We're ready to tackle electromagnetic radiation, the most important of the four energy transport processes (it's the most important because it can carry energy through empty space).  We'll spend the next two lectures on this topic. 

To really understand EM radiation you first need to know something about electric fields.  To understand electric fields we need to quickly review a basic rule concerning static electricity. 


I have tried a short "static electricity" demonstration in the classroom version of this course with mixed success.  The demonstration consists of blowing bubbles toward a Van de Graaff generator.  The metal ball at the top of the generator gets charged up with electricity (I'm not sure what polarity it is).  If the bubbles can pick up the same polarity of charge (and this is the difficult part of the demonstration) the bubbles will be repelled by the charge on the dome of the Van de Graaff generator.  Here's a video that shows what is supposed to happen.

Electric field arrows (or just the E field) show you the direction and give you an idea of the strength of the electrical force that would be exerted on a positive charge located at that point.



In this figure (p. 59 in the ClassNotes) a positive charge has been placed at 3 locations around a center charge.  The electric field arrow shows the direction of the force that would be exerted on each of the charges.  The force arrow is shown in blue.

The E field arrows tell you what will happen to a + charge.  You can use the arrows to determine what will happen to a - charge also. 


For a negative charge the force will point in a direction opposite the E field arrow.

Here's a figure to test your understanding.

The direction and strength of the E field near the ground during fair weather and under a thunderstorm are shown.  Show the directions of the forces that would be exerted on the charges shown in the figure. You'll find the answer at the end of today's notes.


We'll use electric field arrows to illustrate and explain the transport of energy in the form of electromagnetic radiation.
We imagine turning on a source of EM radiation and then a short time later we take a snapshot.  The EM radiation is a wavy pattern of electric and magnetic field arrows.  We'll ignore the magnetic field lines.  The E field lines sometimes point up, sometimes down.  The pattern of electric field arrows repeats itself. 


Textbooks often represent EM radiation with a wavy line like shown above. But what does that represent?

The wavy line just connects the tips of a bunch of electric field arrows.

Now back to the earlier picture.  Note the + charge near the right side of the picture.  At the time this picture was taken an upward pointing electric field arrow shows that the EM radiation exerts a fairly strong upward force on the + charge.



This picture was taken a short time after the first snapshot when the radiation had traveled a little further to the right.  The EM radiation now exerts a somewhat weaker downward force on the + charge.

The + charge is now being pushed upward again.  A movie of the + charge, rather than just a series of snapshots, would show the charge bobbing up and down much like a swimmer in the ocean would do as waves passed by.


The wavy pattern used to depict EM radiation can be described spatially in terms of its wavelength, the distance between identical points on the pattern.  By spatially we mean you look at different parts of the radiation at one particular instant frozen in time.


Or you can describe the radiation temporally using the frequency of oscillation (number of up and down cycles completed by an oscillating charge per second).  By temporally we mean you at one particular point for a certain period of time.

EM radiation can be created when you cause a charge to move up and down. If you move a charge up and down slowly (upper left in the figure above) you would produce long wavelength radiation that would propagate out to the right at the speed of light.  If you move the charge up and down more rapidly you produce short wavelength radiation that propagates at the same speed.

Once the EM radiation encounters the charges at the right side of the figure above the EM radiation causes those charges to oscillate up and down.  In the case of the long wavelength radiation the charge at right oscillates slowly.  This is low frequency and low energy motion.  The short wavelength causes the charge at right to oscillate more rapidly - high frequency and high energy.

These three characteristics: long wavelength / low frequency / low energy go together. So do short wavelength / high frequency / high energy.  Note that the two different types of radiation both propagate at the same speed.

The following figure illustrates how energy can be transported from one place to another (even through empty space) in the form of electromagnetic (EM) radiation.


You add energy when you cause an electrical charge to move up and down and create the EM radiation (top left).

In the middle figure, the EM radiation then travels out to the right (it could be through empty space or through something like the atmosphere). 

Once the EM radiation encounters an electrical charge at another location (bottom right), the energy reappears as the radiation causes the charge to move.  Energy has been transported from left to right.


This is really just a partial list of some of the different types of EM radiation.  In the top list, shortwave length and high energy forms of EM radiation are on the left (gamma rays and X-rays for example).  Microwaves and radiowaves are longer wavelength, lower energy forms of EM radiation.

We will mostly be concerned with just ultraviolet light (UV), visible light (VIS), and infrared light (IR).  Note the micrometer (millionths of a meter) units used for wavelength for these kinds of light.  The visible portion of the spectrum falls between 0.4 and 0.7 micrometers (UV and IR light are both invisible).  All of the vivid colors shown above are just EM radiation with slightly different wavelengths.  When you see all of these colors mixed together, you see white light.

Here's the answer to the question found earlier in the notes about electric fields and forces at the earth's surface

Electrical forces will move positive charges in the direction of the electric field.  Negative charges will move in a direction opposite the electric field.