The thermal conductivities of various substances are listed in Table . . These values vary slightly with temperature, but can be considered to be constant over a normal temperature range. Compare the relatively large thermal conductivities of good thermal conductors and, metals, with the relatively small thermal conductivities of some good thermal insulators, such as wood and glass wool.
You may have noticed that some cooking pots have copper coating on the bottom. Being a good conductor of heat, copper promotes the distribution of heat over the bottom of a pot for uniform cooking. Plastic foams, on the other hand, are good insulators, mainly because they contain pockets of air. Recall that gases are poor conductors, and note the low thermal conductivity of air in the Table .
. Heat retention and transfer are important in many other applications. Houses made of concrete roofs get very hot during summer days because thermal conductivity of concrete (though much smaller than that of a metal) is still not small enough. Therefore, people, usually, prefer to give a layer of earth or foam insulation on the ceiling so that heat transfer is prohibited and keeps the room cooler.
In some situations, heat transfer is critical. In a nuclear reactor, for example, elaborate heat transfer systems need to be installed so that the enormous energy produced by nuclear fission in the core transits out sufficiently fast, thus preventing the core from overheating. Table . Thermal conductivities of some material Material Thermal conductivity (J s – m – K – ) Metals Silver Copper Aluminium Brass Steel .
Lead . Mercury . Non-metals Insulating brick . Concrete .
Body fat . Felt . Glass . Ice .
Glass wool . Wood . Water . Gases Air .
Argon . Hydrogen . Example . What is the temperature of the steel-copper junction in the steady state of the system shown in Fig.
. . Length of the steel rod = . cm, length of the copper rod = .
cm, temperature of the furnace = ° C, temperature of the other end =