Wednesday Feb. 26, 2014

A couple of new songs from Suzanne Vega before class this morning:  "Laying on of Hands / Stoic 2" and "Fool's Complaint".  Both are from her new "Tales from the Realm of the Queen of Pentacles" album.

The 1S1P Assignment #2 Surface Weather Map Analysis papers were collected today.



I used a propane torch to heat up a piece of a broken graduated cylinder at the start of class today.  The glass gets so hot that you can see it starting to glow red.  Still the fact that glass is a relatively poor conductor of energy meant that I could hold onto the cylinder just a couple of inches from the end.

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 (the glass would probably shatter).  That would be an example of latent heat energy transport, we'll also 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).



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 the curry powder demonstration in class on Monday.  Diffusion 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 curry 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.  




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 free convection and its something you can see. 




You could see (with difficulty) 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 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 another example



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 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 semester or two 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.

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.   Cool windy weather is being forecast for the weekend, you can check this out for yourself.

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 (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.

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.


We spent the remainder of the class period looking at latent heat energy transport.  This is the 3rd and the next to most important energy transport process that we will cover. 

If you had an object that you wanted to cool off quickly you could blow on it.  That might take a minute or two (maybe more).  Or you could stick it into some water, that would cool it off pretty quickly because water will conduct energy more rapidly than air.  With a really hot object 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.  The cooling in this case takes only a few seconds.
Latent heat energy transport is sometimes a little hard to visualize or understand because the energy is "hidden" in water vapor or water.




You should be able to name each of these phase changes sketched above (this is p. 55 in the ClassNotes).  You should also be able to indicate whether energy must be added or removed in order for each phase change to take place.  I.e.  do you need to add energy to ice or take energy from a piece of ice to cause it to melt.


Latent heat energy transport is associated with changes of phase (solid to liquid, water to water vapor, that sort of thing) 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 probably the best example of sublimation.  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 cold (below freezing) day they will eventually dry.  The clothes would first freeze but then the ice would slowly sublimate 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.

Here's the simplest example I can think of




You put an ice cube in a glass of warm water.


Energy will naturally flow from hot to cold; in this case from the water (room temperature would be about 70 F) to the ice (32 F).  This transport of energy would occur via conduction.


Once the ice had absorbed enough energy it would melt.  Energy taken from the water would cause the water to cool.  The energy that needed to be added to the ice would be taken from the surroundings (the water) and would cause the surroundings to cool.

Here's another example you should be very familiar with.
When you step out of the shower in the morning you're covered with water.  Some of the water evaporates.  It does so whether you want it to or not.  Evaporation requires energy and it gets that energy from your body.  Because your body is losing energy your body feels 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 (I have been using  Tedy Bruschi as an example for several years) 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.  It would take 8 people running at 20 MPH to evaporate the resulting ice water. 


Phase changes can also go in the other direction.



Try to again name the phase changes and show whether energy flows in or out of the water vapor or water when they change phase.
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 orange energy arrows have turned around and are pointing from the material toward the surroundings.  It's kind of like a genie coming out of a magic lamp.  One Tedy Bruschi worth of kinetic energy is released when enough water freezes to make an ice cube.  Many genies, many Tedy Bruschis, 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 much too warm body.  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.

A can of cold drink will warm more quickly in warm moist surroundings than in warm dry surroundings.  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 to keep water vapor in the air from condensing onto the can.

Now two figures to illustrate how latent heat energy transport works.




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 from your body), is free to move from the bathroom to the kitchen where a cold can is 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. 

Energy has effectively been transported from your warm body in the bathroom to a cold can in the kitchen.


The story in this picture starts at left in the tropics where there is often an abundance or 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.