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Q & A: Heat Conduction of Metals

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Most recent answer: 11/14/2007
Q:
Why do certain metals conduct heat better than others?
- Andrew Harless (age 11)
Franklin middle School, Springfield Illinios
A:
In ordinary metals, the same electrons which carry electrical current are also responsible for most of the heat conduction. The reasons for the variation in thermal conductivity are the same as the reasons for the variation in electrical conductivity. Roughly speaking, there are three factors which determine these conductivities:

1. the density of conduction electrons
2. the typical speeds of conduction electrons
3. the typical distances that the electrons travel before they bump into something and change directions.

In ordinary metals, the third factor is the most variable one. One of the biggest reasons that it varies is that the electrons actually travel as waves which don't bounce off the atoms in good crystals, the same as light waves don't bounce off atoms as they travel through a good transparent crystal. Some ordinary metals (say brass) are alloys of different types of atoms, so the electrons bounce off the irregular patterns of different types of atoms. Others, like copper, silver, and aluminum, are pure elements. There will be a little electron scattering even in perfect crystals of pure elements, since the atoms are always jiggling a little out of position because of the energy connected with temperature. This 'thermal' scattering varies substantially between different metals. The scattering off alloys, defects, etc. can be much larger.

mike w

(published on 10/22/2007)

Follow-Up #1: glass heat conduction

Q:
why glass is a bad conductor of heat ?
- deepti (age 19)
delhi
A:
Ordinary glasses have no free electrons, so those arenít around to provide the sort of heat conduction you find in metals. That leaves sound waves as the main heat conduction mechanism. Glasses have irregular atomic positions, unlike crystals, so that tends to scatter the sound waves, preventing them from travelling very far in a straight line. As a result, glasses tend to have a bit lower thermal conductivity than crystals.

Mike W.

(published on 11/14/2007)

Follow-up on this answer.