Wednesday, Feb. 21, 2018

Lucius "Madness" (4:20), "Two of Us on the Run" (4:51), "Turn it Around" (4:20), "Goodbye" (2:42), "Almost Makes Me Wish for Rain" (4:05),
"Free as a Bird" (3:20), "How Loud Your Heart Gets" (6:30)

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.  I don't have double pane windows in my house.  As a matter of fact I leave a window open so my cats can get in and out of the house (that's not particularly energy efficient).  It also means there are lots of mosquitoes in the house in the summer.

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.

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 p. 48a in the ClassNotes)
Now some surprisingly practical applications, I think, of what we have learned about conductive and convective energy transport.  Energy transport really does show up in a lot more everyday real life 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.

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.  


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 National Geographic Magazine that lists some of the limits of human survival.  I can't just scan the original and add it to the notes without violating copyright laws.  But if you click on the link above you'll find all of the same information online in the form of a quiz.

10. Latent heat energy transport

This is the 3rd and the next-to-most important energy transport process (probably also the hardest to understand).  We were just able to get started on this section.

If you had a hot object that you wanted to cool off quickly the best thing might 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 p. 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.  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 play the role of 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.

I'm moving the rest of the notes on latent heat energy transport to the Friday, Feb. 23 notes.

Here's a cleaned up version of the topics summary started in Monday's lecture and continued in today's class.
You might try writing the numbered headings down on a couple of sheets of blank paper and then see which of the details you could fill in from memory..