# Good conductor of electricty => Good conductor of heat?

1. Jun 21, 2012

### jobsism

I've noticed that almost every good conductor of electricity, is also a good conductor of heat, except for certain exceptions like Graphite. Why is this so? I can't seem to associate electron flow with heat conduction.

2. Jun 21, 2012

### Gordianus

Check the Wiedemann-Franz law that explains how the ratio of thermal to electrical conductivity in metals is almost a constant

3. Jun 22, 2012

### jobsism

Re: Good conductor of electricity ==> Good conductor of heat?

Ah, thank you for your reply! I understand the law, but I'm still confused on how electrical conduction contributes to thermal conduction. I'm thinking, that in good electricity conductors, there are more number of free electrons, the motion of which causes them to acquire higher thermal energy. But is this what allows higher heat conduction?

4. Jun 22, 2012

### Cleonis

Re: Good conductor of electricity ==> Good conductor of heat?

I confirm it's about the electrons.
In metals the valence band and the conductance band are overlapping.

In any material the internal energy (the heat) redistributes over the available degrees of freedom. In a metal heat redistributes to the electron population too. The nature of a metal is that the electrons can have any energy within a certain extended band. In an insulator only a big jump in energy will free an electron to move away from its current atom. At normal temperatures the available energy is not enough for that jump, so there is high electric resistence.

Coming back to metals: part of the total internal energy is in the electron population, and the electrons can move freely through the metal, transporting the internal energy to other parts.

For more information look up things like 'valence band' and 'conductance band'.

5. Jun 22, 2012

### jobsism

Thank you, Cleonis! I think I understand now.

6. Jun 22, 2012

### Antiphon

Not always one-to-one the other way though. The best thermal conductor is diamond and it's a great insulator. Sapphire is similar.

Actually graphite is an ok conductor of both current and heat.