MadSci Network: Physics

Re: Why does aluminum foil keep a hot potato hot?

Date: Sat Nov 3 15:49:31 2001
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 1004141120.Ph

Greetings, Spencer:

I think I have to start this Answer by taking issue with your statement
about "wrapping up a hot potato".  It is my understanding that the
aluminum foil is wrapped around the potato before it is cooked, not
afterward.  The reason is that the foil, like the nail you described,
helps the potato to cook faster.

And now that I've probably confused you even more, I need to review the
three different ways that heat energy is transported from Point A to
Point B.  First is conduction, about which you seem to be comfortably
knowledgeable.  Each hot molecule near A bumps into a cooler molecule
located slightly closer to B, transfering some kinetic/heat energy.
For as long as heat pours out of A (the heating element in an oven, for
example), the molecules near A can continue to transfer heat toward B
(the potato, of course), by simply bumping other molecules.

Second is convection, which requires the molecules near A to physically
move toward B.  They generally acquire heat at A by conduction, and
deposit heat at B by conduction, but there is very little wastage of
time and energy, heating up lots of other molecules.  A good example of
natural convection in action can be found on a cold day, when a house
is warm and the windows are single panes of glass.  In this case the
whole face of the window conducts heat from inside the house to outside
-- but if that was all, the heating costs for that house would be quite
less than is actually the case.  See, as air molecules inside the house
next to the window are cooled, they tend to sink; as air molecules
outside the house next to the window are warmed, they tend to rise.

Well, if those molecules move to become no-longer-adjacent to the
window, what takes their place?  Inside the house, warm air flows
toward the top of the window; outside the house, cold air flows toward
the bottom of the window.  Thus, inside the house, we can imagine a
loop made of moving air:  At the top of the window the warm air begins
conducting heat into the glass, and being cooled by that, so it sinks
-- and encounters more cool glass, so conducts some more heat into the
glass, and sinks some more....  At the bottom of the window the quite
cool air now flows across the floor of the house, where the heating
system may warm it up again, causing it to rise, and eventually flow
back toward the window.  This is the essence of the convective process.

Meanwhile, outside the house, cold air at the bottom of the window has
some heat conducted into it, so it rises a little, encounterings glass
that is a bit warmer, so even more heat can be conducted into it, so
it rises some more....   At the top of the window it just keeps on
rising, encountering a sky full of cold air.  And there is plenty more
cold air to move toward the bottom of the window.  The total amount of
heat that can be lost from any house in this fashion is expensive!

Anyone interested in cutting down on heating costs should be willing
to do what they can to stop the convective flows.  Inside the house
curtains can be placed snugly against the wall surrounding the window,
keeping air from FLOWING toward and away from the glass.  Outside the
house shutters might be installed, for much the same reason.  One could
also invest in double-paned windows -- the gap between the two panes is
too small to allow a convective loop to exist, and so heat must waste
time merely conducting through the air (a relatively poor conductor it
is) separating the panes.

If you have ever seen a "convection oven", you might have noticed that
it is generally smaller than the average kitchen stove.  It features a
fan to push hot air from the heating element toward the potato, and
cooks it faster because very hot air begins arriving at the food rather
soon after the oven is turned on.  Forced convection is an extremely
efficient way to move heat from Point A to Point B.  Most restaurants
use convection ovens these days, because shorter cooking time is almost
always a benefit to the customers, and energy costs are lower, too.

The third and final heat-transference mechanism is radiation.  Every
type of substance is able to absorb and emit photons of pure energy.
Some substances are more efficient at emitting; some are more efficient
absorbing.  The glow you see coming from any heating element is pure
energy that, simply because it went straight from the heating element
to your eyeballs, isn't helping to cook any food.  But no matter how
much glow you actually see, you should be aware that the spectrum of
radiant energy includes a great deal that you cannot see at all --
in fact that heating element is radiating far more invisible infrared
energy than it is radiating visible energy.  Meanwhile, most ovens are
made of metal these days, and metal is a good reflector of radiant heat
energy.  This means that photons can bounce around inside the oven
until they get absorbed by something -- usually the food.

And now to tackle the question of how a wrapping of aluminum foil helps
a potato to cook faster.  As you probably know, aluminum is a pretty
good reflector of radiant energy, and so it certainly does not help to
cook the potato by reflecting photons emitted by the heating element.
However, those photons eventually all get absorbed somewhere inside the
oven, usually by the very air.  Hotter air means that it can bounce all
the better off the aluminum foil, transferring heat by conduction.  And
as you already noted, the aluminum will do a good job of conducting the
heat toward the potato.

So the potato heats up.  What happens then?  It starts to radiate
photons!  Which cannot escape, because of the layer of aluminum foil!
Instead the photons bounce back and get re-absorbed by the potato, even
as more heat is conducted to it from the air in the oven by that same
layer of foil.  Thus the potato cooks faster, because it cannot leak
very much of the heat that is getting conducted into it.

Finally, when the fully-cooked potato is removed from the oven, the
layer of foil continues to prevent the escape of heat-energy photons.
It is to be acknowledged that the foil can conduct heat from the skin
of the potato to the exterior air, but inside the potato, heat must
transfer to the skin before the aluminum can help.  This cannot happen
via convection, because the potato is a reasonably solid object, and
convection only works in gases and liquids.  It certainly happens via
conduction, but the body of a potato is a poor conductor.  Finally,
heat can radiate from the interior of the potato.  Much of that will
probably be absorbed by the body of the potato before it reaches the
skin, but some will make it that far -- only to be bounced back into
the body of the potato by the aluminum foil.

Therefore, because conduction operates poorly INSIDE the potato,
convection can only exist outside the potato, and radiation is foiled,
that is why the potato stays hot.

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