If you had a heat sink of zero mass, infinite conductivity and zero thermal
resistance btween sink and air, it would work perfectly, no matter what size it
was. So mass itself doesn't matter: the implication is that greater mass
equates to greater area and lower thermal resistance. After all, which is going
to give best results - 500 grams (OK, 1 pound in the US!) of depleted uranium
or 500 grams (1 pound) of aluminium? The aluminium obviously has a greater
volume, and thus a greater surface area.
In this imperfect world, the mass times the specific heat tells you how many
calories are needed to raise the sink temperature above ambient by some amount.
The power being dissipated at 4.2 Joules/calorie tells you how long it takes to
do it. The Theta sink-to-air tells you how much heat the sink is losing. The
complication then occurs because the sink is not generally at equal
temperature all over. In any case, all you're really interested in at the end
of the day is Theta-junction-to-ambient. From memory, you can end up with set
of simultaneous second order differential equations trying to work it all out
from first principles, and those are things that I avoid!
73
Peter G3RZP
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