MadSci Network: Physics

Re: how come a aluminium foil is cool when u remove it from the oven?

Date: Sat Nov 27 02:40:45 2004
Posted By: Frank Berauer, Technology Transfer Engineer Microelectronics
Area of science: Physics
ID: 1101339900.Ph

Hi Calvin,

Thanks for this interesting question.
The reason why the foil feels cool is not its specific heat
capacity (which is heat energy stored per volume per degree).
It also is not caused by its heat conductivity (which is heat
energy transported per area per degree temperature difference).
It is the ratio between volume and surface area!
I will explain this a bit more:
Heat energy is stored in the volume of the material (given by
its specific heat capacity). 
Heat energy is transported out of the material through its
surface (given by the heat conductivity).
A thin foil has a low volume and thus stores only little heat
energy. Yet it has a big surface area and thus cools down very
Another way to visualise this is by comparing the thickness of
the foil with the thichness of you outer skin (where you don't
have nerve receptors and thus won't feel any heat): The skin
is thicker. Thus by the time the heat from the foil has moved
all through your skin, the foil is already much colder, and you
won't feel much heat! 
It's a different story if you touch a hot stone (from the same
oven), which has a much higher volume-to-surface-ratio (and is
thicker than your skin): It cools down slower and will burn
your finger.
I hope this helps to understand the problem.
Keep up your inquisitive spirit - I am looking forward to more
such questions!


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