[time-nuts] Mass vs BTU Function

Mike S mikes at flatsurface.com
Thu Jan 27 22:19:45 UTC 2011

At 04:18 PM 1/27/2011, J. Forster wrote...
>If you are considering conductivity for dynamic reasons, the correct
>figure of merit is "Thermal Diffusivity"
>= (Specific Heat) / (Thermal Conductivity)

If you want a thermal mass to help control temperature swings, the more 
heat capacity is good. Isn't more thermal conductivity also desired? It 
seems like a substance with low conductivity wouldn't gather/release 
heat well.

If more of both is desired, shouldn't the figure of merit should then 
be (specific heat * thermal conductivity), since you want more of both?

In answer to the original question, which asked for heat capacity per 
volume. One need only multiply the specific heat by the density. For 
the examples given, plus iron and water:

(substance) (specific heat) (density) (heat capacity?)
( ) (kJ/kg K) (g/ml^3)(kJ/l K)
Al 0.91 2.7 2.5
Cu 0.39 9.96 3.9
Pb 0.13 11.36 1.5
Fe 0.46 7.87 3.6
H2O 4.2 1.0 4.2

So, copper is best, but iron (steel shouldn't be much different) is 
pretty close and very much cheaper. Water is better and cheaper still, 
but can be a bit messy.

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