Monday Feb. 22, 2016

Buyepongo "Tropical Potpourri" (3:29), "Monte Verde" (5:34)

The Optional Assignment on the Station Model and Surface Weather Maps has been graded and was returned in class today.

Quick review: temperature & heat, temperature scales

When you add energy to something its temperature usually increases.  The figure below shows you what happens inside an object when it's temperature changes.



Temperature provides a measure of the average kinetic energy of the atoms or molecules in a material.
The atoms or molecules inside the warmer object will be moving more rapidly (they'll be moving freely in a gas, just "jiggling" around while still bonded to each other in a solid).   Since kinetic energy is energy of motion, temperature gives you an idea of the average speed of the moving atoms or molecules in a material.

You need to be careful what temperature scale you use when using temperature as a measure of average kinetic energy.  You must use the Kelvin temperature scale because it does not go below zero (0 K is known as absolute zero). The smallest kinetic energy you can have is zero kinetic energy.  There is no such thing as negative kinetic energy.

You can think of heat (heat energy) as being the total kinetic energy of all the molecules or atoms in a material.
This is illustrated below.  The figure was drawn so that all of the atoms or molecules had about the same average kinetic energy.  There are fewer atoms or molecules in the figure at left.  So the total of all the kinetic energies is less than in the figure at right.
The atoms or molecules in the examples below have the same temperatures,
the same average kinetic energies
The total kinetic energy of all the atoms or molecules is lower in this example.
 More atoms or molecules means the total kinetic energy, the heat energy, is this example is higher.


There are three temperature scales that we might have occasion to use in this class.  They're shown below.  There are two temperatures that you should try to remember for each scale. 



The boiling and freezing points of water on both the Celsius and the Fahrenheit scales (the freezing point of water and the melting point of ice are the same).  Remember that the Kelvin scale doesn't go below zero.  0 K is referred to as absolute zero, it's as cold as you can get.  A nice round number of the average temperature of the earth is 300 K, that's the last temperature value to remember.

Here's some additional temperature data that I'm including just in case you're interested.



You certainly don't need to try to remember all these numbers.  The world high temperature record value of 136 F above was measured in Libya at a location that was only about 35 miles from the Mediterranean coast.  Water, as we have seen, moderates climate so it seemed odd that such a high temperature would have been recorded there.  The World Meteorological Organization recently decided the 136 F reading was invalid and the new world record is the 134 F measurement made in Death Valley.

The continental US cold temperature record of -70 F was set in Montana and the -80 F value in Alaska.  The world record -129 F was measured at Vostok station in Antarctica.  This unusually cold reading was the result of three factors: high latitude, high altitude, and location in the middle of land rather than being near or surrounded by ocean (again water moderates climate, both hot and cold).  

Liquid nitrogen is very cold but it is still quite a bit warmer than absolute zero.  Liquid helium gets within a few degrees of absolute zero, but it's expensive and there's only a limited amount of helium available.  So I would feel guilty bringing some to class and I don't think it would look any different than liquid nitrogen.

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 copper tubing 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 have collided with the object and picked up energy from the object.  This is reflected by the increased speed of motion or increased kinetic energy of these molecules or atoms (these guys are colored orange). 

In the middle picture the initial layer of energetic molecules have collided with some of their neighbors and shared energy with them (these are pink).  The neighbor molecules have gained energy though they don't have as much energy as the molecules next to the hot object.

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.

Here's another neater sketch of conduction that was shown in class.





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.


Transport of energy by conduction is similar to the transport of a strong smell throughout a classroom by diffusion.  Small eddies of wind in the classroom blow in random directions and move the odor throughout the room.  For a demonstration you need something that has a strong smell but is safe to breathe.

I've tried a variety of things such as curry powder and Vicks VapoRub in the past.   This semester I tried some garlic.  The classroom is too large and the ventilation system too efficient so the smell doesn't get very far.  The demonstration is still instructive, I think, because you can visualize what should happen.  


Also we can add something else to the demonstration that might help you to understand the difference between energy transport by conduction and by convection.  




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 (rain coats yes but not winter coats).



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 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.




Energy transport by convection





I used the torch to heat up the broken glass graduated cylinder again.  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).  I'm pretty sure I forgot to mention that convection works in liquids and gases but not solids (the atoms or molecules in a solid can't move freely).



At Point 1 in the picture above a thin layer of air surrounding a hot object has been heated by conduction. Then at Point 2 a person is blowing the blob of warm air off to the right.  The warm air molecules are moving away at Point 3 from the hot object together as a group (that's the organized part of the motion).  At Point 4 cooler air moves in and surrounds the hot object and the whole process repeats itself. 

Think back to garlic demonstration earlier in class.  Diffusion alone wasn't able to spread the smell very far into the classroom.  To try to spread the smell somewhat further, we could put a small fan behind the ground up garlic powder and try to blow the smell further into the classroom.  That would be more like forced convection and would be more effective than just diffusion.

And actually you don't need to force convection, it will often happen on its own.
A thin layer of air at Point 1 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 (Point 3).  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 at Point 4 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.

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.  A piece of aluminum and a piece of wood (oak) were passed around 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 for 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)

This was as far as we got in class today.  There's one more small section before we move onto latent heat energy transport.

Wind chill
Wind chill is a really good example of energy transport by convection.  As a matter of fact I'm hoping that if I mention energy transport by convection that 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'd last about 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.