

Positive Charge
 

Negative Charge
 
 

 

 

 

 

 

 

 

 

 

 


As long as these charges remain at rest, nothing too exciting happens with the electric forces, and the magnetic forces are zero for charges at rest.  Even if these charges move with uniform motion (constant speed in a straight line), then they do not set up waves that carry energy from one place to another.  But if a particle with either a positive or negative charge is allowed to accelerate, then it will emit electromagnetic radiation of some kind which will carry energy away from the emitting particle off into space to be absorbed somewhere else.

 

 

A DESCRIPTION OF THE WAVES

 

To give a very rough idea of how this works, we need to know that electromagnetic waves actually consist of pulses of electric and magnetic forces moving from one place to another.  To keep this from becoming too complicated, we will forget about the magnetic forces and just talk a little bit about the electric forces.  The picture below is a picture of an electromagnetic wave.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The height of the wave represents the strength of the electric forces, which first point one direction and then the other.  The arrows represent these forces.  Since they constantly change directions, then as the wave hits the negatively charged electron, it will push the electron alternately up and down thus making it oscillate.  If the wave is a radio wave, and if the electron is in a wire, then it will oscillate back and forth in the wire because of the radio wave.  So will any other electron that is free to move in the wire.  Moving electrons constitute an electric current, so we have a model of a radio wave making a current flow in an antenna.

 

There is no need for a medium such as a spring or the air or “ether”.  Electric and magnetic forces can exist without one.

 

 

MAKING WAVES

 

The wave is created by some system of accelerating particles.  You may remember that acceleration is any deviation from uniform motion.  Uniform motion is motion at a constant speed in a straight line.  Therefore an electron could emit an electromagnetic wave if it speeds up, if it slows down, if it moves in a circle, if it oscillates, or if it hits a target and comes suddenly to rest.

 

To imagine a little more about the process of creating electromagnetic waves, look at the picture of the two positive charges just below.

 

 

 

 

 

Here is the force on the right hand charge due to the left hand charge.
 
 

 

 

 


If the left hand charge is at rest, then there is a nice, stable force on the right hand charge.  But suppose we shake the left hand charge:

 

 

 

 

 

 

 

 

 

 

 

 

 


When the left hand charge is shaken, the forces that it would cause on another charged particle change.  But, and here is the important point, it takes time for the information about the new position of the charge to penetrate through space to the right hand charge.  So there is a region of space around the right hand charge where the force is still the same, old force caused by the old configuration of electric charge.  As time goes by, the new information moves farther away from the shaken charge, and finally the right hand charge will be pushed in a new direction.

 

So there is a pulse of changing electric force moving through space because the charge creating this force has been shaken.  As it happens, this pulse moves at the speed of light, and it constitutes a pulse of electromagnetic radiation.

 

The above picture talks about the force that would be caused by a charge if there were another charge there to receive the force.  This “force that would be there if only there were a charge to receive it” is called an electric field.  So you can think of an electromagnetic wave as a changing electric field advancing at the speed of light.  If the accelerated charge that is creating the wave is being shaken continuously, then there will be a continuous train of waves moving away from it.

 

 

ENERGY IN WAVES

 

Since these waves carry energy, here is a good place for a preliminary description of energy.  There will be more about it later.

 

We can think of energy roughly as the ability to move something.  The advancing electromagnetic wave carries energy because it can move any electron that it hits.  It turns out that energy is conserved, which means that no one can either create it or destroy it.  Therefore if something such as a wave gains energy, then something else had to lose the same amount of energy.  When the wave is created, it does gain energy.  So the charged particle that has been accelerated to create the wave has to lose energy.  Its energy might be replenished by whatever is shaking the charge.  Likewise, the wave can transfer energy to the electron in the wire that it hits.  Then the wave will lose energy and this target electron will gain it.  So the wave is transferring energy from its source to its target.

 

The energy it a wave is determined partly by the amplitude of the wave.  The picture below shows what amplitude means.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


One of the waves in this picture has four times the amplitude as the other.  It stands to reason that a wave with larger amplitude could move an electron to a greater degree.  So a wave with larger amplitude carries more energy.

 

It is also true that a wave with a higher frequency carries more energy.  The reasons for this are imbedded in quantum theory at least for electromagnetic waves.  However, a wave of higher frequency could certainly make a target electron vibrate back and forth more times per second thus making it move to a greater degree. 

 

An electromagnetic wave with a higher frequency has a shorter wavelength, as discussed in the previous article.  So the short wavelength end of the list of electromagnetic wave types (ultraviolet, x-ray, gamma ray) carry more energy, hit harder, and, other things being equal, do more damage.