# Thermal conductivity

1. Jul 16, 2009

### quanchi

Help!

Which has higher heat conductivity: water or air?

And could you give me a proof as well?

Regards,
Quan Chi

2. Jul 16, 2009

### cepheid

Staff Emeritus
Have you tried looking it up?

Proof??? Wouldn't this be an experimental result? Granted, it is probably possible for someone to use theoretical physics arguments to predict what the heat conductivity of a substance depends upon and therefore which one would be higher. But that would be a lot of work and would seem to be unecessary if you just want the answer.

EDIT: Unless somebody has asked YOU to provide such a derivation, in which case asking somebody else to do it for you is cheating.

3. Jul 16, 2009

### quanchi

Just an answer would be enough. I have tried looking it up without success so I thought I might just ask some professional help. I have no time to do a research and english is not my native language.
And the reason I am asking this question is a quarrel I had with my friends. I thought the correct answer was water but I could not prove it to them so easily.

4. Jul 16, 2009

### cepheid

Staff Emeritus
5. Jul 16, 2009

### quanchi

Thank you very much :)

6. Jul 17, 2009

### Andy Resnick

Just to throw a wrench in the works, the data is not corrected for density. For example, there are two ways to quantify viscosity- kinematic and dynamic- water and air have about the same dynamic viscosities.

The Prandtl number (ratio of viscous to thermal diffusion) does take this into account, and is about 0.7 for air and 7 for water.

7. Jul 17, 2009

### cfung

If I were to explain why water is a better thermal conductor than air, I would say because liquid are denser than air (molecular distances are small) thus giving water more opportunity to contact with the heat source.

There are rarely proofs in physics theories, if any. But explanation like those can be taken as self-evident. And in some other cases, we can only feel confident about an explanation because it's grounded on suggestive evidence and logic that's been rigorously examined.

Last edited: Jul 17, 2009