Wednesday, Feb. 22, 2006

An online version of the Controls of Temperature topic covered in Chapter 3 is now available.  An Optional Assignment covering this material will be distributed in class on Friday.

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Energy transport by EM radiation is the most important of the 4 energy transport processes because it is capable of carrying energy through empty space; that is something the other three can't do.  We'll briefly review static electricity, learn something about electric fields, and then use these concepts to gain a little understanding of EM radiation.

Two charges of the same polarity (like charges) would be pushed apart if they are placed near each other.  Two charges of opposite polarity would be attracted. The figure above and the one below can be found on p. 59 in the photocopied class notes.


The figure above shows the pattern of electric field arrows around a central + charge.  The E field arrows show what would happen if a second + charge were placed somewhere in the picture.  Three + charges (in yellow) placed above, to the right, and below the center charge are all pushed outward and away from the center charge.  The outward force becomes weaker when the distance between the two charges increases.  A negative charge placed to the left of the center charge is attracted to the center positive charge.

Click here for some sample questions about electric fields and static electricity.

The figures on p. 60 in the photocopied have been broken into 3 parts below for clarity.

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 arrows repeats itself. 

Note the + charge near the right of the picture.  At the time this picture was taking the EM radiation exerts a fairly strong upward force on the + charge.

This picture was taken a short time later and the rediation has traveled a little further to the right.  The EM radiation now exerts a relatively weak downward force on the + charge.


The + charge is now being pushed upward again.  A movie of the + charge would show it 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 (the wavy line connects the heads of the electric field arrows) can be described spatially in terms of its wavelength, the distance between identical points on the pattern.  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)



One way of producing is to cause electrical charge to move up and down.  You supply energy to cause the charge to oscillate and EM radiation is produced and propagates to the right.  The EM radiation encounters a charge somewhere else and causes it to begin to move up and down.  Energy has been transported to the charge.

Slow up and down oscillation produces long wavelength, low energy radiation.
Fast up and down oscillation produces shorter wavelength, higher energy radiation.

This relationship can be seen more clearly in the animation shown in class.  Use your mouse to move the slider to the left to lower the spring tension.  Then use the mouse to pull downward on the right hand charge.  It will begin oscillating slowly.  Long wavelength radiation will be produced that propagates to the left and causes the left hand charge to begin to oscillate at low frequency.  Moving the slider to the right will increase the spring tension.  Pulling down on the right hand charge will produce shorter wavelength radiation. 

Note the speed of progation (in empty space) of long and short wavelength radiation is the same.

Gamma rays, X-rays, and ultraviolet light are high-energy, short wavelength forms of EM radiation.  Infrared light, microwaves, and radio waves are longer wavelength, lower energy forms of radiation.

The different colors of visible light have wavelengths that range from 0.4 to 0.7 micrometers.  Mixing all the colors of visible light together produces white light.