Mon., Feb. 18 notes

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

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.

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.