Objects having differing temperatures try to come into equilibrium, by moving energy from the warmer to to cooler one. There are several different ways to transport the energy:

• Conduction through a material
• Convection of a fluid (a gas or a liquid)
• Radiation of light
• Changes of phase: evaporation or melting

The word "conductor" can refer to several things. Electrical conductors are used in wires. Orchestra conductors wave batons. Train conductors were the people who took your ticket. We are interested in conductors of thermal energy, which are different from those other conductors (in particular, good thermal conductors are not necessarily good electrical conductors).

Conduction of thermal energy takes place within matter (which can be in solid, liquid, or gaseous form). The energy moves through the material without the material itself moving noticeably. We should distinguish three ranges of thermal conductivity:

• Metals are generally good thermal conductors, for the same reason that they are generally electrical conductors: they contain electrons that can move independently of the atoms, carrying both charge and energy. So if one side of a metal rod is hot, thermal energy from that side is carried quickly by electrons to the other side, making it hot, too.
• Thermal conduction also takes place in materials that are not electrical conductors. In nonmetallic materials, it takes place by a sort of bucket-brigade: when one side of an object is warmer than the other, it contains more thermal energy, and the small scale (invisible) vibrations of the material are larger there. The motion of one region gives rise to motion in neighboring regions, and in this way the energy moves from place to place. This method of moving energy around is 100 times slower than what happens in a metal, but the thermal conduction through a solid material like glass is still appreciable: that is why we use double pane glass, storm windows, and curtains when it is important to keep the house warm.
• Air and other gases are poor thermal conductors. There's very little matter there, and the bucket brigade doesn't have very many buckets.

Thermal conduction of energy from one object to another requires that they be in contact. The rate of energy transfer is larger when the contact area is larger, and when the temperature difference is larger. However, the rate of energy transfer is smaller when the distance between the hot and cold parts is larger.

The opposite of a thermal conductor is a thermal insulator. An insulator is a very poor conductor. Because we are warm-blooded (and like to stay warm), insulators are important. When we touch a cool piece of foam insulation, it does not take energy away from us because it is not a good conductor. It may be very cold, but it does not feel very cool because it does not remove energy from us quickly.

Thermal conduction requires the presence of matter, and so the very best insulator is vacuum. Sometimes this is used -- a thermos bottle is a double bottle with vacuum between them.

Air and other gases are good insulators, because their density is low (so there isn't much material present to transport the energy). Most of the materials we use for insulation are really just air held in place: fur, hair, feathers, styrofoam, double-pane glass and storm windows, and woven cloth are all examples of this idea. Robins know all about this: they puff up their feathers when it is cold.

Robin in April	Robin in February

Thermal conduction affects whether objects at room temperature feel cold, warm or neither to us. Our body temperature is fixed, and usually well above that of our environment and the objects in that environment. When we touch an object that is cooler than we are, thermal energy flows from us to the object. How quickly energy flows depends on what the object is made of. If it's a thermal conductor, it quickly conducts energy away, leaving our hand feeling cool. Stone and glass are good thermal conductors and so they feel cool. A thermal insulator feels much different. Think of touching a cloth napkin. Even though it's at the same temperature as a smooth glass table top it rests on, it feels warmer. When we touch the napkin energy flows only slowly between us and it, and so we think it's warmer. Touching an object that is hotter than we are produces results that also depend on whether the item is a conductor or insulator. If we encounter a sun-warmed metal seat belt buckle that is hotter than we are, energy flows from the metal to us. This is again because it is a good conductor, and if it's really hot, we get burned. Meanwhile, the fabric on the car seat can be at the same temperature as the metal buckle, but it does not burn us because it's an insulator and transfers energy to us only very slowly.

Check the box when you are done:  

Discussion of thermal conduction