Wed., Feb. 20 notes

Latent heat energy transport involves changes in phase or state. You need to be able to add two types of information to this picture (this is page 55 in the ClassNotes): (i) You should be able to name each of the phase changes shown above and (ii) You should also be able to indicate whether energy must be added to or removed from the material in order for each phase change to take place (does the red "energy arrow" point into the material or point outward away from the material) And actually there is a third thing, (iii), that we'll get to in a minute.

A solid to liquid phase change is melting, liquid to gas is evaporation, and sublimation is a solid to gas phase change.

Dry ice is the best example of sublimation that I can think of. When placed in a warm room, dry ice turns directly from solid carbon dioxide to gaseous carbon dioxide without melting first. If you wash clothes and stick them outside on a dry cold (below freezing) day they will eventually dry. The clothes would first freeze but then the ice would slowly sublime away.

In each case above energy must be added to the material changing phase. You can consciously add or supply the energy (such as when you put water in a pan and put the pan on a hot stove and cause it to boil).

That much is pretty clear. The confusing part of this topic is when phase changes occur without you playing any role. Energy is still required to melt ice; in this case the needed energy will be taken from the surroundings. It is not always obvious what the "surroundings" are.

Here is the third thing to understand, (iii). When energy is taken from the surroundings, what effect will that have on the surroundings? When you take energy from the surroundings, the surroundings will cool.

Here's an example where you are the surroundings. You'll be able to feel what happens when energy is taken from your body and used to evaporate some water.

When you step out of the shower in the morning you're covered with water. Some of the water evaporates. It doesn't ask permission, it just evaporates whether you want it to or not. The energy needed for evaporation is taken from the surroundings, from your body. Because your body is losing energy you feel cold.

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.


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

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.

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.

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.

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.