Wednesday, October 2

Today we will be using page 46a, page 47a, page 47b, page 48a, page 48b, page 55, page 56, page 57, and page 58 from the ClassNotes.

We pick up from where we left off on Monday ...
6. Energy transport processes




By far the most important process is at the bottom of the list above.  Energy transport in the form of electromagnetic radiation (sunlight for example) is the only process that can carry energy through empty space.  Electromagnetic radiation travels both to the earth (from the sun) and away from the earth back into space.  Electromagnetic radiation is also responsible for about 80% of the energy transported between the ground and atmosphere.

You might be surprised to learn that latent heat is the second most important transport process.  The term latent heat can refer to both a type of energy and an energy transport process (the energy is hidden in the water vapor, the water vapor can move around and carry that energy with it).

Rising parcels of warm air and sinking parcels of cold air are examples of free convection.  Because of convection you feel colder or a cold windy day than on a cold calm day (the wind chill effect).  Ocean currents are also an example of convection. 

Convection is also one of the ways of rising air motions in the atmosphere (convergence into centers of low pressure and fronts are two other ways we've encountered so far).  Caution: convection is 1 of 4 energy transport processes and 1 of 4 processes that cause rising air motions.

Conduction is the least important energy transport at least in the atmosphere.  Air is such a poor conductor of energy that it makes a very good insulator.

7. Energy transport by conduction

Conduction is the first of four energy transport processes that we will cover (and the least important transport process in the atmosphere).  The figure below illustrates this process.  Imagine heating the end of a piece of something just so you can visualize a hot object.  If you held the object in air it would slowly lose energy by conduction and cool off.


How does that happen?  In the top picture some of the atoms or molecules near the hot object are colliding with the object are picking up energy.

In the middle picture the initial layer of molecules are colored orange.  They are moving faster than they were.  They begin to collide with their outer neighbors and start to share energy with them.

In the third picture molecules further out (yellow) have now gained some energy.  The random motions and collisions between molecules is carrying energy from the hot object out into the colder surrounding air.

Conduction transports energy from hot to cold.  The rate of energy transport depends first on the temperature gradient or temperature difference between the hot object and the cooler surroundings.  If the object in the picture had been warm rather than hot, less energy would flow and energy would flow at a slower into the surrounding air.  If there were no temperature difference there wouldn't be any energy transport at all.

The rate of energy transport also depends on the material transporting energy (air in the example above).  Thermal conductivities of some common materials are listed.  Air is a very poor conductor of energy and is generally regarded as an insulator. 

Water is a little bit better conductor.  Metals are generally very good conductors (cooking pans are often made of stainless steel but have aluminum or copper bottoms to evenly spread out heat when placed on a stove).  Diamond has a very high thermal conductivity (apparently the highest of all known solids).  Diamonds are sometimes called "ice."  They feel cold when you touch them.  The cold feeling is due to the fact that they conduct energy very quickly away from your warm fingers when you touch them.

I brought a propane torch (2 of them actually, one to serve as a backup) to class to demonstrate the behavior of materials with different thermal conductivities.  Here's what I wanted to illustrate

Copper is a good conductor.  You must move your fingers several inches away from the end to keep from getting burned.
 Glass has much lower thermal conductivity.  You can hold onto the glass just a couple of inches from the flame and not feel any heat.  Because energy is not being carried away from the end of the piece of glass, the glass can get hot enough to begin to glow red.
 You can put your finger alongside the flame with just 1/2 inch or so of separation.  Air is a very poor conductor.  Don't put your finger above the flame though.


Because air has such a low thermal conductivity it is often used as an insulator. 
It is important, however, to keep the air trapped in small pockets or small volumes so that it isn't able to move and transport energy by convection (we'll look at convection momentarily).  Here are some examples of insulators that use air:



Foam is often used as an insulator.  Foam is filled with lots of small air bubbles, that's what provides the insulation.
You can safely hold onto a foam cup filled with liquid nitrogen (-320 F) because the foam does such a good job insulating your fingers from the cold liquid inside.


Thin insulating layer of air in a double pane window. 

We really haven't needed winter coats yet in Tucson this semester.
Down feathers are often used in coats and sleeping bags.  Packing together a bunch of the "clusters" produces very good insulation provided the feathers stay "fluffed up" and trap air.  source of this image Synthetic fibers (Primaloft - Synergy are shown above in a microphotograph) have some advantages over down.  There is still some insulation when wet and the material is hypoallergenic.  source of this image

A photograph of aerogel (image source), sometimes known as solid air.  It's an excellent insulator because it is mostly air.  The very small particles in the aerogel are scattering light in the same way air molecules do.  That's why it has this sky blue color. 
A scanning electron microscope photograph of asbestos which was once widely used as insulation.  Asbestos fibers can cause lung cancer and other damage to your lungs when inhaled.  The white bar at the top left edge of the image is 50 um across.  You can find this image and read more about asbestos here.


8. Energy transport by convection





I used the torch again to heat up the broken glass graduated cylinder.  The glass gets so hot that you can see it starting to glow red.

How would you cool off a hot object like this?  You could just hold onto it and it would eventually cool by conduction.  If you were in a little  bit more of a hurry you could blow on it.  That's forced convection, the energy transport process we will be covering next.  Or you could stick the hot end of the cylinder into some water (you'd hear a short hissing sound and the glass would probably shatter).  The hissing would mean the hot piece of glass had evaporated some water.  That would be an example of latent heat energy transport which we'll be discussing later in the period.




Rather than moving about randomly, the atoms or molecules move together as a group (organized motion). Convection works in liquids and gases but not solids (the atoms or molecules in a solid can't move freely).


In the middle of the picture above a thin layer of air surrounding a hot object has been heated by conduction. Then a person is blowing the blob of warm air off to the right.  The warm air molecules are moving away from the hot object together as a group (that's the organized part of the motion).  Cooler air moves in and surrounds the hot object and the whole process repeats itself. 

And actually you don't need to force convection, it will often happen on its own.



A thin layer of air in the figure above (lower left) is heated by conduction.  Then because hot air is also low density air, it actually isn't necessary to blow on the hot object, the warm air will rise by itself..  Energy is being transported away from the hot object into the cooler surrounding air.  This is called free convection.  Cooler air moves in to take the place of the rising air and the cycle repeats itself.

The example at upper right is also free convection.  Room temperature air in contact with a cold object loses energy and becomes cold high density air.  The sinking air motions that would be found around a cold object have the effect of transporting energy from the room temperature surroundings to the colder object.

In both examples of free convection, energy is being transported from hot toward cold.  See if you can draw in some arrows in the two cases above showing the direction of energy transport then click here.

I could put my finger alongside the flame from the propane torch without any problem.  There's very little energy transported sideways through air by conduction.  





Be careful if you put your finger or hand above the torch.  That's because there's a lot of very hot air rising from the torch.  This is energy transport by free convection and its something you can sometimes see. 





Up at the front of the classroom you might have been able to see (barely) the shimmering of hot rising air when I held the torch in front of the projector screen.  There is a technique, called Schlieren photography, that can better catch these barely visible air motions (it is able to see and photograph the differences in air density).  The photo at right is an example and shows the hot rising air above a candle.  The photo was taken by Gary Settles from Penn State University and can be found at this site.

9. Real world examples of energy transport by conduction and convection (see page 48a in the ClassNotes)

Now some examples of conductive and convective energy transport.  They really do show up in a lot more everyday situations than you might expect.




Note first of all there is a temperature difference between your hand and a room temperature (70 F) object.  Energy will flow from your warm hand to the colder object.  Metals are better conductors than wood.  If you touch a piece of 70 F metal it will feel much colder than a piece of 70 F wood, even though they both have the same temperature.  A piece of 70 F diamond would feel even colder because it is an even better conductor than metal.  I brought a piece of aluminum and a piece of wood (oak) to class so that you could check this out for yourself.

Something that feels cold may not be as cold as it seems.

Our perception of cold
is more an indication of how quickly our body or hand is losing energy
than a reliable measurement of temperature.

Here's another example



It's pleasant standing outside on a nice day in 70 F air, it doesn't feel warm or cold.  But if you jump into 70 F pool water you will feel cold, at least until you "get used to" the water temperature (your body might reduce blood flow to your extremities and skin to try to reduce energy loss).

Air is a poor conductor.  If you go out in 40 F weather you will feel cold largely because there is a larger temperature difference between you and your surroundings (and temperature difference is one of the factors that affect rate of energy transport by conduction). 



If you stick your hand into a bucket of 40 F water, it will feel very cold (your hand will actually soon begin to hurt).  Keep some warm water nearby to warm up your hand. 

Water is a much better conductor than air.  Energy flows much more rapidly from your hand into the cold water.  I mentioned in class that I thought this might be good for you.  The reason is that successive application of hot and then cold is sometimes used to treat arthritis joint pain (it used to work wonders on my Dad's knee).



You can safely stick your hand into liquid nitrogen for a fraction of a second.  There is an enormous temperature difference between your hand and the liquid nitrogen which would ordinarily cause energy to leave your hand at a dangerously high rate (which could cause your hand to freeze solid).  It doesn't feel particularly cold though and doesn't feel wet.  The reason is that some of the liquid nitrogen evaporates and quickly surrounds your hand with a layer of nitrogen gas.  Just like air, nitrogen is a poor conductor (air is mostly nitrogen).  The nitrogen gas insulates your hand from the cold for a very short time (the gas is a poor conductor but a conductor nonetheless).  If you leave your hand in the liquid nitrogen for even a few seconds it would freeze.  That would cause irreparable damage.

You can hold onto a Styrofoam cup of liquid nitrogen longer.  That is because the air can't freely move; it's trapped in little air pockets in the foam.  When air is free to move, convection will begin to transport energy at a more rapid rate than conduction alone. 

A question came up in class a few semesters ago about sticking you hand (or maybe just the tip of one finger) into molten lead.  I've never seen it done and certainly haven't tried it myself.  But I suspected that you would first need to wet your hand.  Then once you stick it into the lead the water would vaporize and surround your hand with a thin layer of gas, water vapor.  The water vapor is a poor conductor just like the nitrogen and oxygen in air, and that protects your hand, for a short time, from the intense heat.  Here's a video (and water does play a critical role)

Wind chill
Wind chill is a really good example of energy transport by convection.  As a matter of fact I'm hoping that whenever you hear of energy transport by convection you'll first think of wind chill.  Wind chill is also a reminder that our perception of cold is an indication of how quickly our body is losing energy rather than an accurate measurement of temperature.  

Before we get into the details, here's a question: 
It's 40 F outside,
the wind is blowing at 30 MPH,
and the wind chill temperature is 28 F. 
What temperature would you measure with a thermometer?



Your body works hard to keep its core temperature around 98.6 F.  If you go outside on a 40 F day (calm winds) you will feel cool; your body is losing energy to the colder surroundings (by conduction mainly).  Your body will be able to keep you warm for a little while (perhaps indefinitely, I don't know).  The 5 arrows represent the rate at which your body is losing energy. 

A thermometer behaves differently, it is supposed to cool to the temperature of the surroundings.  Once it reaches 40 F and has the same temperature as the air around it the energy loss will stop.  If your body cools to 40 F you will die.



If you go outside on a 40 F day with 30 MPH winds your body will lose energy at a more rapid rate (because convection together with conduction are transporting energy away from your body).  Note the additional arrows drawn on the figures above indicating the greater heat loss.  This higher rate of energy loss will make it feel colder than a 40 F day with calm winds.  

Actually, in terms of the rate at which your body loses energy, the windy 40 F day would feel the same as a 28 F day without any wind.  Your body is losing energy at the same rate in both cases (9 arrows in both cases).  The combination 40 F and 30 MPH winds results in a wind chill temperature of 28 F.



You would feel colder on a 40 F day with 30 MPH winds but the actual temperature is still 40 F.  The thermometer will again cool to the temperature of its surroundings, it will just cool more quickly on a windy day.  Once the thermometer reaches 40 F there won't be any additional energy flow or further cooling.   The thermometer would measure 40 F on both the calm and the windy day.

Standing outside on a 40 F day is not an immediate life threatening situation.  Falling into 40 F water is, you might last 30 minutes (though you might lose consciousness before that and die by drowning).




Energy will be conducted away from your body more quickly than your body can replace it.  Your core body temperature will drop and bring on hypothermia.

Be sure not to confuse hypothermia with hyperthermia which can bring on heatstroke and is a serious outdoors risk in S. Arizona in the summer.

Talk of how long you would last in 40 F water reminds me of a page from the National Geographic Magazine that lists some of the limits of human survival.  I'm trying to find a complete citation.

Here's a summary of the main points about energy so far.











Latent heat energy transport

This is the 3rd and the next-to-most important energy transport process (and perhaps the most confusing).

If you had a hot object that you wanted to cool off quickly the best thing would be to stick it into some water.  That would work first because water will conduct energy more rapidly than air.  Also, and this is the most important part, when a really hot object is immersed in water, you'd probably hear a brief sizzling sound, the sound of boiling water.  A lot of energy would be taken quickly from the hot object and used to boil (evaporate) the water.  A phase change means latent heat energy transport is involved.  The cooling in this case takes only a few seconds.  Latent heat is a very potent energy transport process.
Latent heat energy transport is sometimes a little hard to visualize or understand because the energy is "hidden" in water vapor or water.



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. 

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.