Monday, October 7

We'll review and finish latent heat energy transport page 58.  Then we'll move into the first part of the section on electromagnetic radiation - page 59, page 60, page 61 and page 62.
 
Latent heat energy transport -- Examples of how energy transport takes place.

Two more figures to illustrate how latent heat energy transport can carry energy from location to another.  This first one is my favorite, it ties everything together.




1.  You've just stepped out of the shower and are covered with water.  The water is evaporating and energy is being taken from your body. 

2.  The water vapor (containing the energy taken from your body), drifts into the kitchen where it finds a cold can sitting on a table. 

3.  Water vapor comes into contact with the cold can and condenses.  The hidden latent heat energy in the water vapor is released into the can and warms the drink inside. 

Without you even leaving the bathroom,
energy has effectively been transported from your warm body to the cold can in the kitchen.

Here's what happens on a much grander scale in the atmosphere.


We start in this picture in the tropics where there is often a surplus of sunlight energy.  Some of the incoming sunlight evaporates ocean water.  The resulting water vapor moves somewhere else and carries hidden latent heat energy with it. This hidden energy reappears when something (air running into a mountain and rising, expanding, and cooling) causes the water vapor to condense.  The condensation releases energy into the surrounding atmosphere.  This would warm the air.

Energy arriving in sunlight in the tropics has effectively been transported to the atmosphere in a place like Tucson.

Now we get started on the 4th and most important energy transport, electromagnetic radiation

Energy transport by electromagnetic radiation
It's time to tackle electromagnetic (EM) radiation, the 4th and most important of the energy transport processes (it's the most important because it can transport energy through empty space (outer space)).



Many introductory textbooks depict EM radiation with a wavy line like shown above.  They don't usually explain what the wavy line represents.

The wavy line just connects the tips of a bunch of "electric field arrows". But what exactly are electric field arrows?




An electric field arrow (vector)
 just shows the direction and
gives you an idea of the strength
of the electrical force
that would be exerted on a positive charge at that position.
It's just like an arrow painted on a street showing you what direction to drive.

Electromagnetic (EM) radiation
Now we'll use what we know about electric field arrows (electric field for short)  to start to understand electromagnetic radiation.  How is it able to carry energy from one place to another.  You'll find most of the following on p. 60 in the photocopied ClassNotes. 



We imagine turning on a source of EM radiation and then a very short time later we take a snapshot.  In that time the EM radiation has traveled to the right (at the speed of light).  The EM radiation is a wavy pattern of electric and magnetic field arrows.  We'll ignore the magnetic field arrows.  The E field arrows sometimes point up, sometimes down.  The pattern of electric field arrows repeats itself.
 
Note the + charge near the right side of the picture.  At the time this picture was taken the electric field at the position of the + charge points upward.  There is a fairly strong upward pointing force being exerted on the + charge.


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

A 3rd snapshot taken a short time later.  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.







Wavelength and frequency

Just as light (which is EM radiation) comes in different colors, there are types of EM radiation.  A couple are sketched below.


How would you describe the difference you see above in words?

The wavy pattern can be described spatially (what you would see at a particular time if the EM radiation where spread out in a snapshot) 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).  By temporally we mean you look at one particular fixed point and look at how things change with time. 


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 that is produced 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.


We'll quickly review a couple of figures and then add a third with some new information

 2 ways of describing or differentiating between different types of EM radiation: wavelength & frequency


You can describe the radiation spatially using the wavelength.  In this case you're looking at the EM radiation at different locations at one particular time.

Or you can describe the EM radiation temporally using the frequency.  Frequency is the number of up and down cycles a charge would complete per second. 

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 that is produced 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.

Wavelength, frequency, and energy
There's an important association between wavelength, frequency, and the energy in EM radiation.




 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 electromagnetic spectrum
The EM spectrum is just a list of the different kinds of EM radiation.  A partial list is shown below.





In the top list, shortwave wavelength/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).  These are shown on an expanded scale below.  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.

I've tried to demonstrate colors mixing together to make white light using laser pointers.



But it's too hard to get them adjusted so that the small spots of colored light all fall on top of each other on the screen at the front of the room.  And even if you do the small spot of light is so small that it's hard to see clearly in a large classroom (you need to do the experiment on a piece of paper a few feet away).

Here's the basic idea, you mix red green and blue light together.  You see white light were the three colors overlap and mix in the center of the picture above.  Doesn't it seem odd that green and red mix to produce yellow?