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 pink). 

In the middle picture the initial bunch of energetic molecules have collided with some of their neighbors and shared energy with them (these are orange).  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 have now (the yellow ones) gained some energy.  The random motions and collisions between molecules is carrying energy from the hot object out into the colder material.

Conduction transports energy from hot to cold.  The rate of energy transport depends first on the material (air in the example above).  Thermal conductivities of some common materials are listed.  Air is a very poor conductor of energy.  Air 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.  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.

The rate of energy transport also depends on temperature difference.  If the object in the picture had been warm rather than hot, less energy would flow or energy would flow at a slower into the surrounding material.

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 our demonstration we used curry powder.

With time the smell seemed to have spread throughout about half the room.  A second pile of curry powder was placed in the back of the room so that everyone would get a chance to smell it.

---

Convection was the next energy transport process we had a look at.  Rather than moving about randomly, the atoms or molecules move as a group.  Convection works in liquids and gases but not solids. 

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 (yes, that is a drawing of a person's head) 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 can repeat itself. 

This is forced convection.  If you have a hot object in your hand you could just hold onto it and let it cool by conduction.  That might take a while because air is a poor conductor.  Or you could blow on the hot object and force it to cool more quickly.  I had wanted to put a small fan behind the curry powder to help spread the smell further out into the classroom, but I forgot to bring the fan.

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 and represents another way of causing rising air motions in the atmosphere.  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.

Now some practical applications 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.

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 sublimates when placed in a warm room, it turns directly from solid carbon dioxide to gaseous carbon dioxide). 

In each case 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) or the phase change can occur without you playing any role.  In that case the needed energy will be taken from the surroundings.  When you step out of the shower in the morning, the water takes energy from your body and evaporates.  Because your body is losing energy your body feels cold. 

Then choose one of the three arrows at the bottom of the picture to show both the direction and the force that would be exerted on each charge.

Here's the answer.  The closer the charge is to the center, the greater the strength of the outward force.  With just a little thought you can see that if you were to place + charges at other positions you would quickly end up with a figure that looks like the pattern at the bottom of p. 59 in the photocopied ClassNotes.