Friday, Feb. 23, 2018

Dom La Nena "Llegare" (3:50), "Vivo No Mare" (4:57), Alison Krauss & Robert Plant "Sister Rosetta Goes Before Us" (3:20), Barcelona Gypsy Klezmer Orchestra "Djelem Djelem" (5:35)


We'll first finish up the section on latent heat energy transport.




The object of this figure is to give you some appreciation for the amount of energy involved in phase changes.  A 240 pound man or woman running at 20 MPH has just enough kinetic energy (if you could capture it) to be able to melt an ordinary ice cube (I have been using  Tedy Bruschi as an example for several years but he's now retired so I have switched to Scooby Wright).  It would take 8 people running at 20 MPH to evaporate the resulting ice water. 

Latent heat energy is energy that is hidden in water or water vapor.   We can now start to visualize what that means and how that works.
Energy added to melt the ice is hidden in the water that results
 Energy added to evaporate the water is added to the energy already in the water and is hidden in the water vapor


Phase changes can go in the other direction


Again (i) try to name each phase change and (ii) show the direction of energy flow (into or out of the material) when the phase change occurs
You might not have heard of deposition before when a gas changes directly to a solid.  The formation of frost is an example of deposition. 

You can consciously remove energy from water vapor to make it condense.  You take energy out of water to cause it to freeze (you could put water in a freezer;  energy would flow from the relatively warm water to the colder surroundings).  If one of these phase changes occurs, without you playing a role, energy will be released into the surroundings (causing the surroundings to warm). 

Note the direction of the energy arrows - energy is being released into the surroundings (warming the surroundings).  It's kind of like a genie coming out of a magic lamp.  One Scooby Wright worth of kinetic energy is released when enough water freezes to make an ice cube.  Many Scooby Wrights are released when water vapor condenses.

 This release of energy into the surroundings and the warming of the surroundings is a little harder for us to appreciate because it never really happens to us in a way that we can feel.  Have you ever stepped out of an air conditioned building into warm moist air outdoors and had your glasses or sunglasses "steam up"?  Water vapor never condenses onto your body (your body is too warm).  However if it did you would feel warm.  It would be just the opposite of the cold feeling when you step out of the shower or a pool and the water on your body evaporates.  You know how cold the evaporation can make you feel, the same amount of condensation would produce a lot of warming.  I suspect we'd be surprised at how much warming it produces.


Alternate view showing the latent heat energy in water vapor and water coming out of hiding during a phase change and being released into the surroundings.

Here's a practical application of what we have been learning.


Cans of a cold drink are taken out of the refrigerator and placed on the kitchen table on a warm dry day and a warm humid day.  Except for the differences in the amount of moisture in the air everything else is the same.  Moisture has condensed onto the can above at right.  Do the two cans warm up at the same rate or does one warm up more quickly than the other.  In the latter case which can warms up most rapidly.


The can on the right will warm more quickly.  Equal amounts of heat will flow from the warm air into the cold cans in both cases.  Condensation of water vapor is an additional source of energy and will warm that can more rapidly.  I suspect that the condensation may actually be the dominant process.



The foam "cozy", "koozie", or whatever you want to call it, that you can put around a can of soda or beer is designed to insulate the can from the warmer surroundings but also, and probably more importantly, to keep water vapor in the air from condensing onto the can (source of the image above)
We're beating this concept to death but we're almost done.  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.


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
The wavy pattern used to depict EM radiation can be described spatially (what you would see 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. 

Wavelength, frequency, and energy




 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 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.


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.