Thu., Oct. 3 notes

Kinetic energy is energy of motion. Some examples (both large and microscopic scale) are mentioned and sketched above. This is a relatively easy to visualize and understand form of energy.

Radiant energy This is probably the most important form of energy that we'll be dealing with. Electromagnetic radiation is another name for radiant energy. Sunlight is an example of radiant energy.

Radiant energy is something that we can see and feel (you feel warm when you stand in sunlight). Something that is not quite so obvious is that everyone in the classroom is emitting radiant energy. This is infrared light, an invisible form of radiant energy. And actually it's not just the people; the walls, ceiling, floor and even the air in the classroom are also emitting infrared light. We can't see it and, because it's there all the time, I'm not sure whether we can feel it or not.

Latent heat energy is an important, under-appreciated, and rather confusing type of energy. The word latent refers to energy that is hidden. That's part of the problem. Another part of what makes latent heat energy hard to visualize and appreciate is that the energy is hidden or stored in water vapor or water - that seems like an unlikely place to find energy.

2. We'll get the topic of energy units out of the way

Joules are the units of energy that you would probably encounter in a physics class. Your electric bill shows the amount of energy that you have used in a month's time, the units are kilowatt-hours. We'll usually be using calories as units of energy. 1 calorie is the energy need to warm 1 gram of water 1 C (there are about 5 grams of water in a teaspoon).

3. Temperature provides a measure of the average kinetic energy of the atoms or molecules in a material. Much of what follows is on page 44 in the ClassNotes)

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 amount of kinetic energy you can have is zero kinetic energy. There is no such thing as negative kinetic energy.

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 on 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 will see, moderates climate, it reduces the extremes, so it seems 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. There is also some question about the 134 F Death Valley value (see this article in Wikipedia). There seems to be some agreement that 129 F is the highest reliable measurement of temperature. Temperatures that hot have been measured at multiple locations.

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; plus I don't think it would look any different than liquid nitrogen.

We started class today at this point
4. Energy, temperature, and specific heat

When you add energy to an object, the object will usually warm up (or if you take energy from an object the object will cool). It is relatively easy to come up with an equation that allows you to figure out what the temperature change will be (this is another equation I'll try to remember to write on the board before the next quiz. Try to understand it, you don't have to memorize it.

The temperature change, ΔT, will first depend on how much energy was added, ΔE. This is a direct proportionality, so ΔE is in the numerator of the equation (ΔE and ΔT are both positive when energy is added, negative when energy is removed)

When you add equal amounts of energy to large and small pans of water, the water in the small pan will get hotter. The temperature change, ΔT, will depend on the amount of water, the mass. A small mass will mean a large ΔT, so mass should go in the denominator of the equation.

Specific heat is what we use to account for the fact that different materials react differently when energy is added to them. A material with a large specific heat will warm more slowly than a material with a small specific heat. Specific heat has the same kind of effect on ΔT as mass. Specific heat is sometimes called "thermal mass" or "thermal capacity."

Here's an important example that will show the effect of specific heat (see page 45b in the ClassNotes).

Equal amounts of energy (500 calories) are added to equal masses (100 grams) of water and soil. We use water and soil in the example because most of the earth's surface is either ocean or land. Before we do the calculation, try to guess which material will warm up the most. Everything is the same except for the specific heats. Will water, with its 4 times larger specific heat, warm up more or less than the soil?

The details of the calculation are shown below.

With its higher specific heat, the water doesn't heat up nearly as much as the soil. If we had been removing energy the water wouldn't cool off as much as the soil would.

5. Water moderates climate
These different rates of warming of water and soil have important effects on regional climate.

Oceans moderate the climate. Cities near a large body of water won't warm as much in the summer and won't cool as much during the winter compared to a city that is surrounded by land. Water's ΔT is smaller than land's because water has a higher specific heat.

The yearly high and low monthly average temperatures are shown at two locations above. The city on the coast has a 30^o F annual range of temperature (range is the difference between the summer and winter temperatures). The city further inland (assumed to be at the same latitude and altitude) has an annual range of 60^o F. Note that both cities have the same 60^o F annual average temperature.

Water moderates climates - it reduces the difference between summertime high and wintertime low temperatures.

Growing tomatoes in the desert - practical application
Here's another situation where you can take advantage of water's high specific heat to moderate climate on a smaller scale (it fits better in the Spring semester edition of the class than the Fall semester).

This is my winter vegetable garden (brocolli plants can be seen in the background). Tomatoes (foreground) are a summer vegetable. in Tucson you need to get tomatoes started early so that they can produce fruit before it gets too hot. I usually start mine in February and you need to protect the plants from frost.

Here's one way of doing that. You moderate the climate and surround each plant with a "wall o water" - a teepee like arrangement that surrounds each plant. The cylinders are filled with water and they take advantage of the high specific heat of water and won't cool as much as the air or soil would during a cold night. The walls of water produce a warm moist micro climate that the tomato seedlings love. The plastic is transparent so plenty of sunlight can get through. Note the brocolli growing in the background, it isn't nearly as sensitive to the cold and doesn't require protection.

6. Energy transport processes

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.

Conduction is some ways is like diffusion. I sprinkled a little curry powder on the table in the front of the classroom. I hoped that random air motions could carry (diffuse) the curry smell to the back of the room.


I hoped the smell of curry was strong enough that air currents in the room would carry the smell to the back of the room.	The smell really only made it to the first or second row. ILC 150 is too big and the ventilation system is too good.

The demonstration doesn't really work. That's OK though because I'll add a 2nd element to the demonstration that I hope will help you understand the difference between conduction and convection.

The rate of energy transport by conduction 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

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.


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.

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.

At this point I put a small fan behind the curry powder with the idea that the fan would carry the curry smell further back in the room. I'm not sure how far back the smell traveled.

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.

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.

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.


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.