Good and poor conductors of heat

What happens if you use a polystyrene cup instead of a glass cup to hold hot water? You will find that heat is less easily transferred through the cup, and you will feel more comfortable on holding it. Clearly some materials are better conductors of heat than others.

 
  Fig. 1-5 You feel cold when touching a metal surface. Do you think the temperature of the metal is lower than its surrounding?

Do you have the experience of touching a metal surface handle on a cold day? Your hand will feel very cold. Is it because the metal is originally colder than its surrounding? Not really. Actually this is related to the fact that metals are good conductors of heat. Try the following activity to see if you can explain your feeling on touching different objects in terms of conduction.

Activity: Touching different materials in the kitchen

But what makes metals good conductors of heat? This is closely related to the atomic structure of metals. Metals have free electrons that are not bounded to the atoms. These electrons are free to move around within the metal, colliding with the metal atoms and transferring heat to them efficiently. This makes metals better conductors of heat than most other materials.

 
  Fig. 1-6 An experiment which demonstrates different metals have different conductivity.

Even different metals would have different abilities of heat conduction. The following video shows an experiment to determine whether aluminum is a better conductor of heat than brass. Two rods of the same size, one made of aluminum and one of brass, are heated by the same lighter at one end. Candles are stick evenly along both rods. Starting from the heated end, these candles would fall one by one as the heat conducted through the rods is sufficient to melt the base of the candles. An earlier time of fall thus indicates that the metal is a better conductor of heat. Look at the video to find out which metal conducts heat better.

Comparing the ability of heat conduction of brass and aluminum
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