Monday Oct. 1, 2012

Songs from the Beatles are often short so I was able to squeeze 4 in before class today.  You heard "I'll Cry Instead", "Things We Said Today", "You Can't Do That", and "I'll Be Back" all from the "A Hard Day's Night" album.

The Surface Weather Map optional assignment was collected today.  I'll try to get those graded and returned to you this week.

The Upper Level Charts optional assignment is due at the beginning of class on Wednesday.

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.  I heated up the end of a piece of copper tubing just so you could 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 bunch 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.

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 to class to demonstrate the behavior of materials with different thermal conductivities.

A piece of copper tubing is held in the flame in the picture at left.  Copper is a good conductor.  Energy is transported from the flame by the copper and you must grab the tubing several inches from the end to keep from burning your fingers.  Part of a glass graduated cylinder is held in the flame in the center picture.  You could comfortably hold onto the cylinder just a couple of inches from the end because glass is a relatively poor conductor.  The end of the glass tubing got so hot that it began to glow (its is emitting radiant energy, the 4th of the energy transport processes we will discuss).  Air is such a poor conductor that it is safe to hold your finger just half an inch from the hot flame and still not feel any heat coming from the flame.

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 smells throughout the room.  For a demonstration you need something that has a strong smell but is safe to breathe.

I chose curry powder.

With time I was hoping the smell would spread throughout the room.  But ILC 150 is too large and the ventilation system is too good.  It quickly replaces air in the classroom with fresh air from outside (if mercury were ever spilled I'm guessing the ventilation system won't allow the vapor to build up the dangerous levels).  Still we'll add another element to this demonstration and try to spread the curry further into the room.

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 shortly).  Here are some examples of insulators that use air:

Foam is filled with lots of small air bubbles, they're what provides the insulation.

Thin insulating layer of air in a double pane windowI don't have double pane windows in my house.  As a matter of fact I leave a window open so the cats can get in and out of the house (that's not particularly energy efficient).  And the stray cats have found out about it and come in to eat my cat's food).  Maybe sprinkling curry powder on the carpet will keep the stray cats out.

Hollow fibers (Hollofil) filled with air used in sleeping bags and winter coats.  Goose feathers (goosedown) work in a similar way.  I thought of another example after class, Fiberglas insulation.  It works so well as an insulator first because it is glass which has low thermal conductivity and also because it traps lots of little pockets of air.

Convection was the next energy transport process we had a look at.

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).
That's something I forgot to mention in class.

I heated up the piece of copper tubing again.  How would you cool it off?  The first thing that might come to mind is to blow on it.  That's forced convection.

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.

To try to spread the curry smell further into the room, I put a small fan behind the curry powder.

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.   I'm very careful if I put my fingers 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 convection.

I held the torch in front of one of the screens and you could barely see some of the shimmering rising air from the torch (see sketch at left).  Schlieren photography can capture this hot, rising, low-density air (the photo above at right, not shown in class, is from this site was taken by Gary Settles from Penn State University.  You'll find some other examples if you click on the link).

Free convection is a 3rd way of causing rising air motions (together with convergence into centers of low pressure and fronts).  They're sketched below together with the 4th process.

Wind is blowing up and over an obstacle in the fourth sketch above.  This is orographic or topographic lifting.  Clouds would be found on the upslope side of the mountain.  The sinking air on the downwind side warms and keeps clouds from forming.  One side of a moutain can be green and lush because of abundant precipitation.  The other side could be dry and covered less and different vegetation.  This is the rainshadow effect and will come up again later in the course.

Now some fairly 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 or whatever is losing energy than a reliable measurement of temperature.

Here's a similar situation.

It's pleasant standing outside on a nice day in 70 F air.  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 extremeties and skin to try to reduce energy loss).

Air is a poor conductor.  If you go out in 40 F weather you will feel colder 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 (I probably shouldn't, but I will suggest you try this), it will feel very cold (your hand will actually soon begin to hurt).  Water is a much better conductor than air.  Energy flows much more rapidly from your hand into the cold water.  Successive application of hot and then cold is sometimes used to treat arthritis joint pain.

You can safely stick your hand in 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 why is that some of the liquid nitrogen evaporates and quickly surrounds your hand with a layer of nitrogen gas.  This gas is a poor conductor and 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 second it would begin to freeze and cause irreparable damage.

Wind chill is a good example of energy transport by convection and.  As a matter of fact I'm hoping that if I mention energy transport by convection that you'll first think of wind chill.  It 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 anyway (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 probably 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.

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 (you'd probably go unconscious 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.